Journal of Physics: Conference Series OPEN ACCESS HCI Issues in Tokamak Fusion Plasmas To cite this article: HP Winter 2007 J. Phys.: Conf. Ser. 58 33 View the article online for updates and enhancements. You may also like Spectroscopic diagnostic of Langmuir turbulence in magnetic fusion plasmas Eugene Oks - Magnetic fusion development for global warming suppression Jiangang Li, Jie Zhang and Xuru Duan - Economic, safety and environmental prospects of fusion reactors R.W. Conn, J.P. Holdren, S. Sharafat et al. - This content was downloaded from IP address 142.197.250.254 on 26/02/2022 at 13:54
Transcript
Journal of Physics Conference Series
OPEN ACCESS
HCI Issues in Tokamak Fusion PlasmasTo cite this article HP Winter 2007 J Phys Conf Ser 58 33
View the article online for updates and enhancements
You may also likeSpectroscopic diagnostic of Langmuirturbulence in magnetic fusion plasmasEugene Oks
-
Magnetic fusion development for globalwarming suppressionJiangang Li Jie Zhang and Xuru Duan
-
Economic safety and environmentalprospects of fusion reactorsRW Conn JP Holdren S Sharafat et al
-
This content was downloaded from IP address 142197250254 on 26022022 at 1354
HCI Issues in Tokamak Fusion Plasmas
HP Winter
Institut fuumlr Allgemeine Physik Technische Universitaumlt Wien Austria
E-mail winteriaptuwienacat
Abstract Thermonuclear fusion powers our universe and is a most promising option for electricity and hydrogen fuel generation within a future sustainable and environmentally benign energy scenario We shortly describe the current status of magnetic fusion research on its way to the international tokamak experiment ITER for which construction at the European site Cadarache (France) starts in the near future and first plasma operation is expected in about ten years from now In the adopted EU strategy for international fusion RampD ITER is the first part of a lsquobroader approach to fusionrsquo toward a fusion demonstration reactor (lsquoDEMOrsquo) which can supply electricity into a power grid before the mid of this century We then discuss the role of highly charged ions (HCI) for magnetic fusion (plasma transport heating edge and divertor plasmas and diagnostics) Finally we present two illustrative examples from our own work on HCI atomic and surface collisions with relevance for magnetic fusion plasmas
1 Introduction Highly charged ions (HCI) and fusion plasmas (a) In the present context an ion Zq+ should be called lsquohighly chargedrsquo if in a given situation its charge q is considerably higher than its equilibrium charge This can also be stated from its potential vs kinetic energy since the former strongly depends on q For example a total ionization energy of 79 eV for producing He2+ from neutral ground state He is of no concern inside a hot D-T plasma (15 keV temperature) where alpha particles with 35 MeV kinetic energy are born from fusion reactions However after many elastic collisions such alphas slow down to a kinetic energy of less than ten eV in the plasma edge and become lsquohighly chargedrsquo because in contact with the material boundary their potential energy decides on inelastic surface processes as electron emission molecular desorption etc
(b) We need to distinguish between free HCI and those embedded in a plasma environment In the latter case the balance between collisional excitation and ionization on the one hand and radiative and collisional deexcitation and recombination on the other hand strongly depends on the composition density and temperature of the host plasma Such a situation can only very approximately be described by closed plasma models and is nowadays treated by involved numerical modelling for which respective precise atomic data is a prerequisite [1 2] but by no means sufficient Likewise information from spectroscopic measurements depends strongly on the optical thickness of the plasma where the respective HCI reside upon their radiative decay
(c) Research with highly charged ions (HCI) has for many years been justified by its interest for fusion RampD as eg by DH Crandall [3] in his summary talk for the first HCI conference 1982 in Stockholm He referred to the importance of fusion plasma spectroscopy and inelastic HCI collisions with electrons (ionization excitation recombination) and atoms (charge transfer)
IOP Publishing Journal of Physics Conference Series 58 (2007) 33ndash40doi1010881742-6596581005 13th International Conference on the Physics of Highly Charged Ions
33copy 2007 IOP Publishing Ltd
It is thus puzzling that during the whole HCI conference series (with only two exceptions [4 5]) this allegedly important relation between HCI and fusion research has never been analyzed in detail
(d) However DH Crandall [3] also stressed the relation between HCI source development and experiments on slow HCI collisions During the last twenty five years the electron beam ion source (EBIS [6]) and the electron cyclotron resonance ion source (ECRIS [7]) have rapidly matured The EBIS is useful for injection of short HCI pulses into synchrotrons but in a modified version as electron beam ion trap (EBIT) [8] has become an important tool for HCI spectroscopy and internal and external low-energy atomic collision experiments ECRIS is a direct offspring from fusion plasma research with magnetic mirror machines but now has become the working horse for slow HCI studies in many laboratories Special conference series deal with those multicharged ion sources and their ubiquity and simple handling has strongly promoted new fields as slow HCI-surface interactions [9] in particular surface engineering [10] by means of lsquopotential sputteringrsquo [11] or the storage and cooling of HCI in traps and rings for QED tests atomic clock development and high-precision atomic mass measurement [12 13] Actually these more recent attractive applications make a justification of HCI related research by fusion alone obsolete On the other hand HCI definitely play an important role for fusion research in many respects but a really good understanding of this role is impossible without a strong context with fusion plasma physics In section 3 we attempt to explain this role by reference to central aspects of current research on magnetically confined tokamak plasmas
2 The status of magnetic fusion - JET ITER and the lsquofast trackrsquo to DEMO European fusion research in the last fifty years has often been criticized as too slow and costly Actually it is a great success story not only because of the achieved scientific progress but also for the increasingly integrated approach in which this research is being conducted This is a shining example for an lsquoEuropean Research Arearsquo (we recommend the interesting historical account [14] and the most recent IAEA status report [15])
In 1955 the British engineer JD Lawson [16] formulated his famous criterion according to which for net energy gain from a D-T fusion plasma with an ion temperature Ti = 10 keV one needs to achieve a lsquofusion productrsquo ni E 1020 sm3 for plasma ion density ni and energy confinement time E
Figure 1 Fusion triple product ni ETi vs Ti from different tokamak experiments since 1969 Parameters for expected ITER operation are highlighted (with permission of ITER)
Between about 10 and 20 keV the Lawson criterion can also be formulated for the lsquofusion triple productrsquo ni Ti vs Ti (cf figure 1) In principle it applies for any kind of fusion plasma including inertial confinement (ICF) schemes However until today the most successful approach has been made by magnetic plasma confinement in the so-called lsquotokamakrsquo configuration
A tokamak (Russian acronym for lsquocurrent in magnetized chamberrsquo) basically comprises a ring shaped (toroidal) plasma in a strong toroidal magnetic field (Bt) For stability reasons the magnetic
34
field lines have to be slightly twisted (rotational transform) by superposition with a poloidal magnetic field produced by a strong plasma current Ip The latter is inductively driven through the ring plasma as the secondary winding of a transformer Consequently the original tokamak scheme permits only pulsed operation However in lsquoadvanced tokamaksrsquo (see section 3) after inductive start-up the plasma current is sustained by a self-organized diffusion current (lsquobootstraprsquo effect) and by electromagnetic waves which permits continuous operation schemes
The Joint European Torus (JET) in CulhamUK is so far the largest and most successful tokamak experiment JETrsquos life started in 1983 and will continue at least until 2010 under the lsquoEuropean Fusion Development AgreementEFDArsquo [17] The JET plasma features a large radius R = 3 m with elongated cross section a plasma surface of 200 m2 and volume of about 150 m3 and Bt and Ip may reach 3 T and 3 MA respectively To achieve a sufficiently high plasma temperature ohmic heating by Ip has to be supported by other techniques as the injection of neutral fuel atom beams and of electromagnetic waves with electron cyclotron resonance (ECR) and ion cyclotron resonance (ICR) frequencies In future fusion reactors a so-called lsquoburning plasmarsquo will mainly be heated by the 35 MeV alpha particles from D-T fusion reactions
JET has been run in several campaigns with D-T fuel and in 1997 reached for about 05 s a peak fusion power of 16 MW a world record until today This figure has to be compared with the total heating power applied for plasma operation giving a peak ratio of fusion to plasma heating power (lsquoplasma amplification factorrsquo Q) of 65 JET together with numerous other tokamak experiments has generated sufficient expertise on the achievable plasma energy confinement times within the plasma parameter space (density geometry Bt and Ip) to permit a reliable empirical scaling for extrapolation toward still larger experiments in particular ITER (see figure 2)
Figure 2 Experimentally observed (vertical axis) vs empirically determined energy confinement time from various tokamak experiments extrapolated toward ITER (with permission of ITER)
The design of ITER (in Latin lsquothe wayrsquo and an officially no more valid acronym for lsquoInternational Thermonuclear Experimental Reactorrsquo) was already started in 1988 but had to be thoroughly revised in 1998 mainly for cost reasons The new design was finished in 2001 (cf figure 3 details given on the ITER webpage [18]) As to ITERrsquos main parameters R = 62 m Bt = 53 T (from superconducting coils) Ip 15 MA and plasma volume and surface are 840 m3 and ca 1000 m2 respectively
ITERrsquos principal mission is the demonstration of fusion reactor relevant burning plasmas with Q 10 during a discharge time of 500 s and possibly in steady-state operation ITER will be built by industrial companies from the present seven partners (EU Japan RF USA PR of China South Korea and India more countries have expressed their strong interest to join)
The total budget for building ITER is 47 billion euro Construction at the European site Cadarache in France will start in 2007 and first plasma operation is expected for 2016
35
In the bilateral ITER negotiations between the EU and Japan who both had offered sites a so-called lsquoBroader Approach to Fusionrsquo has been adopted based on a lsquoFast Trackrsquo strategy proposed in 2001 (parallel development of fusion plasma physics and technology) It includes support from the EU both for a next large tokamak and the first DEMO site in Japan Another bilateral collaboration agreement has recently been concluded for six years to carry out lsquoEngineering Validation and Engineering Design ActivitiesEVEDArsquo in preparation of the planned International Fusion Material Irradiation Facility (IFMIF) This accelerator-based fusion dedicated facility could be built within ten years after successful conclusion of EVEDA IFMIF shall provide the fusion reactor relevant neutron irradiation environment for development and testing of structural materials needed for the construction of DEMO Such materials and relevant technology have to be available at a time when sufficiently conclusive results from ITER have arrived ITER IFMIF and DEMO are the three principal parts of the proposed lsquoFast Trackrsquo for making fusion energy a reality before the mid of this century
3 Why are HCI important for magnetically confined fusion plasmas The presence of HCI in fusion plasmas is commonly seen just as a nuisance Neutral atoms with nuclear charge Z gt 2 from plasma-wall interaction (PWI) [19 20] diffuse into the hot plasma core where they become ionized Fully stripped ions contribute to the global plasma bremsstrahlung with a radiation power in proportion to Z2 and an approximate much stronger Z45-dependence holds for the line radiation from the incompletely ionized collisionally excited ions With a non-negligible impurity content this radiation constitutes an important power loss which needs to be balanced by stronger plasma heating and improved confinement The common figure for characterization of the plasma impurity content is the lsquoeffective ion chargersquo
Zeff Z 2nZ ne (1) Impurities including the lsquoHe ashrsquo from fusion reactions dilute the D-T plasma fuel thanks to the
plasma quasi-neutrality condition ne ZnZ (2) (ne and nZ are the electron and ion densities Z 1) the majority (D T) ion density is already
strongly reduced for a relatively small impurity content (eg by 16 for only 1 admixed fully stripped oxygen) This radiative power loss inhibits the plasma start-up strongly influences the achievable plasma density limit and also varies the neutral beam power deposition profile
However plasma impurities also play a useful and in fact rather important role not only because various plasma diagnostic techniques are based on the exploitation of impurity radiation Still more relevant is their ability to modify the plasma radiation loss profile in particular in the plasma edge region where also low-Z impurities (C O) are not yet fully ionized An important part of the kinetic energy of the plasma constituents is converted into radiation by interaction with impurities This considerably dampens the PWI and actually makes the operation of D-T-fusion reactors possible For a given impurity species and plasma temperature the radiated power strongly depends on the ion charge Carefully administered impurity seeding can thus influence the plasma balance in a rather positive way (radiation cooling by lsquoseededrsquo Ne Si Ar or other species) Problems associated with impurity production by PWI are reduced by a magnetic lsquodivertorrsquo Magnetic field lines are diverted by means of external poloidal coils toward a separated region where the plasma is partially neutralized and PWI products are rapidly pumped away in order to minimize contamination of the main plasma Most common are lsquosingle-nullrsquo poloidal divertors as also planned for ITER (see figures 3 and 4)
Another important aspect of PWI are transient lsquoedge localized modesELMsrsquo which result from particular plasma instabilities [21] During an ELM up to 10 of the total plasma thermal energy can erupt as particle flux within less than 1 ms toward first wall components This inflicts significant material damage with strong impurity influx ELMs are a special concern for ITER because the ratio of plasma energy content to first wall surface strongly increases with plasma size A possible remedy for deleterious lsquogiant ELMSrsquo is the injection of frozen fuel pellets On the other hand mild ELMs seem to be indispensable for regular expulsion of plasma impurities in particular the He ash
36
Figure 3 Cutaway view of ITER (total height 24 m with permission of ITER)
Figure 4 Cross section of ITER vacuum vessel with divertor region on bottom (with permission of ITER)
The plasma profile can be carefully tailored by local radiation cooling which influences the plasma transport [22] and in particular permits the formation of an lsquointernal transport barrierrsquo (ITB) for improved plasma confinement [23] The impurity profile is especially relevant for development of advanced tokamak regimes (non-inductive current drive for steady-state operation simultaneous control of plasma current and pressure profile by active feedback MHD stability control for optimized plasma performance [24]) All the here mentioned features are interdependent and self-consistently interacting which makes their study by integrated plasma modeling a rather difficult task However the further rapid increase of computing power will lead to increasingly realistic results in such studies
4 Some experimental studies with slow HCI for magnetic fusion plasmas The first case concerns beam emission spectroscopy (BES) for edge plasma diagnostics If a fast (10 ndash 50 keV) neutral Li beam is injected into the plasma boundary collisions with plasma electrons and ions (majority species and impurities) give rise to characteristic Li I line emission Measurement of the Li I line intensity along the injected diagnostic beam delivers the plasma electron density profile and electron capture from Li by impurity ions leads to the impurity ion density and temperature profiles
This technique has first been demonstrated at TEXTOR (Juelich) [25] [26] and later at ASDEX Upgrade (IPP Garching) [27] and it is now implemented at JET as well Evaluation of plasma properties requires extensive modeling of the Li beam state composition with the help of a dedicated atomic collision data base [28] Li-BES is also of interest for measuring correlation functions of electron density fluctuations [29] Since the applicability of Li-BES is limited by the available neutral Li beam intensity switching to Na injection will probably be useful More recently we have also applied fast He beams [30] for BES Combined line emission measurements from He I singlet and triplet transitions possibly permit evaluation of plasma electron temperature profiles First exploratory measurements have been conducted at JET (cf figures 5ab) and ASDEX Upgrade [31] [32] but until now the obtained results remain inconclusive which is primarily caused by our still incomplete atomic collision data base for neutral He beam modeling
37
Figure 5 (a) Extreme flux surface positions during a 12 cm horizontal sweep of JET plasma during 80 keV He-BES measurements (b) Time development of He I 588 nm triplet line intensity showing the changing signal from passive edge plasma emission (upper part) and from the Doppler shifted He-BES emission (lower part) [33])
However very attractive features as the rather simple preparation of intense neutral He beams by He doping of the standard neutral heating beams and the considerably deeper plasma penetration in comparison to Li and Na-BES suggest further continuation of these studies
For the second example we have recently explored [34] the influence of particle induced electron emission (PIEE lsquoparticlersquo designates ions as well as electrons) from a material plasma boundary on the voltage drop across the adjacent plasma sheath Such a situation is of interest for the plasma region near a tokamak divertor plate A new model involving a collisional kinetic sheath consistently coupled to a fluid presheath has been developed which is applicable for plasmas with a Debye length much smaller than the relevant characteristic presheath length (lsquoasymptotic two-scale limitrsquo cf figure 6a) Majority and impurity ions are accelerated by the sheath voltage from the bulk plasma and can release electrons from the material surface which flow back into the bulk plasma (see figure 6b for characteristic PIEE yields for Cq+ ion impact on different graphite surfaces) Moreover fast plasma electrons from the plasma Maxwellian tail can reach the surface to release secondary electrons and these fast electrons will also be partially reflected In a first approach we have neglected the tokamak-generic inclined magnetic field and also the fast electron reflection Under these simplifying assumptions the reduction of the plasma sheath voltage has been self-consistently calculated and was found to reach up to 30 of the sheath voltage for a case with PIEE not taken into account More realistic results will be obtained after adaptation of our model for an inclined magnetic field and with electron reflection
5 Summary and conclusions In this survey we have attempted to elucidate the beneficial role of plasma impurities (both in low and high charge states) for magnetically confined fusion plasmas in particular with respect to ITER European fusion research has taken a strong lead in this field by setting up two task forces on plasma-wall interaction and on integrated plasma modeling In both areas there is a strong need for more complete and more precise data on collisions of HCI (low-Z He Be B C O Ne medium-Z Ar Fe etc high-Z W) with hydrogen atoms and molecules and also with surfaces (Be graphite stainless steel W) The most important role of highly charged plasma impurities is to be seen in the quest of advanced tokamak plasma studies and related plasma modeling for the optimal design of future thermonuclear fusion reactors based on the tokamak configuration
38
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120
(e- io
n)
velocity (10 4 ms)
Cq+ Graphite
C4+CL
C4+HOPG
C2+HOPG
C+HOPG
C5+CL
C2+CL
Figure 6 (a) Schematic layout of new plasma sheath model with PIEE by ions and electrons flowing from a plasma (right side) toward a conducting material wall (left side) (b) Total electron yields for electron emission induced by singly and multiply charged carbon ions from surfaces of carbon fiber-enforced (CFC-CL) and highly oriented pyrolytic graphite (HOPG) respectively (data from [35])
Acknowledgements The author thanks Prof Friedrich Aumayr (TU Wien) and Drs Josef Schweinzer and Elisabeth Wolfrum (IPP Garching) for longstanding cooperation in research activities described in this paper This work has been supported by the European Commission under the contract of Association EURATOM-OEAW and was carried out within the framework of the European Fusion Development Agreement (EFDA) The views and opinions expressed therein do not necessarily reflect those of the European Commission
References [1] Janev R K 1995 Atomic and Molecular Processes in Fusion Edge Plasma[2] Atomic and Plasma-Material Interaction Data for Fusion vol 1 - 12 IAEA Vienna 1991 - 2003 [3] Crandall D H 1983 An overview of the symposium Status and direction of current research
with highly charged ions Physica Scripta T3 249 - 253 [4] Janev R K 1989 Atomic collisions in fusion plasmas J de Physique 50 C1-421 - 443 [5] Fawcett B C 1991 Role of highly-charged ions in astrophysical and laboratory plasmas
ZPhysD 21 S1 - 5 [6] Donets E D Ilyushenko V I and V Alpert 1969 Ultrahigh vacuum electron beam source of
highly stripped ions Proc 1 Conf Int sur les Sources dIons Saclay France 635 - 642 [7] Bliman S et al A high intensity ECR stripped ion source IEEE TransNuclSci 1971 NS-19
200 - 203 [8] Marrs R E Beiersdorfer P and Schneider D 1994 The electron-beam ion trap Physics Today
October 27 - 34 [9] Arnau A et al 1997 Interaction of slow multicharged ions with solid surfaces Surface Science
Reports 27 113 - 240 [10] Schenkel T et al 1999 Interaction of slow very highly charged ions with surfaces
ProgrSurfSci 61 23 - 84 [11] Aumayr F and Winter HP 2004 Potential sputtering PhilTransRSocLond 362 77 - 102 [12] Stoumlhlker T et al 2003 Test of strong-field QED in The physics of multiply and highly charged
ions (ed F J Currell) vol 1 chapter 11
39
[13] Kluge H-J 2006 Trapped and cooled Atomic physics experiments with highly charged ions in Penning traps (these proceedings)
[14] Braams C M and Stott P E 2002 Nuclear Fusion - half a century of magnetic confinement fusion research IoP Publ Bristol
[15] Kaw P K 2005 Status report on controlled thermonuclear fusion Nucl Fusion 45 A 1 - 28 [16] Lawson J D 1955 Some criteria for a useful thermonuclear reactor AERE GPR 1807 [17] wwwjetefdaorg[18] wwwiterorg[19] Philipps V Roth J and Loarte A 2003 Key issues in plasma-wall interactions for ITER a
European approach Plasma PhysControl Fusion 45 A17 - 30 [20] Samm U 2005 Plasma-Wall Interaction Status and Data Needs in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [21] Loarte A 2005 Energy deposition from ELMS in fusion devices in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [22] Garbet X et al 2004 Physics of transport in tokamaks Plasma PhysControl Fusion 46 B557 -
574[23] Challis C D 2004 The use of internal transport barriers in tokamak plasmas Plasma
PhysControl Fusion 46 B23 - 40 [24] Greenfield C M et al 2004 Advanced tokamak research in DIII-D Plasma PhysControl Fusion
46 B213 - 233 [25] Schorn R P et al 1992 Radial temperature distribution of C6+ ions in the TEXTOR edge plasma
[26] Wolfrum E et al 1993 Fast lithium-beam spectroscopy of tokamak edge plasmas RevSciInstrum 64 2285 - 2292
[27] Brandenburg R et al 1999 Fast lithium beam edge plasma spectroscopy at IPP Garching ndash Status and recent development Fusion technology 36 289 ndash 295
[28] Schweinzer J et al 1999 Database for inelastic collisions of lithium atoms with electrons protons and multiply charged ions At Data NuclData Tables 72 239 - 273
[29] Zoletnik S et al 1998 Determination of electron density fluctuation correlation functions via beam emission spectroscopy Plasma PhysControl Fusion 40 1399 - 1416
[30] Falter H D et al 2000 Helium doped hydrogen or deuterium beam as cost effective and simple tool for plasma spectroscopy RevSciInstrum 71 3723 - 3727
[31] Menhart S et al 2001 Explorative studies for the development of fast He beam plasma diagnostics JournNuclMater 290-293 673 - 677
[32] Galutschek E et al 2003 Development of fast helium beam emission spectroscopy for tokamak plasma density and temperature diagnostics Proc contr papers to 30 EPS Conf on Contr Fusion and Plasma Phys (StPetersburg 7 ndash 11 July 2003)
[33] Proschek M 2001 Towards fast He beam edge plasma diagnostics PhD thesis TU Wien [34] Schupfer N et al 2006 Effect of particle-induced electron emission (PIEE) on the plasma sheath
voltage Plasma PhysControl Fusion 48 1093 - 1103 [35] Cernusca S et al 2003 Edge plasma-relevant ion-surface collision processes Int J Mass
Spectrom 223-224 21 - 36
40
HCI Issues in Tokamak Fusion Plasmas
HP Winter
Institut fuumlr Allgemeine Physik Technische Universitaumlt Wien Austria
E-mail winteriaptuwienacat
Abstract Thermonuclear fusion powers our universe and is a most promising option for electricity and hydrogen fuel generation within a future sustainable and environmentally benign energy scenario We shortly describe the current status of magnetic fusion research on its way to the international tokamak experiment ITER for which construction at the European site Cadarache (France) starts in the near future and first plasma operation is expected in about ten years from now In the adopted EU strategy for international fusion RampD ITER is the first part of a lsquobroader approach to fusionrsquo toward a fusion demonstration reactor (lsquoDEMOrsquo) which can supply electricity into a power grid before the mid of this century We then discuss the role of highly charged ions (HCI) for magnetic fusion (plasma transport heating edge and divertor plasmas and diagnostics) Finally we present two illustrative examples from our own work on HCI atomic and surface collisions with relevance for magnetic fusion plasmas
1 Introduction Highly charged ions (HCI) and fusion plasmas (a) In the present context an ion Zq+ should be called lsquohighly chargedrsquo if in a given situation its charge q is considerably higher than its equilibrium charge This can also be stated from its potential vs kinetic energy since the former strongly depends on q For example a total ionization energy of 79 eV for producing He2+ from neutral ground state He is of no concern inside a hot D-T plasma (15 keV temperature) where alpha particles with 35 MeV kinetic energy are born from fusion reactions However after many elastic collisions such alphas slow down to a kinetic energy of less than ten eV in the plasma edge and become lsquohighly chargedrsquo because in contact with the material boundary their potential energy decides on inelastic surface processes as electron emission molecular desorption etc
(b) We need to distinguish between free HCI and those embedded in a plasma environment In the latter case the balance between collisional excitation and ionization on the one hand and radiative and collisional deexcitation and recombination on the other hand strongly depends on the composition density and temperature of the host plasma Such a situation can only very approximately be described by closed plasma models and is nowadays treated by involved numerical modelling for which respective precise atomic data is a prerequisite [1 2] but by no means sufficient Likewise information from spectroscopic measurements depends strongly on the optical thickness of the plasma where the respective HCI reside upon their radiative decay
(c) Research with highly charged ions (HCI) has for many years been justified by its interest for fusion RampD as eg by DH Crandall [3] in his summary talk for the first HCI conference 1982 in Stockholm He referred to the importance of fusion plasma spectroscopy and inelastic HCI collisions with electrons (ionization excitation recombination) and atoms (charge transfer)
IOP Publishing Journal of Physics Conference Series 58 (2007) 33ndash40doi1010881742-6596581005 13th International Conference on the Physics of Highly Charged Ions
33copy 2007 IOP Publishing Ltd
It is thus puzzling that during the whole HCI conference series (with only two exceptions [4 5]) this allegedly important relation between HCI and fusion research has never been analyzed in detail
(d) However DH Crandall [3] also stressed the relation between HCI source development and experiments on slow HCI collisions During the last twenty five years the electron beam ion source (EBIS [6]) and the electron cyclotron resonance ion source (ECRIS [7]) have rapidly matured The EBIS is useful for injection of short HCI pulses into synchrotrons but in a modified version as electron beam ion trap (EBIT) [8] has become an important tool for HCI spectroscopy and internal and external low-energy atomic collision experiments ECRIS is a direct offspring from fusion plasma research with magnetic mirror machines but now has become the working horse for slow HCI studies in many laboratories Special conference series deal with those multicharged ion sources and their ubiquity and simple handling has strongly promoted new fields as slow HCI-surface interactions [9] in particular surface engineering [10] by means of lsquopotential sputteringrsquo [11] or the storage and cooling of HCI in traps and rings for QED tests atomic clock development and high-precision atomic mass measurement [12 13] Actually these more recent attractive applications make a justification of HCI related research by fusion alone obsolete On the other hand HCI definitely play an important role for fusion research in many respects but a really good understanding of this role is impossible without a strong context with fusion plasma physics In section 3 we attempt to explain this role by reference to central aspects of current research on magnetically confined tokamak plasmas
2 The status of magnetic fusion - JET ITER and the lsquofast trackrsquo to DEMO European fusion research in the last fifty years has often been criticized as too slow and costly Actually it is a great success story not only because of the achieved scientific progress but also for the increasingly integrated approach in which this research is being conducted This is a shining example for an lsquoEuropean Research Arearsquo (we recommend the interesting historical account [14] and the most recent IAEA status report [15])
In 1955 the British engineer JD Lawson [16] formulated his famous criterion according to which for net energy gain from a D-T fusion plasma with an ion temperature Ti = 10 keV one needs to achieve a lsquofusion productrsquo ni E 1020 sm3 for plasma ion density ni and energy confinement time E
Figure 1 Fusion triple product ni ETi vs Ti from different tokamak experiments since 1969 Parameters for expected ITER operation are highlighted (with permission of ITER)
Between about 10 and 20 keV the Lawson criterion can also be formulated for the lsquofusion triple productrsquo ni Ti vs Ti (cf figure 1) In principle it applies for any kind of fusion plasma including inertial confinement (ICF) schemes However until today the most successful approach has been made by magnetic plasma confinement in the so-called lsquotokamakrsquo configuration
A tokamak (Russian acronym for lsquocurrent in magnetized chamberrsquo) basically comprises a ring shaped (toroidal) plasma in a strong toroidal magnetic field (Bt) For stability reasons the magnetic
34
field lines have to be slightly twisted (rotational transform) by superposition with a poloidal magnetic field produced by a strong plasma current Ip The latter is inductively driven through the ring plasma as the secondary winding of a transformer Consequently the original tokamak scheme permits only pulsed operation However in lsquoadvanced tokamaksrsquo (see section 3) after inductive start-up the plasma current is sustained by a self-organized diffusion current (lsquobootstraprsquo effect) and by electromagnetic waves which permits continuous operation schemes
The Joint European Torus (JET) in CulhamUK is so far the largest and most successful tokamak experiment JETrsquos life started in 1983 and will continue at least until 2010 under the lsquoEuropean Fusion Development AgreementEFDArsquo [17] The JET plasma features a large radius R = 3 m with elongated cross section a plasma surface of 200 m2 and volume of about 150 m3 and Bt and Ip may reach 3 T and 3 MA respectively To achieve a sufficiently high plasma temperature ohmic heating by Ip has to be supported by other techniques as the injection of neutral fuel atom beams and of electromagnetic waves with electron cyclotron resonance (ECR) and ion cyclotron resonance (ICR) frequencies In future fusion reactors a so-called lsquoburning plasmarsquo will mainly be heated by the 35 MeV alpha particles from D-T fusion reactions
JET has been run in several campaigns with D-T fuel and in 1997 reached for about 05 s a peak fusion power of 16 MW a world record until today This figure has to be compared with the total heating power applied for plasma operation giving a peak ratio of fusion to plasma heating power (lsquoplasma amplification factorrsquo Q) of 65 JET together with numerous other tokamak experiments has generated sufficient expertise on the achievable plasma energy confinement times within the plasma parameter space (density geometry Bt and Ip) to permit a reliable empirical scaling for extrapolation toward still larger experiments in particular ITER (see figure 2)
Figure 2 Experimentally observed (vertical axis) vs empirically determined energy confinement time from various tokamak experiments extrapolated toward ITER (with permission of ITER)
The design of ITER (in Latin lsquothe wayrsquo and an officially no more valid acronym for lsquoInternational Thermonuclear Experimental Reactorrsquo) was already started in 1988 but had to be thoroughly revised in 1998 mainly for cost reasons The new design was finished in 2001 (cf figure 3 details given on the ITER webpage [18]) As to ITERrsquos main parameters R = 62 m Bt = 53 T (from superconducting coils) Ip 15 MA and plasma volume and surface are 840 m3 and ca 1000 m2 respectively
ITERrsquos principal mission is the demonstration of fusion reactor relevant burning plasmas with Q 10 during a discharge time of 500 s and possibly in steady-state operation ITER will be built by industrial companies from the present seven partners (EU Japan RF USA PR of China South Korea and India more countries have expressed their strong interest to join)
The total budget for building ITER is 47 billion euro Construction at the European site Cadarache in France will start in 2007 and first plasma operation is expected for 2016
35
In the bilateral ITER negotiations between the EU and Japan who both had offered sites a so-called lsquoBroader Approach to Fusionrsquo has been adopted based on a lsquoFast Trackrsquo strategy proposed in 2001 (parallel development of fusion plasma physics and technology) It includes support from the EU both for a next large tokamak and the first DEMO site in Japan Another bilateral collaboration agreement has recently been concluded for six years to carry out lsquoEngineering Validation and Engineering Design ActivitiesEVEDArsquo in preparation of the planned International Fusion Material Irradiation Facility (IFMIF) This accelerator-based fusion dedicated facility could be built within ten years after successful conclusion of EVEDA IFMIF shall provide the fusion reactor relevant neutron irradiation environment for development and testing of structural materials needed for the construction of DEMO Such materials and relevant technology have to be available at a time when sufficiently conclusive results from ITER have arrived ITER IFMIF and DEMO are the three principal parts of the proposed lsquoFast Trackrsquo for making fusion energy a reality before the mid of this century
3 Why are HCI important for magnetically confined fusion plasmas The presence of HCI in fusion plasmas is commonly seen just as a nuisance Neutral atoms with nuclear charge Z gt 2 from plasma-wall interaction (PWI) [19 20] diffuse into the hot plasma core where they become ionized Fully stripped ions contribute to the global plasma bremsstrahlung with a radiation power in proportion to Z2 and an approximate much stronger Z45-dependence holds for the line radiation from the incompletely ionized collisionally excited ions With a non-negligible impurity content this radiation constitutes an important power loss which needs to be balanced by stronger plasma heating and improved confinement The common figure for characterization of the plasma impurity content is the lsquoeffective ion chargersquo
Zeff Z 2nZ ne (1) Impurities including the lsquoHe ashrsquo from fusion reactions dilute the D-T plasma fuel thanks to the
plasma quasi-neutrality condition ne ZnZ (2) (ne and nZ are the electron and ion densities Z 1) the majority (D T) ion density is already
strongly reduced for a relatively small impurity content (eg by 16 for only 1 admixed fully stripped oxygen) This radiative power loss inhibits the plasma start-up strongly influences the achievable plasma density limit and also varies the neutral beam power deposition profile
However plasma impurities also play a useful and in fact rather important role not only because various plasma diagnostic techniques are based on the exploitation of impurity radiation Still more relevant is their ability to modify the plasma radiation loss profile in particular in the plasma edge region where also low-Z impurities (C O) are not yet fully ionized An important part of the kinetic energy of the plasma constituents is converted into radiation by interaction with impurities This considerably dampens the PWI and actually makes the operation of D-T-fusion reactors possible For a given impurity species and plasma temperature the radiated power strongly depends on the ion charge Carefully administered impurity seeding can thus influence the plasma balance in a rather positive way (radiation cooling by lsquoseededrsquo Ne Si Ar or other species) Problems associated with impurity production by PWI are reduced by a magnetic lsquodivertorrsquo Magnetic field lines are diverted by means of external poloidal coils toward a separated region where the plasma is partially neutralized and PWI products are rapidly pumped away in order to minimize contamination of the main plasma Most common are lsquosingle-nullrsquo poloidal divertors as also planned for ITER (see figures 3 and 4)
Another important aspect of PWI are transient lsquoedge localized modesELMsrsquo which result from particular plasma instabilities [21] During an ELM up to 10 of the total plasma thermal energy can erupt as particle flux within less than 1 ms toward first wall components This inflicts significant material damage with strong impurity influx ELMs are a special concern for ITER because the ratio of plasma energy content to first wall surface strongly increases with plasma size A possible remedy for deleterious lsquogiant ELMSrsquo is the injection of frozen fuel pellets On the other hand mild ELMs seem to be indispensable for regular expulsion of plasma impurities in particular the He ash
36
Figure 3 Cutaway view of ITER (total height 24 m with permission of ITER)
Figure 4 Cross section of ITER vacuum vessel with divertor region on bottom (with permission of ITER)
The plasma profile can be carefully tailored by local radiation cooling which influences the plasma transport [22] and in particular permits the formation of an lsquointernal transport barrierrsquo (ITB) for improved plasma confinement [23] The impurity profile is especially relevant for development of advanced tokamak regimes (non-inductive current drive for steady-state operation simultaneous control of plasma current and pressure profile by active feedback MHD stability control for optimized plasma performance [24]) All the here mentioned features are interdependent and self-consistently interacting which makes their study by integrated plasma modeling a rather difficult task However the further rapid increase of computing power will lead to increasingly realistic results in such studies
4 Some experimental studies with slow HCI for magnetic fusion plasmas The first case concerns beam emission spectroscopy (BES) for edge plasma diagnostics If a fast (10 ndash 50 keV) neutral Li beam is injected into the plasma boundary collisions with plasma electrons and ions (majority species and impurities) give rise to characteristic Li I line emission Measurement of the Li I line intensity along the injected diagnostic beam delivers the plasma electron density profile and electron capture from Li by impurity ions leads to the impurity ion density and temperature profiles
This technique has first been demonstrated at TEXTOR (Juelich) [25] [26] and later at ASDEX Upgrade (IPP Garching) [27] and it is now implemented at JET as well Evaluation of plasma properties requires extensive modeling of the Li beam state composition with the help of a dedicated atomic collision data base [28] Li-BES is also of interest for measuring correlation functions of electron density fluctuations [29] Since the applicability of Li-BES is limited by the available neutral Li beam intensity switching to Na injection will probably be useful More recently we have also applied fast He beams [30] for BES Combined line emission measurements from He I singlet and triplet transitions possibly permit evaluation of plasma electron temperature profiles First exploratory measurements have been conducted at JET (cf figures 5ab) and ASDEX Upgrade [31] [32] but until now the obtained results remain inconclusive which is primarily caused by our still incomplete atomic collision data base for neutral He beam modeling
37
Figure 5 (a) Extreme flux surface positions during a 12 cm horizontal sweep of JET plasma during 80 keV He-BES measurements (b) Time development of He I 588 nm triplet line intensity showing the changing signal from passive edge plasma emission (upper part) and from the Doppler shifted He-BES emission (lower part) [33])
However very attractive features as the rather simple preparation of intense neutral He beams by He doping of the standard neutral heating beams and the considerably deeper plasma penetration in comparison to Li and Na-BES suggest further continuation of these studies
For the second example we have recently explored [34] the influence of particle induced electron emission (PIEE lsquoparticlersquo designates ions as well as electrons) from a material plasma boundary on the voltage drop across the adjacent plasma sheath Such a situation is of interest for the plasma region near a tokamak divertor plate A new model involving a collisional kinetic sheath consistently coupled to a fluid presheath has been developed which is applicable for plasmas with a Debye length much smaller than the relevant characteristic presheath length (lsquoasymptotic two-scale limitrsquo cf figure 6a) Majority and impurity ions are accelerated by the sheath voltage from the bulk plasma and can release electrons from the material surface which flow back into the bulk plasma (see figure 6b for characteristic PIEE yields for Cq+ ion impact on different graphite surfaces) Moreover fast plasma electrons from the plasma Maxwellian tail can reach the surface to release secondary electrons and these fast electrons will also be partially reflected In a first approach we have neglected the tokamak-generic inclined magnetic field and also the fast electron reflection Under these simplifying assumptions the reduction of the plasma sheath voltage has been self-consistently calculated and was found to reach up to 30 of the sheath voltage for a case with PIEE not taken into account More realistic results will be obtained after adaptation of our model for an inclined magnetic field and with electron reflection
5 Summary and conclusions In this survey we have attempted to elucidate the beneficial role of plasma impurities (both in low and high charge states) for magnetically confined fusion plasmas in particular with respect to ITER European fusion research has taken a strong lead in this field by setting up two task forces on plasma-wall interaction and on integrated plasma modeling In both areas there is a strong need for more complete and more precise data on collisions of HCI (low-Z He Be B C O Ne medium-Z Ar Fe etc high-Z W) with hydrogen atoms and molecules and also with surfaces (Be graphite stainless steel W) The most important role of highly charged plasma impurities is to be seen in the quest of advanced tokamak plasma studies and related plasma modeling for the optimal design of future thermonuclear fusion reactors based on the tokamak configuration
38
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120
(e- io
n)
velocity (10 4 ms)
Cq+ Graphite
C4+CL
C4+HOPG
C2+HOPG
C+HOPG
C5+CL
C2+CL
Figure 6 (a) Schematic layout of new plasma sheath model with PIEE by ions and electrons flowing from a plasma (right side) toward a conducting material wall (left side) (b) Total electron yields for electron emission induced by singly and multiply charged carbon ions from surfaces of carbon fiber-enforced (CFC-CL) and highly oriented pyrolytic graphite (HOPG) respectively (data from [35])
Acknowledgements The author thanks Prof Friedrich Aumayr (TU Wien) and Drs Josef Schweinzer and Elisabeth Wolfrum (IPP Garching) for longstanding cooperation in research activities described in this paper This work has been supported by the European Commission under the contract of Association EURATOM-OEAW and was carried out within the framework of the European Fusion Development Agreement (EFDA) The views and opinions expressed therein do not necessarily reflect those of the European Commission
References [1] Janev R K 1995 Atomic and Molecular Processes in Fusion Edge Plasma[2] Atomic and Plasma-Material Interaction Data for Fusion vol 1 - 12 IAEA Vienna 1991 - 2003 [3] Crandall D H 1983 An overview of the symposium Status and direction of current research
with highly charged ions Physica Scripta T3 249 - 253 [4] Janev R K 1989 Atomic collisions in fusion plasmas J de Physique 50 C1-421 - 443 [5] Fawcett B C 1991 Role of highly-charged ions in astrophysical and laboratory plasmas
ZPhysD 21 S1 - 5 [6] Donets E D Ilyushenko V I and V Alpert 1969 Ultrahigh vacuum electron beam source of
highly stripped ions Proc 1 Conf Int sur les Sources dIons Saclay France 635 - 642 [7] Bliman S et al A high intensity ECR stripped ion source IEEE TransNuclSci 1971 NS-19
200 - 203 [8] Marrs R E Beiersdorfer P and Schneider D 1994 The electron-beam ion trap Physics Today
October 27 - 34 [9] Arnau A et al 1997 Interaction of slow multicharged ions with solid surfaces Surface Science
Reports 27 113 - 240 [10] Schenkel T et al 1999 Interaction of slow very highly charged ions with surfaces
ProgrSurfSci 61 23 - 84 [11] Aumayr F and Winter HP 2004 Potential sputtering PhilTransRSocLond 362 77 - 102 [12] Stoumlhlker T et al 2003 Test of strong-field QED in The physics of multiply and highly charged
ions (ed F J Currell) vol 1 chapter 11
39
[13] Kluge H-J 2006 Trapped and cooled Atomic physics experiments with highly charged ions in Penning traps (these proceedings)
[14] Braams C M and Stott P E 2002 Nuclear Fusion - half a century of magnetic confinement fusion research IoP Publ Bristol
[15] Kaw P K 2005 Status report on controlled thermonuclear fusion Nucl Fusion 45 A 1 - 28 [16] Lawson J D 1955 Some criteria for a useful thermonuclear reactor AERE GPR 1807 [17] wwwjetefdaorg[18] wwwiterorg[19] Philipps V Roth J and Loarte A 2003 Key issues in plasma-wall interactions for ITER a
European approach Plasma PhysControl Fusion 45 A17 - 30 [20] Samm U 2005 Plasma-Wall Interaction Status and Data Needs in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [21] Loarte A 2005 Energy deposition from ELMS in fusion devices in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [22] Garbet X et al 2004 Physics of transport in tokamaks Plasma PhysControl Fusion 46 B557 -
574[23] Challis C D 2004 The use of internal transport barriers in tokamak plasmas Plasma
PhysControl Fusion 46 B23 - 40 [24] Greenfield C M et al 2004 Advanced tokamak research in DIII-D Plasma PhysControl Fusion
46 B213 - 233 [25] Schorn R P et al 1992 Radial temperature distribution of C6+ ions in the TEXTOR edge plasma
[26] Wolfrum E et al 1993 Fast lithium-beam spectroscopy of tokamak edge plasmas RevSciInstrum 64 2285 - 2292
[27] Brandenburg R et al 1999 Fast lithium beam edge plasma spectroscopy at IPP Garching ndash Status and recent development Fusion technology 36 289 ndash 295
[28] Schweinzer J et al 1999 Database for inelastic collisions of lithium atoms with electrons protons and multiply charged ions At Data NuclData Tables 72 239 - 273
[29] Zoletnik S et al 1998 Determination of electron density fluctuation correlation functions via beam emission spectroscopy Plasma PhysControl Fusion 40 1399 - 1416
[30] Falter H D et al 2000 Helium doped hydrogen or deuterium beam as cost effective and simple tool for plasma spectroscopy RevSciInstrum 71 3723 - 3727
[31] Menhart S et al 2001 Explorative studies for the development of fast He beam plasma diagnostics JournNuclMater 290-293 673 - 677
[32] Galutschek E et al 2003 Development of fast helium beam emission spectroscopy for tokamak plasma density and temperature diagnostics Proc contr papers to 30 EPS Conf on Contr Fusion and Plasma Phys (StPetersburg 7 ndash 11 July 2003)
[33] Proschek M 2001 Towards fast He beam edge plasma diagnostics PhD thesis TU Wien [34] Schupfer N et al 2006 Effect of particle-induced electron emission (PIEE) on the plasma sheath
voltage Plasma PhysControl Fusion 48 1093 - 1103 [35] Cernusca S et al 2003 Edge plasma-relevant ion-surface collision processes Int J Mass
Spectrom 223-224 21 - 36
40
It is thus puzzling that during the whole HCI conference series (with only two exceptions [4 5]) this allegedly important relation between HCI and fusion research has never been analyzed in detail
(d) However DH Crandall [3] also stressed the relation between HCI source development and experiments on slow HCI collisions During the last twenty five years the electron beam ion source (EBIS [6]) and the electron cyclotron resonance ion source (ECRIS [7]) have rapidly matured The EBIS is useful for injection of short HCI pulses into synchrotrons but in a modified version as electron beam ion trap (EBIT) [8] has become an important tool for HCI spectroscopy and internal and external low-energy atomic collision experiments ECRIS is a direct offspring from fusion plasma research with magnetic mirror machines but now has become the working horse for slow HCI studies in many laboratories Special conference series deal with those multicharged ion sources and their ubiquity and simple handling has strongly promoted new fields as slow HCI-surface interactions [9] in particular surface engineering [10] by means of lsquopotential sputteringrsquo [11] or the storage and cooling of HCI in traps and rings for QED tests atomic clock development and high-precision atomic mass measurement [12 13] Actually these more recent attractive applications make a justification of HCI related research by fusion alone obsolete On the other hand HCI definitely play an important role for fusion research in many respects but a really good understanding of this role is impossible without a strong context with fusion plasma physics In section 3 we attempt to explain this role by reference to central aspects of current research on magnetically confined tokamak plasmas
2 The status of magnetic fusion - JET ITER and the lsquofast trackrsquo to DEMO European fusion research in the last fifty years has often been criticized as too slow and costly Actually it is a great success story not only because of the achieved scientific progress but also for the increasingly integrated approach in which this research is being conducted This is a shining example for an lsquoEuropean Research Arearsquo (we recommend the interesting historical account [14] and the most recent IAEA status report [15])
In 1955 the British engineer JD Lawson [16] formulated his famous criterion according to which for net energy gain from a D-T fusion plasma with an ion temperature Ti = 10 keV one needs to achieve a lsquofusion productrsquo ni E 1020 sm3 for plasma ion density ni and energy confinement time E
Figure 1 Fusion triple product ni ETi vs Ti from different tokamak experiments since 1969 Parameters for expected ITER operation are highlighted (with permission of ITER)
Between about 10 and 20 keV the Lawson criterion can also be formulated for the lsquofusion triple productrsquo ni Ti vs Ti (cf figure 1) In principle it applies for any kind of fusion plasma including inertial confinement (ICF) schemes However until today the most successful approach has been made by magnetic plasma confinement in the so-called lsquotokamakrsquo configuration
A tokamak (Russian acronym for lsquocurrent in magnetized chamberrsquo) basically comprises a ring shaped (toroidal) plasma in a strong toroidal magnetic field (Bt) For stability reasons the magnetic
34
field lines have to be slightly twisted (rotational transform) by superposition with a poloidal magnetic field produced by a strong plasma current Ip The latter is inductively driven through the ring plasma as the secondary winding of a transformer Consequently the original tokamak scheme permits only pulsed operation However in lsquoadvanced tokamaksrsquo (see section 3) after inductive start-up the plasma current is sustained by a self-organized diffusion current (lsquobootstraprsquo effect) and by electromagnetic waves which permits continuous operation schemes
The Joint European Torus (JET) in CulhamUK is so far the largest and most successful tokamak experiment JETrsquos life started in 1983 and will continue at least until 2010 under the lsquoEuropean Fusion Development AgreementEFDArsquo [17] The JET plasma features a large radius R = 3 m with elongated cross section a plasma surface of 200 m2 and volume of about 150 m3 and Bt and Ip may reach 3 T and 3 MA respectively To achieve a sufficiently high plasma temperature ohmic heating by Ip has to be supported by other techniques as the injection of neutral fuel atom beams and of electromagnetic waves with electron cyclotron resonance (ECR) and ion cyclotron resonance (ICR) frequencies In future fusion reactors a so-called lsquoburning plasmarsquo will mainly be heated by the 35 MeV alpha particles from D-T fusion reactions
JET has been run in several campaigns with D-T fuel and in 1997 reached for about 05 s a peak fusion power of 16 MW a world record until today This figure has to be compared with the total heating power applied for plasma operation giving a peak ratio of fusion to plasma heating power (lsquoplasma amplification factorrsquo Q) of 65 JET together with numerous other tokamak experiments has generated sufficient expertise on the achievable plasma energy confinement times within the plasma parameter space (density geometry Bt and Ip) to permit a reliable empirical scaling for extrapolation toward still larger experiments in particular ITER (see figure 2)
Figure 2 Experimentally observed (vertical axis) vs empirically determined energy confinement time from various tokamak experiments extrapolated toward ITER (with permission of ITER)
The design of ITER (in Latin lsquothe wayrsquo and an officially no more valid acronym for lsquoInternational Thermonuclear Experimental Reactorrsquo) was already started in 1988 but had to be thoroughly revised in 1998 mainly for cost reasons The new design was finished in 2001 (cf figure 3 details given on the ITER webpage [18]) As to ITERrsquos main parameters R = 62 m Bt = 53 T (from superconducting coils) Ip 15 MA and plasma volume and surface are 840 m3 and ca 1000 m2 respectively
ITERrsquos principal mission is the demonstration of fusion reactor relevant burning plasmas with Q 10 during a discharge time of 500 s and possibly in steady-state operation ITER will be built by industrial companies from the present seven partners (EU Japan RF USA PR of China South Korea and India more countries have expressed their strong interest to join)
The total budget for building ITER is 47 billion euro Construction at the European site Cadarache in France will start in 2007 and first plasma operation is expected for 2016
35
In the bilateral ITER negotiations between the EU and Japan who both had offered sites a so-called lsquoBroader Approach to Fusionrsquo has been adopted based on a lsquoFast Trackrsquo strategy proposed in 2001 (parallel development of fusion plasma physics and technology) It includes support from the EU both for a next large tokamak and the first DEMO site in Japan Another bilateral collaboration agreement has recently been concluded for six years to carry out lsquoEngineering Validation and Engineering Design ActivitiesEVEDArsquo in preparation of the planned International Fusion Material Irradiation Facility (IFMIF) This accelerator-based fusion dedicated facility could be built within ten years after successful conclusion of EVEDA IFMIF shall provide the fusion reactor relevant neutron irradiation environment for development and testing of structural materials needed for the construction of DEMO Such materials and relevant technology have to be available at a time when sufficiently conclusive results from ITER have arrived ITER IFMIF and DEMO are the three principal parts of the proposed lsquoFast Trackrsquo for making fusion energy a reality before the mid of this century
3 Why are HCI important for magnetically confined fusion plasmas The presence of HCI in fusion plasmas is commonly seen just as a nuisance Neutral atoms with nuclear charge Z gt 2 from plasma-wall interaction (PWI) [19 20] diffuse into the hot plasma core where they become ionized Fully stripped ions contribute to the global plasma bremsstrahlung with a radiation power in proportion to Z2 and an approximate much stronger Z45-dependence holds for the line radiation from the incompletely ionized collisionally excited ions With a non-negligible impurity content this radiation constitutes an important power loss which needs to be balanced by stronger plasma heating and improved confinement The common figure for characterization of the plasma impurity content is the lsquoeffective ion chargersquo
Zeff Z 2nZ ne (1) Impurities including the lsquoHe ashrsquo from fusion reactions dilute the D-T plasma fuel thanks to the
plasma quasi-neutrality condition ne ZnZ (2) (ne and nZ are the electron and ion densities Z 1) the majority (D T) ion density is already
strongly reduced for a relatively small impurity content (eg by 16 for only 1 admixed fully stripped oxygen) This radiative power loss inhibits the plasma start-up strongly influences the achievable plasma density limit and also varies the neutral beam power deposition profile
However plasma impurities also play a useful and in fact rather important role not only because various plasma diagnostic techniques are based on the exploitation of impurity radiation Still more relevant is their ability to modify the plasma radiation loss profile in particular in the plasma edge region where also low-Z impurities (C O) are not yet fully ionized An important part of the kinetic energy of the plasma constituents is converted into radiation by interaction with impurities This considerably dampens the PWI and actually makes the operation of D-T-fusion reactors possible For a given impurity species and plasma temperature the radiated power strongly depends on the ion charge Carefully administered impurity seeding can thus influence the plasma balance in a rather positive way (radiation cooling by lsquoseededrsquo Ne Si Ar or other species) Problems associated with impurity production by PWI are reduced by a magnetic lsquodivertorrsquo Magnetic field lines are diverted by means of external poloidal coils toward a separated region where the plasma is partially neutralized and PWI products are rapidly pumped away in order to minimize contamination of the main plasma Most common are lsquosingle-nullrsquo poloidal divertors as also planned for ITER (see figures 3 and 4)
Another important aspect of PWI are transient lsquoedge localized modesELMsrsquo which result from particular plasma instabilities [21] During an ELM up to 10 of the total plasma thermal energy can erupt as particle flux within less than 1 ms toward first wall components This inflicts significant material damage with strong impurity influx ELMs are a special concern for ITER because the ratio of plasma energy content to first wall surface strongly increases with plasma size A possible remedy for deleterious lsquogiant ELMSrsquo is the injection of frozen fuel pellets On the other hand mild ELMs seem to be indispensable for regular expulsion of plasma impurities in particular the He ash
36
Figure 3 Cutaway view of ITER (total height 24 m with permission of ITER)
Figure 4 Cross section of ITER vacuum vessel with divertor region on bottom (with permission of ITER)
The plasma profile can be carefully tailored by local radiation cooling which influences the plasma transport [22] and in particular permits the formation of an lsquointernal transport barrierrsquo (ITB) for improved plasma confinement [23] The impurity profile is especially relevant for development of advanced tokamak regimes (non-inductive current drive for steady-state operation simultaneous control of plasma current and pressure profile by active feedback MHD stability control for optimized plasma performance [24]) All the here mentioned features are interdependent and self-consistently interacting which makes their study by integrated plasma modeling a rather difficult task However the further rapid increase of computing power will lead to increasingly realistic results in such studies
4 Some experimental studies with slow HCI for magnetic fusion plasmas The first case concerns beam emission spectroscopy (BES) for edge plasma diagnostics If a fast (10 ndash 50 keV) neutral Li beam is injected into the plasma boundary collisions with plasma electrons and ions (majority species and impurities) give rise to characteristic Li I line emission Measurement of the Li I line intensity along the injected diagnostic beam delivers the plasma electron density profile and electron capture from Li by impurity ions leads to the impurity ion density and temperature profiles
This technique has first been demonstrated at TEXTOR (Juelich) [25] [26] and later at ASDEX Upgrade (IPP Garching) [27] and it is now implemented at JET as well Evaluation of plasma properties requires extensive modeling of the Li beam state composition with the help of a dedicated atomic collision data base [28] Li-BES is also of interest for measuring correlation functions of electron density fluctuations [29] Since the applicability of Li-BES is limited by the available neutral Li beam intensity switching to Na injection will probably be useful More recently we have also applied fast He beams [30] for BES Combined line emission measurements from He I singlet and triplet transitions possibly permit evaluation of plasma electron temperature profiles First exploratory measurements have been conducted at JET (cf figures 5ab) and ASDEX Upgrade [31] [32] but until now the obtained results remain inconclusive which is primarily caused by our still incomplete atomic collision data base for neutral He beam modeling
37
Figure 5 (a) Extreme flux surface positions during a 12 cm horizontal sweep of JET plasma during 80 keV He-BES measurements (b) Time development of He I 588 nm triplet line intensity showing the changing signal from passive edge plasma emission (upper part) and from the Doppler shifted He-BES emission (lower part) [33])
However very attractive features as the rather simple preparation of intense neutral He beams by He doping of the standard neutral heating beams and the considerably deeper plasma penetration in comparison to Li and Na-BES suggest further continuation of these studies
For the second example we have recently explored [34] the influence of particle induced electron emission (PIEE lsquoparticlersquo designates ions as well as electrons) from a material plasma boundary on the voltage drop across the adjacent plasma sheath Such a situation is of interest for the plasma region near a tokamak divertor plate A new model involving a collisional kinetic sheath consistently coupled to a fluid presheath has been developed which is applicable for plasmas with a Debye length much smaller than the relevant characteristic presheath length (lsquoasymptotic two-scale limitrsquo cf figure 6a) Majority and impurity ions are accelerated by the sheath voltage from the bulk plasma and can release electrons from the material surface which flow back into the bulk plasma (see figure 6b for characteristic PIEE yields for Cq+ ion impact on different graphite surfaces) Moreover fast plasma electrons from the plasma Maxwellian tail can reach the surface to release secondary electrons and these fast electrons will also be partially reflected In a first approach we have neglected the tokamak-generic inclined magnetic field and also the fast electron reflection Under these simplifying assumptions the reduction of the plasma sheath voltage has been self-consistently calculated and was found to reach up to 30 of the sheath voltage for a case with PIEE not taken into account More realistic results will be obtained after adaptation of our model for an inclined magnetic field and with electron reflection
5 Summary and conclusions In this survey we have attempted to elucidate the beneficial role of plasma impurities (both in low and high charge states) for magnetically confined fusion plasmas in particular with respect to ITER European fusion research has taken a strong lead in this field by setting up two task forces on plasma-wall interaction and on integrated plasma modeling In both areas there is a strong need for more complete and more precise data on collisions of HCI (low-Z He Be B C O Ne medium-Z Ar Fe etc high-Z W) with hydrogen atoms and molecules and also with surfaces (Be graphite stainless steel W) The most important role of highly charged plasma impurities is to be seen in the quest of advanced tokamak plasma studies and related plasma modeling for the optimal design of future thermonuclear fusion reactors based on the tokamak configuration
38
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120
(e- io
n)
velocity (10 4 ms)
Cq+ Graphite
C4+CL
C4+HOPG
C2+HOPG
C+HOPG
C5+CL
C2+CL
Figure 6 (a) Schematic layout of new plasma sheath model with PIEE by ions and electrons flowing from a plasma (right side) toward a conducting material wall (left side) (b) Total electron yields for electron emission induced by singly and multiply charged carbon ions from surfaces of carbon fiber-enforced (CFC-CL) and highly oriented pyrolytic graphite (HOPG) respectively (data from [35])
Acknowledgements The author thanks Prof Friedrich Aumayr (TU Wien) and Drs Josef Schweinzer and Elisabeth Wolfrum (IPP Garching) for longstanding cooperation in research activities described in this paper This work has been supported by the European Commission under the contract of Association EURATOM-OEAW and was carried out within the framework of the European Fusion Development Agreement (EFDA) The views and opinions expressed therein do not necessarily reflect those of the European Commission
References [1] Janev R K 1995 Atomic and Molecular Processes in Fusion Edge Plasma[2] Atomic and Plasma-Material Interaction Data for Fusion vol 1 - 12 IAEA Vienna 1991 - 2003 [3] Crandall D H 1983 An overview of the symposium Status and direction of current research
with highly charged ions Physica Scripta T3 249 - 253 [4] Janev R K 1989 Atomic collisions in fusion plasmas J de Physique 50 C1-421 - 443 [5] Fawcett B C 1991 Role of highly-charged ions in astrophysical and laboratory plasmas
ZPhysD 21 S1 - 5 [6] Donets E D Ilyushenko V I and V Alpert 1969 Ultrahigh vacuum electron beam source of
highly stripped ions Proc 1 Conf Int sur les Sources dIons Saclay France 635 - 642 [7] Bliman S et al A high intensity ECR stripped ion source IEEE TransNuclSci 1971 NS-19
200 - 203 [8] Marrs R E Beiersdorfer P and Schneider D 1994 The electron-beam ion trap Physics Today
October 27 - 34 [9] Arnau A et al 1997 Interaction of slow multicharged ions with solid surfaces Surface Science
Reports 27 113 - 240 [10] Schenkel T et al 1999 Interaction of slow very highly charged ions with surfaces
ProgrSurfSci 61 23 - 84 [11] Aumayr F and Winter HP 2004 Potential sputtering PhilTransRSocLond 362 77 - 102 [12] Stoumlhlker T et al 2003 Test of strong-field QED in The physics of multiply and highly charged
ions (ed F J Currell) vol 1 chapter 11
39
[13] Kluge H-J 2006 Trapped and cooled Atomic physics experiments with highly charged ions in Penning traps (these proceedings)
[14] Braams C M and Stott P E 2002 Nuclear Fusion - half a century of magnetic confinement fusion research IoP Publ Bristol
[15] Kaw P K 2005 Status report on controlled thermonuclear fusion Nucl Fusion 45 A 1 - 28 [16] Lawson J D 1955 Some criteria for a useful thermonuclear reactor AERE GPR 1807 [17] wwwjetefdaorg[18] wwwiterorg[19] Philipps V Roth J and Loarte A 2003 Key issues in plasma-wall interactions for ITER a
European approach Plasma PhysControl Fusion 45 A17 - 30 [20] Samm U 2005 Plasma-Wall Interaction Status and Data Needs in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [21] Loarte A 2005 Energy deposition from ELMS in fusion devices in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [22] Garbet X et al 2004 Physics of transport in tokamaks Plasma PhysControl Fusion 46 B557 -
574[23] Challis C D 2004 The use of internal transport barriers in tokamak plasmas Plasma
PhysControl Fusion 46 B23 - 40 [24] Greenfield C M et al 2004 Advanced tokamak research in DIII-D Plasma PhysControl Fusion
46 B213 - 233 [25] Schorn R P et al 1992 Radial temperature distribution of C6+ ions in the TEXTOR edge plasma
[26] Wolfrum E et al 1993 Fast lithium-beam spectroscopy of tokamak edge plasmas RevSciInstrum 64 2285 - 2292
[27] Brandenburg R et al 1999 Fast lithium beam edge plasma spectroscopy at IPP Garching ndash Status and recent development Fusion technology 36 289 ndash 295
[28] Schweinzer J et al 1999 Database for inelastic collisions of lithium atoms with electrons protons and multiply charged ions At Data NuclData Tables 72 239 - 273
[29] Zoletnik S et al 1998 Determination of electron density fluctuation correlation functions via beam emission spectroscopy Plasma PhysControl Fusion 40 1399 - 1416
[30] Falter H D et al 2000 Helium doped hydrogen or deuterium beam as cost effective and simple tool for plasma spectroscopy RevSciInstrum 71 3723 - 3727
[31] Menhart S et al 2001 Explorative studies for the development of fast He beam plasma diagnostics JournNuclMater 290-293 673 - 677
[32] Galutschek E et al 2003 Development of fast helium beam emission spectroscopy for tokamak plasma density and temperature diagnostics Proc contr papers to 30 EPS Conf on Contr Fusion and Plasma Phys (StPetersburg 7 ndash 11 July 2003)
[33] Proschek M 2001 Towards fast He beam edge plasma diagnostics PhD thesis TU Wien [34] Schupfer N et al 2006 Effect of particle-induced electron emission (PIEE) on the plasma sheath
voltage Plasma PhysControl Fusion 48 1093 - 1103 [35] Cernusca S et al 2003 Edge plasma-relevant ion-surface collision processes Int J Mass
Spectrom 223-224 21 - 36
40
field lines have to be slightly twisted (rotational transform) by superposition with a poloidal magnetic field produced by a strong plasma current Ip The latter is inductively driven through the ring plasma as the secondary winding of a transformer Consequently the original tokamak scheme permits only pulsed operation However in lsquoadvanced tokamaksrsquo (see section 3) after inductive start-up the plasma current is sustained by a self-organized diffusion current (lsquobootstraprsquo effect) and by electromagnetic waves which permits continuous operation schemes
The Joint European Torus (JET) in CulhamUK is so far the largest and most successful tokamak experiment JETrsquos life started in 1983 and will continue at least until 2010 under the lsquoEuropean Fusion Development AgreementEFDArsquo [17] The JET plasma features a large radius R = 3 m with elongated cross section a plasma surface of 200 m2 and volume of about 150 m3 and Bt and Ip may reach 3 T and 3 MA respectively To achieve a sufficiently high plasma temperature ohmic heating by Ip has to be supported by other techniques as the injection of neutral fuel atom beams and of electromagnetic waves with electron cyclotron resonance (ECR) and ion cyclotron resonance (ICR) frequencies In future fusion reactors a so-called lsquoburning plasmarsquo will mainly be heated by the 35 MeV alpha particles from D-T fusion reactions
JET has been run in several campaigns with D-T fuel and in 1997 reached for about 05 s a peak fusion power of 16 MW a world record until today This figure has to be compared with the total heating power applied for plasma operation giving a peak ratio of fusion to plasma heating power (lsquoplasma amplification factorrsquo Q) of 65 JET together with numerous other tokamak experiments has generated sufficient expertise on the achievable plasma energy confinement times within the plasma parameter space (density geometry Bt and Ip) to permit a reliable empirical scaling for extrapolation toward still larger experiments in particular ITER (see figure 2)
Figure 2 Experimentally observed (vertical axis) vs empirically determined energy confinement time from various tokamak experiments extrapolated toward ITER (with permission of ITER)
The design of ITER (in Latin lsquothe wayrsquo and an officially no more valid acronym for lsquoInternational Thermonuclear Experimental Reactorrsquo) was already started in 1988 but had to be thoroughly revised in 1998 mainly for cost reasons The new design was finished in 2001 (cf figure 3 details given on the ITER webpage [18]) As to ITERrsquos main parameters R = 62 m Bt = 53 T (from superconducting coils) Ip 15 MA and plasma volume and surface are 840 m3 and ca 1000 m2 respectively
ITERrsquos principal mission is the demonstration of fusion reactor relevant burning plasmas with Q 10 during a discharge time of 500 s and possibly in steady-state operation ITER will be built by industrial companies from the present seven partners (EU Japan RF USA PR of China South Korea and India more countries have expressed their strong interest to join)
The total budget for building ITER is 47 billion euro Construction at the European site Cadarache in France will start in 2007 and first plasma operation is expected for 2016
35
In the bilateral ITER negotiations between the EU and Japan who both had offered sites a so-called lsquoBroader Approach to Fusionrsquo has been adopted based on a lsquoFast Trackrsquo strategy proposed in 2001 (parallel development of fusion plasma physics and technology) It includes support from the EU both for a next large tokamak and the first DEMO site in Japan Another bilateral collaboration agreement has recently been concluded for six years to carry out lsquoEngineering Validation and Engineering Design ActivitiesEVEDArsquo in preparation of the planned International Fusion Material Irradiation Facility (IFMIF) This accelerator-based fusion dedicated facility could be built within ten years after successful conclusion of EVEDA IFMIF shall provide the fusion reactor relevant neutron irradiation environment for development and testing of structural materials needed for the construction of DEMO Such materials and relevant technology have to be available at a time when sufficiently conclusive results from ITER have arrived ITER IFMIF and DEMO are the three principal parts of the proposed lsquoFast Trackrsquo for making fusion energy a reality before the mid of this century
3 Why are HCI important for magnetically confined fusion plasmas The presence of HCI in fusion plasmas is commonly seen just as a nuisance Neutral atoms with nuclear charge Z gt 2 from plasma-wall interaction (PWI) [19 20] diffuse into the hot plasma core where they become ionized Fully stripped ions contribute to the global plasma bremsstrahlung with a radiation power in proportion to Z2 and an approximate much stronger Z45-dependence holds for the line radiation from the incompletely ionized collisionally excited ions With a non-negligible impurity content this radiation constitutes an important power loss which needs to be balanced by stronger plasma heating and improved confinement The common figure for characterization of the plasma impurity content is the lsquoeffective ion chargersquo
Zeff Z 2nZ ne (1) Impurities including the lsquoHe ashrsquo from fusion reactions dilute the D-T plasma fuel thanks to the
plasma quasi-neutrality condition ne ZnZ (2) (ne and nZ are the electron and ion densities Z 1) the majority (D T) ion density is already
strongly reduced for a relatively small impurity content (eg by 16 for only 1 admixed fully stripped oxygen) This radiative power loss inhibits the plasma start-up strongly influences the achievable plasma density limit and also varies the neutral beam power deposition profile
However plasma impurities also play a useful and in fact rather important role not only because various plasma diagnostic techniques are based on the exploitation of impurity radiation Still more relevant is their ability to modify the plasma radiation loss profile in particular in the plasma edge region where also low-Z impurities (C O) are not yet fully ionized An important part of the kinetic energy of the plasma constituents is converted into radiation by interaction with impurities This considerably dampens the PWI and actually makes the operation of D-T-fusion reactors possible For a given impurity species and plasma temperature the radiated power strongly depends on the ion charge Carefully administered impurity seeding can thus influence the plasma balance in a rather positive way (radiation cooling by lsquoseededrsquo Ne Si Ar or other species) Problems associated with impurity production by PWI are reduced by a magnetic lsquodivertorrsquo Magnetic field lines are diverted by means of external poloidal coils toward a separated region where the plasma is partially neutralized and PWI products are rapidly pumped away in order to minimize contamination of the main plasma Most common are lsquosingle-nullrsquo poloidal divertors as also planned for ITER (see figures 3 and 4)
Another important aspect of PWI are transient lsquoedge localized modesELMsrsquo which result from particular plasma instabilities [21] During an ELM up to 10 of the total plasma thermal energy can erupt as particle flux within less than 1 ms toward first wall components This inflicts significant material damage with strong impurity influx ELMs are a special concern for ITER because the ratio of plasma energy content to first wall surface strongly increases with plasma size A possible remedy for deleterious lsquogiant ELMSrsquo is the injection of frozen fuel pellets On the other hand mild ELMs seem to be indispensable for regular expulsion of plasma impurities in particular the He ash
36
Figure 3 Cutaway view of ITER (total height 24 m with permission of ITER)
Figure 4 Cross section of ITER vacuum vessel with divertor region on bottom (with permission of ITER)
The plasma profile can be carefully tailored by local radiation cooling which influences the plasma transport [22] and in particular permits the formation of an lsquointernal transport barrierrsquo (ITB) for improved plasma confinement [23] The impurity profile is especially relevant for development of advanced tokamak regimes (non-inductive current drive for steady-state operation simultaneous control of plasma current and pressure profile by active feedback MHD stability control for optimized plasma performance [24]) All the here mentioned features are interdependent and self-consistently interacting which makes their study by integrated plasma modeling a rather difficult task However the further rapid increase of computing power will lead to increasingly realistic results in such studies
4 Some experimental studies with slow HCI for magnetic fusion plasmas The first case concerns beam emission spectroscopy (BES) for edge plasma diagnostics If a fast (10 ndash 50 keV) neutral Li beam is injected into the plasma boundary collisions with plasma electrons and ions (majority species and impurities) give rise to characteristic Li I line emission Measurement of the Li I line intensity along the injected diagnostic beam delivers the plasma electron density profile and electron capture from Li by impurity ions leads to the impurity ion density and temperature profiles
This technique has first been demonstrated at TEXTOR (Juelich) [25] [26] and later at ASDEX Upgrade (IPP Garching) [27] and it is now implemented at JET as well Evaluation of plasma properties requires extensive modeling of the Li beam state composition with the help of a dedicated atomic collision data base [28] Li-BES is also of interest for measuring correlation functions of electron density fluctuations [29] Since the applicability of Li-BES is limited by the available neutral Li beam intensity switching to Na injection will probably be useful More recently we have also applied fast He beams [30] for BES Combined line emission measurements from He I singlet and triplet transitions possibly permit evaluation of plasma electron temperature profiles First exploratory measurements have been conducted at JET (cf figures 5ab) and ASDEX Upgrade [31] [32] but until now the obtained results remain inconclusive which is primarily caused by our still incomplete atomic collision data base for neutral He beam modeling
37
Figure 5 (a) Extreme flux surface positions during a 12 cm horizontal sweep of JET plasma during 80 keV He-BES measurements (b) Time development of He I 588 nm triplet line intensity showing the changing signal from passive edge plasma emission (upper part) and from the Doppler shifted He-BES emission (lower part) [33])
However very attractive features as the rather simple preparation of intense neutral He beams by He doping of the standard neutral heating beams and the considerably deeper plasma penetration in comparison to Li and Na-BES suggest further continuation of these studies
For the second example we have recently explored [34] the influence of particle induced electron emission (PIEE lsquoparticlersquo designates ions as well as electrons) from a material plasma boundary on the voltage drop across the adjacent plasma sheath Such a situation is of interest for the plasma region near a tokamak divertor plate A new model involving a collisional kinetic sheath consistently coupled to a fluid presheath has been developed which is applicable for plasmas with a Debye length much smaller than the relevant characteristic presheath length (lsquoasymptotic two-scale limitrsquo cf figure 6a) Majority and impurity ions are accelerated by the sheath voltage from the bulk plasma and can release electrons from the material surface which flow back into the bulk plasma (see figure 6b for characteristic PIEE yields for Cq+ ion impact on different graphite surfaces) Moreover fast plasma electrons from the plasma Maxwellian tail can reach the surface to release secondary electrons and these fast electrons will also be partially reflected In a first approach we have neglected the tokamak-generic inclined magnetic field and also the fast electron reflection Under these simplifying assumptions the reduction of the plasma sheath voltage has been self-consistently calculated and was found to reach up to 30 of the sheath voltage for a case with PIEE not taken into account More realistic results will be obtained after adaptation of our model for an inclined magnetic field and with electron reflection
5 Summary and conclusions In this survey we have attempted to elucidate the beneficial role of plasma impurities (both in low and high charge states) for magnetically confined fusion plasmas in particular with respect to ITER European fusion research has taken a strong lead in this field by setting up two task forces on plasma-wall interaction and on integrated plasma modeling In both areas there is a strong need for more complete and more precise data on collisions of HCI (low-Z He Be B C O Ne medium-Z Ar Fe etc high-Z W) with hydrogen atoms and molecules and also with surfaces (Be graphite stainless steel W) The most important role of highly charged plasma impurities is to be seen in the quest of advanced tokamak plasma studies and related plasma modeling for the optimal design of future thermonuclear fusion reactors based on the tokamak configuration
38
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120
(e- io
n)
velocity (10 4 ms)
Cq+ Graphite
C4+CL
C4+HOPG
C2+HOPG
C+HOPG
C5+CL
C2+CL
Figure 6 (a) Schematic layout of new plasma sheath model with PIEE by ions and electrons flowing from a plasma (right side) toward a conducting material wall (left side) (b) Total electron yields for electron emission induced by singly and multiply charged carbon ions from surfaces of carbon fiber-enforced (CFC-CL) and highly oriented pyrolytic graphite (HOPG) respectively (data from [35])
Acknowledgements The author thanks Prof Friedrich Aumayr (TU Wien) and Drs Josef Schweinzer and Elisabeth Wolfrum (IPP Garching) for longstanding cooperation in research activities described in this paper This work has been supported by the European Commission under the contract of Association EURATOM-OEAW and was carried out within the framework of the European Fusion Development Agreement (EFDA) The views and opinions expressed therein do not necessarily reflect those of the European Commission
References [1] Janev R K 1995 Atomic and Molecular Processes in Fusion Edge Plasma[2] Atomic and Plasma-Material Interaction Data for Fusion vol 1 - 12 IAEA Vienna 1991 - 2003 [3] Crandall D H 1983 An overview of the symposium Status and direction of current research
with highly charged ions Physica Scripta T3 249 - 253 [4] Janev R K 1989 Atomic collisions in fusion plasmas J de Physique 50 C1-421 - 443 [5] Fawcett B C 1991 Role of highly-charged ions in astrophysical and laboratory plasmas
ZPhysD 21 S1 - 5 [6] Donets E D Ilyushenko V I and V Alpert 1969 Ultrahigh vacuum electron beam source of
highly stripped ions Proc 1 Conf Int sur les Sources dIons Saclay France 635 - 642 [7] Bliman S et al A high intensity ECR stripped ion source IEEE TransNuclSci 1971 NS-19
200 - 203 [8] Marrs R E Beiersdorfer P and Schneider D 1994 The electron-beam ion trap Physics Today
October 27 - 34 [9] Arnau A et al 1997 Interaction of slow multicharged ions with solid surfaces Surface Science
Reports 27 113 - 240 [10] Schenkel T et al 1999 Interaction of slow very highly charged ions with surfaces
ProgrSurfSci 61 23 - 84 [11] Aumayr F and Winter HP 2004 Potential sputtering PhilTransRSocLond 362 77 - 102 [12] Stoumlhlker T et al 2003 Test of strong-field QED in The physics of multiply and highly charged
ions (ed F J Currell) vol 1 chapter 11
39
[13] Kluge H-J 2006 Trapped and cooled Atomic physics experiments with highly charged ions in Penning traps (these proceedings)
[14] Braams C M and Stott P E 2002 Nuclear Fusion - half a century of magnetic confinement fusion research IoP Publ Bristol
[15] Kaw P K 2005 Status report on controlled thermonuclear fusion Nucl Fusion 45 A 1 - 28 [16] Lawson J D 1955 Some criteria for a useful thermonuclear reactor AERE GPR 1807 [17] wwwjetefdaorg[18] wwwiterorg[19] Philipps V Roth J and Loarte A 2003 Key issues in plasma-wall interactions for ITER a
European approach Plasma PhysControl Fusion 45 A17 - 30 [20] Samm U 2005 Plasma-Wall Interaction Status and Data Needs in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [21] Loarte A 2005 Energy deposition from ELMS in fusion devices in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [22] Garbet X et al 2004 Physics of transport in tokamaks Plasma PhysControl Fusion 46 B557 -
574[23] Challis C D 2004 The use of internal transport barriers in tokamak plasmas Plasma
PhysControl Fusion 46 B23 - 40 [24] Greenfield C M et al 2004 Advanced tokamak research in DIII-D Plasma PhysControl Fusion
46 B213 - 233 [25] Schorn R P et al 1992 Radial temperature distribution of C6+ ions in the TEXTOR edge plasma
[26] Wolfrum E et al 1993 Fast lithium-beam spectroscopy of tokamak edge plasmas RevSciInstrum 64 2285 - 2292
[27] Brandenburg R et al 1999 Fast lithium beam edge plasma spectroscopy at IPP Garching ndash Status and recent development Fusion technology 36 289 ndash 295
[28] Schweinzer J et al 1999 Database for inelastic collisions of lithium atoms with electrons protons and multiply charged ions At Data NuclData Tables 72 239 - 273
[29] Zoletnik S et al 1998 Determination of electron density fluctuation correlation functions via beam emission spectroscopy Plasma PhysControl Fusion 40 1399 - 1416
[30] Falter H D et al 2000 Helium doped hydrogen or deuterium beam as cost effective and simple tool for plasma spectroscopy RevSciInstrum 71 3723 - 3727
[31] Menhart S et al 2001 Explorative studies for the development of fast He beam plasma diagnostics JournNuclMater 290-293 673 - 677
[32] Galutschek E et al 2003 Development of fast helium beam emission spectroscopy for tokamak plasma density and temperature diagnostics Proc contr papers to 30 EPS Conf on Contr Fusion and Plasma Phys (StPetersburg 7 ndash 11 July 2003)
[33] Proschek M 2001 Towards fast He beam edge plasma diagnostics PhD thesis TU Wien [34] Schupfer N et al 2006 Effect of particle-induced electron emission (PIEE) on the plasma sheath
voltage Plasma PhysControl Fusion 48 1093 - 1103 [35] Cernusca S et al 2003 Edge plasma-relevant ion-surface collision processes Int J Mass
Spectrom 223-224 21 - 36
40
In the bilateral ITER negotiations between the EU and Japan who both had offered sites a so-called lsquoBroader Approach to Fusionrsquo has been adopted based on a lsquoFast Trackrsquo strategy proposed in 2001 (parallel development of fusion plasma physics and technology) It includes support from the EU both for a next large tokamak and the first DEMO site in Japan Another bilateral collaboration agreement has recently been concluded for six years to carry out lsquoEngineering Validation and Engineering Design ActivitiesEVEDArsquo in preparation of the planned International Fusion Material Irradiation Facility (IFMIF) This accelerator-based fusion dedicated facility could be built within ten years after successful conclusion of EVEDA IFMIF shall provide the fusion reactor relevant neutron irradiation environment for development and testing of structural materials needed for the construction of DEMO Such materials and relevant technology have to be available at a time when sufficiently conclusive results from ITER have arrived ITER IFMIF and DEMO are the three principal parts of the proposed lsquoFast Trackrsquo for making fusion energy a reality before the mid of this century
3 Why are HCI important for magnetically confined fusion plasmas The presence of HCI in fusion plasmas is commonly seen just as a nuisance Neutral atoms with nuclear charge Z gt 2 from plasma-wall interaction (PWI) [19 20] diffuse into the hot plasma core where they become ionized Fully stripped ions contribute to the global plasma bremsstrahlung with a radiation power in proportion to Z2 and an approximate much stronger Z45-dependence holds for the line radiation from the incompletely ionized collisionally excited ions With a non-negligible impurity content this radiation constitutes an important power loss which needs to be balanced by stronger plasma heating and improved confinement The common figure for characterization of the plasma impurity content is the lsquoeffective ion chargersquo
Zeff Z 2nZ ne (1) Impurities including the lsquoHe ashrsquo from fusion reactions dilute the D-T plasma fuel thanks to the
plasma quasi-neutrality condition ne ZnZ (2) (ne and nZ are the electron and ion densities Z 1) the majority (D T) ion density is already
strongly reduced for a relatively small impurity content (eg by 16 for only 1 admixed fully stripped oxygen) This radiative power loss inhibits the plasma start-up strongly influences the achievable plasma density limit and also varies the neutral beam power deposition profile
However plasma impurities also play a useful and in fact rather important role not only because various plasma diagnostic techniques are based on the exploitation of impurity radiation Still more relevant is their ability to modify the plasma radiation loss profile in particular in the plasma edge region where also low-Z impurities (C O) are not yet fully ionized An important part of the kinetic energy of the plasma constituents is converted into radiation by interaction with impurities This considerably dampens the PWI and actually makes the operation of D-T-fusion reactors possible For a given impurity species and plasma temperature the radiated power strongly depends on the ion charge Carefully administered impurity seeding can thus influence the plasma balance in a rather positive way (radiation cooling by lsquoseededrsquo Ne Si Ar or other species) Problems associated with impurity production by PWI are reduced by a magnetic lsquodivertorrsquo Magnetic field lines are diverted by means of external poloidal coils toward a separated region where the plasma is partially neutralized and PWI products are rapidly pumped away in order to minimize contamination of the main plasma Most common are lsquosingle-nullrsquo poloidal divertors as also planned for ITER (see figures 3 and 4)
Another important aspect of PWI are transient lsquoedge localized modesELMsrsquo which result from particular plasma instabilities [21] During an ELM up to 10 of the total plasma thermal energy can erupt as particle flux within less than 1 ms toward first wall components This inflicts significant material damage with strong impurity influx ELMs are a special concern for ITER because the ratio of plasma energy content to first wall surface strongly increases with plasma size A possible remedy for deleterious lsquogiant ELMSrsquo is the injection of frozen fuel pellets On the other hand mild ELMs seem to be indispensable for regular expulsion of plasma impurities in particular the He ash
36
Figure 3 Cutaway view of ITER (total height 24 m with permission of ITER)
Figure 4 Cross section of ITER vacuum vessel with divertor region on bottom (with permission of ITER)
The plasma profile can be carefully tailored by local radiation cooling which influences the plasma transport [22] and in particular permits the formation of an lsquointernal transport barrierrsquo (ITB) for improved plasma confinement [23] The impurity profile is especially relevant for development of advanced tokamak regimes (non-inductive current drive for steady-state operation simultaneous control of plasma current and pressure profile by active feedback MHD stability control for optimized plasma performance [24]) All the here mentioned features are interdependent and self-consistently interacting which makes their study by integrated plasma modeling a rather difficult task However the further rapid increase of computing power will lead to increasingly realistic results in such studies
4 Some experimental studies with slow HCI for magnetic fusion plasmas The first case concerns beam emission spectroscopy (BES) for edge plasma diagnostics If a fast (10 ndash 50 keV) neutral Li beam is injected into the plasma boundary collisions with plasma electrons and ions (majority species and impurities) give rise to characteristic Li I line emission Measurement of the Li I line intensity along the injected diagnostic beam delivers the plasma electron density profile and electron capture from Li by impurity ions leads to the impurity ion density and temperature profiles
This technique has first been demonstrated at TEXTOR (Juelich) [25] [26] and later at ASDEX Upgrade (IPP Garching) [27] and it is now implemented at JET as well Evaluation of plasma properties requires extensive modeling of the Li beam state composition with the help of a dedicated atomic collision data base [28] Li-BES is also of interest for measuring correlation functions of electron density fluctuations [29] Since the applicability of Li-BES is limited by the available neutral Li beam intensity switching to Na injection will probably be useful More recently we have also applied fast He beams [30] for BES Combined line emission measurements from He I singlet and triplet transitions possibly permit evaluation of plasma electron temperature profiles First exploratory measurements have been conducted at JET (cf figures 5ab) and ASDEX Upgrade [31] [32] but until now the obtained results remain inconclusive which is primarily caused by our still incomplete atomic collision data base for neutral He beam modeling
37
Figure 5 (a) Extreme flux surface positions during a 12 cm horizontal sweep of JET plasma during 80 keV He-BES measurements (b) Time development of He I 588 nm triplet line intensity showing the changing signal from passive edge plasma emission (upper part) and from the Doppler shifted He-BES emission (lower part) [33])
However very attractive features as the rather simple preparation of intense neutral He beams by He doping of the standard neutral heating beams and the considerably deeper plasma penetration in comparison to Li and Na-BES suggest further continuation of these studies
For the second example we have recently explored [34] the influence of particle induced electron emission (PIEE lsquoparticlersquo designates ions as well as electrons) from a material plasma boundary on the voltage drop across the adjacent plasma sheath Such a situation is of interest for the plasma region near a tokamak divertor plate A new model involving a collisional kinetic sheath consistently coupled to a fluid presheath has been developed which is applicable for plasmas with a Debye length much smaller than the relevant characteristic presheath length (lsquoasymptotic two-scale limitrsquo cf figure 6a) Majority and impurity ions are accelerated by the sheath voltage from the bulk plasma and can release electrons from the material surface which flow back into the bulk plasma (see figure 6b for characteristic PIEE yields for Cq+ ion impact on different graphite surfaces) Moreover fast plasma electrons from the plasma Maxwellian tail can reach the surface to release secondary electrons and these fast electrons will also be partially reflected In a first approach we have neglected the tokamak-generic inclined magnetic field and also the fast electron reflection Under these simplifying assumptions the reduction of the plasma sheath voltage has been self-consistently calculated and was found to reach up to 30 of the sheath voltage for a case with PIEE not taken into account More realistic results will be obtained after adaptation of our model for an inclined magnetic field and with electron reflection
5 Summary and conclusions In this survey we have attempted to elucidate the beneficial role of plasma impurities (both in low and high charge states) for magnetically confined fusion plasmas in particular with respect to ITER European fusion research has taken a strong lead in this field by setting up two task forces on plasma-wall interaction and on integrated plasma modeling In both areas there is a strong need for more complete and more precise data on collisions of HCI (low-Z He Be B C O Ne medium-Z Ar Fe etc high-Z W) with hydrogen atoms and molecules and also with surfaces (Be graphite stainless steel W) The most important role of highly charged plasma impurities is to be seen in the quest of advanced tokamak plasma studies and related plasma modeling for the optimal design of future thermonuclear fusion reactors based on the tokamak configuration
38
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120
(e- io
n)
velocity (10 4 ms)
Cq+ Graphite
C4+CL
C4+HOPG
C2+HOPG
C+HOPG
C5+CL
C2+CL
Figure 6 (a) Schematic layout of new plasma sheath model with PIEE by ions and electrons flowing from a plasma (right side) toward a conducting material wall (left side) (b) Total electron yields for electron emission induced by singly and multiply charged carbon ions from surfaces of carbon fiber-enforced (CFC-CL) and highly oriented pyrolytic graphite (HOPG) respectively (data from [35])
Acknowledgements The author thanks Prof Friedrich Aumayr (TU Wien) and Drs Josef Schweinzer and Elisabeth Wolfrum (IPP Garching) for longstanding cooperation in research activities described in this paper This work has been supported by the European Commission under the contract of Association EURATOM-OEAW and was carried out within the framework of the European Fusion Development Agreement (EFDA) The views and opinions expressed therein do not necessarily reflect those of the European Commission
References [1] Janev R K 1995 Atomic and Molecular Processes in Fusion Edge Plasma[2] Atomic and Plasma-Material Interaction Data for Fusion vol 1 - 12 IAEA Vienna 1991 - 2003 [3] Crandall D H 1983 An overview of the symposium Status and direction of current research
with highly charged ions Physica Scripta T3 249 - 253 [4] Janev R K 1989 Atomic collisions in fusion plasmas J de Physique 50 C1-421 - 443 [5] Fawcett B C 1991 Role of highly-charged ions in astrophysical and laboratory plasmas
ZPhysD 21 S1 - 5 [6] Donets E D Ilyushenko V I and V Alpert 1969 Ultrahigh vacuum electron beam source of
highly stripped ions Proc 1 Conf Int sur les Sources dIons Saclay France 635 - 642 [7] Bliman S et al A high intensity ECR stripped ion source IEEE TransNuclSci 1971 NS-19
200 - 203 [8] Marrs R E Beiersdorfer P and Schneider D 1994 The electron-beam ion trap Physics Today
October 27 - 34 [9] Arnau A et al 1997 Interaction of slow multicharged ions with solid surfaces Surface Science
Reports 27 113 - 240 [10] Schenkel T et al 1999 Interaction of slow very highly charged ions with surfaces
ProgrSurfSci 61 23 - 84 [11] Aumayr F and Winter HP 2004 Potential sputtering PhilTransRSocLond 362 77 - 102 [12] Stoumlhlker T et al 2003 Test of strong-field QED in The physics of multiply and highly charged
ions (ed F J Currell) vol 1 chapter 11
39
[13] Kluge H-J 2006 Trapped and cooled Atomic physics experiments with highly charged ions in Penning traps (these proceedings)
[14] Braams C M and Stott P E 2002 Nuclear Fusion - half a century of magnetic confinement fusion research IoP Publ Bristol
[15] Kaw P K 2005 Status report on controlled thermonuclear fusion Nucl Fusion 45 A 1 - 28 [16] Lawson J D 1955 Some criteria for a useful thermonuclear reactor AERE GPR 1807 [17] wwwjetefdaorg[18] wwwiterorg[19] Philipps V Roth J and Loarte A 2003 Key issues in plasma-wall interactions for ITER a
European approach Plasma PhysControl Fusion 45 A17 - 30 [20] Samm U 2005 Plasma-Wall Interaction Status and Data Needs in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [21] Loarte A 2005 Energy deposition from ELMS in fusion devices in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [22] Garbet X et al 2004 Physics of transport in tokamaks Plasma PhysControl Fusion 46 B557 -
574[23] Challis C D 2004 The use of internal transport barriers in tokamak plasmas Plasma
PhysControl Fusion 46 B23 - 40 [24] Greenfield C M et al 2004 Advanced tokamak research in DIII-D Plasma PhysControl Fusion
46 B213 - 233 [25] Schorn R P et al 1992 Radial temperature distribution of C6+ ions in the TEXTOR edge plasma
[26] Wolfrum E et al 1993 Fast lithium-beam spectroscopy of tokamak edge plasmas RevSciInstrum 64 2285 - 2292
[27] Brandenburg R et al 1999 Fast lithium beam edge plasma spectroscopy at IPP Garching ndash Status and recent development Fusion technology 36 289 ndash 295
[28] Schweinzer J et al 1999 Database for inelastic collisions of lithium atoms with electrons protons and multiply charged ions At Data NuclData Tables 72 239 - 273
[29] Zoletnik S et al 1998 Determination of electron density fluctuation correlation functions via beam emission spectroscopy Plasma PhysControl Fusion 40 1399 - 1416
[30] Falter H D et al 2000 Helium doped hydrogen or deuterium beam as cost effective and simple tool for plasma spectroscopy RevSciInstrum 71 3723 - 3727
[31] Menhart S et al 2001 Explorative studies for the development of fast He beam plasma diagnostics JournNuclMater 290-293 673 - 677
[32] Galutschek E et al 2003 Development of fast helium beam emission spectroscopy for tokamak plasma density and temperature diagnostics Proc contr papers to 30 EPS Conf on Contr Fusion and Plasma Phys (StPetersburg 7 ndash 11 July 2003)
[33] Proschek M 2001 Towards fast He beam edge plasma diagnostics PhD thesis TU Wien [34] Schupfer N et al 2006 Effect of particle-induced electron emission (PIEE) on the plasma sheath
voltage Plasma PhysControl Fusion 48 1093 - 1103 [35] Cernusca S et al 2003 Edge plasma-relevant ion-surface collision processes Int J Mass
Spectrom 223-224 21 - 36
40
Figure 3 Cutaway view of ITER (total height 24 m with permission of ITER)
Figure 4 Cross section of ITER vacuum vessel with divertor region on bottom (with permission of ITER)
The plasma profile can be carefully tailored by local radiation cooling which influences the plasma transport [22] and in particular permits the formation of an lsquointernal transport barrierrsquo (ITB) for improved plasma confinement [23] The impurity profile is especially relevant for development of advanced tokamak regimes (non-inductive current drive for steady-state operation simultaneous control of plasma current and pressure profile by active feedback MHD stability control for optimized plasma performance [24]) All the here mentioned features are interdependent and self-consistently interacting which makes their study by integrated plasma modeling a rather difficult task However the further rapid increase of computing power will lead to increasingly realistic results in such studies
4 Some experimental studies with slow HCI for magnetic fusion plasmas The first case concerns beam emission spectroscopy (BES) for edge plasma diagnostics If a fast (10 ndash 50 keV) neutral Li beam is injected into the plasma boundary collisions with plasma electrons and ions (majority species and impurities) give rise to characteristic Li I line emission Measurement of the Li I line intensity along the injected diagnostic beam delivers the plasma electron density profile and electron capture from Li by impurity ions leads to the impurity ion density and temperature profiles
This technique has first been demonstrated at TEXTOR (Juelich) [25] [26] and later at ASDEX Upgrade (IPP Garching) [27] and it is now implemented at JET as well Evaluation of plasma properties requires extensive modeling of the Li beam state composition with the help of a dedicated atomic collision data base [28] Li-BES is also of interest for measuring correlation functions of electron density fluctuations [29] Since the applicability of Li-BES is limited by the available neutral Li beam intensity switching to Na injection will probably be useful More recently we have also applied fast He beams [30] for BES Combined line emission measurements from He I singlet and triplet transitions possibly permit evaluation of plasma electron temperature profiles First exploratory measurements have been conducted at JET (cf figures 5ab) and ASDEX Upgrade [31] [32] but until now the obtained results remain inconclusive which is primarily caused by our still incomplete atomic collision data base for neutral He beam modeling
37
Figure 5 (a) Extreme flux surface positions during a 12 cm horizontal sweep of JET plasma during 80 keV He-BES measurements (b) Time development of He I 588 nm triplet line intensity showing the changing signal from passive edge plasma emission (upper part) and from the Doppler shifted He-BES emission (lower part) [33])
However very attractive features as the rather simple preparation of intense neutral He beams by He doping of the standard neutral heating beams and the considerably deeper plasma penetration in comparison to Li and Na-BES suggest further continuation of these studies
For the second example we have recently explored [34] the influence of particle induced electron emission (PIEE lsquoparticlersquo designates ions as well as electrons) from a material plasma boundary on the voltage drop across the adjacent plasma sheath Such a situation is of interest for the plasma region near a tokamak divertor plate A new model involving a collisional kinetic sheath consistently coupled to a fluid presheath has been developed which is applicable for plasmas with a Debye length much smaller than the relevant characteristic presheath length (lsquoasymptotic two-scale limitrsquo cf figure 6a) Majority and impurity ions are accelerated by the sheath voltage from the bulk plasma and can release electrons from the material surface which flow back into the bulk plasma (see figure 6b for characteristic PIEE yields for Cq+ ion impact on different graphite surfaces) Moreover fast plasma electrons from the plasma Maxwellian tail can reach the surface to release secondary electrons and these fast electrons will also be partially reflected In a first approach we have neglected the tokamak-generic inclined magnetic field and also the fast electron reflection Under these simplifying assumptions the reduction of the plasma sheath voltage has been self-consistently calculated and was found to reach up to 30 of the sheath voltage for a case with PIEE not taken into account More realistic results will be obtained after adaptation of our model for an inclined magnetic field and with electron reflection
5 Summary and conclusions In this survey we have attempted to elucidate the beneficial role of plasma impurities (both in low and high charge states) for magnetically confined fusion plasmas in particular with respect to ITER European fusion research has taken a strong lead in this field by setting up two task forces on plasma-wall interaction and on integrated plasma modeling In both areas there is a strong need for more complete and more precise data on collisions of HCI (low-Z He Be B C O Ne medium-Z Ar Fe etc high-Z W) with hydrogen atoms and molecules and also with surfaces (Be graphite stainless steel W) The most important role of highly charged plasma impurities is to be seen in the quest of advanced tokamak plasma studies and related plasma modeling for the optimal design of future thermonuclear fusion reactors based on the tokamak configuration
38
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120
(e- io
n)
velocity (10 4 ms)
Cq+ Graphite
C4+CL
C4+HOPG
C2+HOPG
C+HOPG
C5+CL
C2+CL
Figure 6 (a) Schematic layout of new plasma sheath model with PIEE by ions and electrons flowing from a plasma (right side) toward a conducting material wall (left side) (b) Total electron yields for electron emission induced by singly and multiply charged carbon ions from surfaces of carbon fiber-enforced (CFC-CL) and highly oriented pyrolytic graphite (HOPG) respectively (data from [35])
Acknowledgements The author thanks Prof Friedrich Aumayr (TU Wien) and Drs Josef Schweinzer and Elisabeth Wolfrum (IPP Garching) for longstanding cooperation in research activities described in this paper This work has been supported by the European Commission under the contract of Association EURATOM-OEAW and was carried out within the framework of the European Fusion Development Agreement (EFDA) The views and opinions expressed therein do not necessarily reflect those of the European Commission
References [1] Janev R K 1995 Atomic and Molecular Processes in Fusion Edge Plasma[2] Atomic and Plasma-Material Interaction Data for Fusion vol 1 - 12 IAEA Vienna 1991 - 2003 [3] Crandall D H 1983 An overview of the symposium Status and direction of current research
with highly charged ions Physica Scripta T3 249 - 253 [4] Janev R K 1989 Atomic collisions in fusion plasmas J de Physique 50 C1-421 - 443 [5] Fawcett B C 1991 Role of highly-charged ions in astrophysical and laboratory plasmas
ZPhysD 21 S1 - 5 [6] Donets E D Ilyushenko V I and V Alpert 1969 Ultrahigh vacuum electron beam source of
highly stripped ions Proc 1 Conf Int sur les Sources dIons Saclay France 635 - 642 [7] Bliman S et al A high intensity ECR stripped ion source IEEE TransNuclSci 1971 NS-19
200 - 203 [8] Marrs R E Beiersdorfer P and Schneider D 1994 The electron-beam ion trap Physics Today
October 27 - 34 [9] Arnau A et al 1997 Interaction of slow multicharged ions with solid surfaces Surface Science
Reports 27 113 - 240 [10] Schenkel T et al 1999 Interaction of slow very highly charged ions with surfaces
ProgrSurfSci 61 23 - 84 [11] Aumayr F and Winter HP 2004 Potential sputtering PhilTransRSocLond 362 77 - 102 [12] Stoumlhlker T et al 2003 Test of strong-field QED in The physics of multiply and highly charged
ions (ed F J Currell) vol 1 chapter 11
39
[13] Kluge H-J 2006 Trapped and cooled Atomic physics experiments with highly charged ions in Penning traps (these proceedings)
[14] Braams C M and Stott P E 2002 Nuclear Fusion - half a century of magnetic confinement fusion research IoP Publ Bristol
[15] Kaw P K 2005 Status report on controlled thermonuclear fusion Nucl Fusion 45 A 1 - 28 [16] Lawson J D 1955 Some criteria for a useful thermonuclear reactor AERE GPR 1807 [17] wwwjetefdaorg[18] wwwiterorg[19] Philipps V Roth J and Loarte A 2003 Key issues in plasma-wall interactions for ITER a
European approach Plasma PhysControl Fusion 45 A17 - 30 [20] Samm U 2005 Plasma-Wall Interaction Status and Data Needs in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [21] Loarte A 2005 Energy deposition from ELMS in fusion devices in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [22] Garbet X et al 2004 Physics of transport in tokamaks Plasma PhysControl Fusion 46 B557 -
574[23] Challis C D 2004 The use of internal transport barriers in tokamak plasmas Plasma
PhysControl Fusion 46 B23 - 40 [24] Greenfield C M et al 2004 Advanced tokamak research in DIII-D Plasma PhysControl Fusion
46 B213 - 233 [25] Schorn R P et al 1992 Radial temperature distribution of C6+ ions in the TEXTOR edge plasma
[26] Wolfrum E et al 1993 Fast lithium-beam spectroscopy of tokamak edge plasmas RevSciInstrum 64 2285 - 2292
[27] Brandenburg R et al 1999 Fast lithium beam edge plasma spectroscopy at IPP Garching ndash Status and recent development Fusion technology 36 289 ndash 295
[28] Schweinzer J et al 1999 Database for inelastic collisions of lithium atoms with electrons protons and multiply charged ions At Data NuclData Tables 72 239 - 273
[29] Zoletnik S et al 1998 Determination of electron density fluctuation correlation functions via beam emission spectroscopy Plasma PhysControl Fusion 40 1399 - 1416
[30] Falter H D et al 2000 Helium doped hydrogen or deuterium beam as cost effective and simple tool for plasma spectroscopy RevSciInstrum 71 3723 - 3727
[31] Menhart S et al 2001 Explorative studies for the development of fast He beam plasma diagnostics JournNuclMater 290-293 673 - 677
[32] Galutschek E et al 2003 Development of fast helium beam emission spectroscopy for tokamak plasma density and temperature diagnostics Proc contr papers to 30 EPS Conf on Contr Fusion and Plasma Phys (StPetersburg 7 ndash 11 July 2003)
[33] Proschek M 2001 Towards fast He beam edge plasma diagnostics PhD thesis TU Wien [34] Schupfer N et al 2006 Effect of particle-induced electron emission (PIEE) on the plasma sheath
voltage Plasma PhysControl Fusion 48 1093 - 1103 [35] Cernusca S et al 2003 Edge plasma-relevant ion-surface collision processes Int J Mass
Spectrom 223-224 21 - 36
40
Figure 5 (a) Extreme flux surface positions during a 12 cm horizontal sweep of JET plasma during 80 keV He-BES measurements (b) Time development of He I 588 nm triplet line intensity showing the changing signal from passive edge plasma emission (upper part) and from the Doppler shifted He-BES emission (lower part) [33])
However very attractive features as the rather simple preparation of intense neutral He beams by He doping of the standard neutral heating beams and the considerably deeper plasma penetration in comparison to Li and Na-BES suggest further continuation of these studies
For the second example we have recently explored [34] the influence of particle induced electron emission (PIEE lsquoparticlersquo designates ions as well as electrons) from a material plasma boundary on the voltage drop across the adjacent plasma sheath Such a situation is of interest for the plasma region near a tokamak divertor plate A new model involving a collisional kinetic sheath consistently coupled to a fluid presheath has been developed which is applicable for plasmas with a Debye length much smaller than the relevant characteristic presheath length (lsquoasymptotic two-scale limitrsquo cf figure 6a) Majority and impurity ions are accelerated by the sheath voltage from the bulk plasma and can release electrons from the material surface which flow back into the bulk plasma (see figure 6b for characteristic PIEE yields for Cq+ ion impact on different graphite surfaces) Moreover fast plasma electrons from the plasma Maxwellian tail can reach the surface to release secondary electrons and these fast electrons will also be partially reflected In a first approach we have neglected the tokamak-generic inclined magnetic field and also the fast electron reflection Under these simplifying assumptions the reduction of the plasma sheath voltage has been self-consistently calculated and was found to reach up to 30 of the sheath voltage for a case with PIEE not taken into account More realistic results will be obtained after adaptation of our model for an inclined magnetic field and with electron reflection
5 Summary and conclusions In this survey we have attempted to elucidate the beneficial role of plasma impurities (both in low and high charge states) for magnetically confined fusion plasmas in particular with respect to ITER European fusion research has taken a strong lead in this field by setting up two task forces on plasma-wall interaction and on integrated plasma modeling In both areas there is a strong need for more complete and more precise data on collisions of HCI (low-Z He Be B C O Ne medium-Z Ar Fe etc high-Z W) with hydrogen atoms and molecules and also with surfaces (Be graphite stainless steel W) The most important role of highly charged plasma impurities is to be seen in the quest of advanced tokamak plasma studies and related plasma modeling for the optimal design of future thermonuclear fusion reactors based on the tokamak configuration
38
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120
(e- io
n)
velocity (10 4 ms)
Cq+ Graphite
C4+CL
C4+HOPG
C2+HOPG
C+HOPG
C5+CL
C2+CL
Figure 6 (a) Schematic layout of new plasma sheath model with PIEE by ions and electrons flowing from a plasma (right side) toward a conducting material wall (left side) (b) Total electron yields for electron emission induced by singly and multiply charged carbon ions from surfaces of carbon fiber-enforced (CFC-CL) and highly oriented pyrolytic graphite (HOPG) respectively (data from [35])
Acknowledgements The author thanks Prof Friedrich Aumayr (TU Wien) and Drs Josef Schweinzer and Elisabeth Wolfrum (IPP Garching) for longstanding cooperation in research activities described in this paper This work has been supported by the European Commission under the contract of Association EURATOM-OEAW and was carried out within the framework of the European Fusion Development Agreement (EFDA) The views and opinions expressed therein do not necessarily reflect those of the European Commission
References [1] Janev R K 1995 Atomic and Molecular Processes in Fusion Edge Plasma[2] Atomic and Plasma-Material Interaction Data for Fusion vol 1 - 12 IAEA Vienna 1991 - 2003 [3] Crandall D H 1983 An overview of the symposium Status and direction of current research
with highly charged ions Physica Scripta T3 249 - 253 [4] Janev R K 1989 Atomic collisions in fusion plasmas J de Physique 50 C1-421 - 443 [5] Fawcett B C 1991 Role of highly-charged ions in astrophysical and laboratory plasmas
ZPhysD 21 S1 - 5 [6] Donets E D Ilyushenko V I and V Alpert 1969 Ultrahigh vacuum electron beam source of
highly stripped ions Proc 1 Conf Int sur les Sources dIons Saclay France 635 - 642 [7] Bliman S et al A high intensity ECR stripped ion source IEEE TransNuclSci 1971 NS-19
200 - 203 [8] Marrs R E Beiersdorfer P and Schneider D 1994 The electron-beam ion trap Physics Today
October 27 - 34 [9] Arnau A et al 1997 Interaction of slow multicharged ions with solid surfaces Surface Science
Reports 27 113 - 240 [10] Schenkel T et al 1999 Interaction of slow very highly charged ions with surfaces
ProgrSurfSci 61 23 - 84 [11] Aumayr F and Winter HP 2004 Potential sputtering PhilTransRSocLond 362 77 - 102 [12] Stoumlhlker T et al 2003 Test of strong-field QED in The physics of multiply and highly charged
ions (ed F J Currell) vol 1 chapter 11
39
[13] Kluge H-J 2006 Trapped and cooled Atomic physics experiments with highly charged ions in Penning traps (these proceedings)
[14] Braams C M and Stott P E 2002 Nuclear Fusion - half a century of magnetic confinement fusion research IoP Publ Bristol
[15] Kaw P K 2005 Status report on controlled thermonuclear fusion Nucl Fusion 45 A 1 - 28 [16] Lawson J D 1955 Some criteria for a useful thermonuclear reactor AERE GPR 1807 [17] wwwjetefdaorg[18] wwwiterorg[19] Philipps V Roth J and Loarte A 2003 Key issues in plasma-wall interactions for ITER a
European approach Plasma PhysControl Fusion 45 A17 - 30 [20] Samm U 2005 Plasma-Wall Interaction Status and Data Needs in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [21] Loarte A 2005 Energy deposition from ELMS in fusion devices in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [22] Garbet X et al 2004 Physics of transport in tokamaks Plasma PhysControl Fusion 46 B557 -
574[23] Challis C D 2004 The use of internal transport barriers in tokamak plasmas Plasma
PhysControl Fusion 46 B23 - 40 [24] Greenfield C M et al 2004 Advanced tokamak research in DIII-D Plasma PhysControl Fusion
46 B213 - 233 [25] Schorn R P et al 1992 Radial temperature distribution of C6+ ions in the TEXTOR edge plasma
[26] Wolfrum E et al 1993 Fast lithium-beam spectroscopy of tokamak edge plasmas RevSciInstrum 64 2285 - 2292
[27] Brandenburg R et al 1999 Fast lithium beam edge plasma spectroscopy at IPP Garching ndash Status and recent development Fusion technology 36 289 ndash 295
[28] Schweinzer J et al 1999 Database for inelastic collisions of lithium atoms with electrons protons and multiply charged ions At Data NuclData Tables 72 239 - 273
[29] Zoletnik S et al 1998 Determination of electron density fluctuation correlation functions via beam emission spectroscopy Plasma PhysControl Fusion 40 1399 - 1416
[30] Falter H D et al 2000 Helium doped hydrogen or deuterium beam as cost effective and simple tool for plasma spectroscopy RevSciInstrum 71 3723 - 3727
[31] Menhart S et al 2001 Explorative studies for the development of fast He beam plasma diagnostics JournNuclMater 290-293 673 - 677
[32] Galutschek E et al 2003 Development of fast helium beam emission spectroscopy for tokamak plasma density and temperature diagnostics Proc contr papers to 30 EPS Conf on Contr Fusion and Plasma Phys (StPetersburg 7 ndash 11 July 2003)
[33] Proschek M 2001 Towards fast He beam edge plasma diagnostics PhD thesis TU Wien [34] Schupfer N et al 2006 Effect of particle-induced electron emission (PIEE) on the plasma sheath
voltage Plasma PhysControl Fusion 48 1093 - 1103 [35] Cernusca S et al 2003 Edge plasma-relevant ion-surface collision processes Int J Mass
Spectrom 223-224 21 - 36
40
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120
(e- io
n)
velocity (10 4 ms)
Cq+ Graphite
C4+CL
C4+HOPG
C2+HOPG
C+HOPG
C5+CL
C2+CL
Figure 6 (a) Schematic layout of new plasma sheath model with PIEE by ions and electrons flowing from a plasma (right side) toward a conducting material wall (left side) (b) Total electron yields for electron emission induced by singly and multiply charged carbon ions from surfaces of carbon fiber-enforced (CFC-CL) and highly oriented pyrolytic graphite (HOPG) respectively (data from [35])
Acknowledgements The author thanks Prof Friedrich Aumayr (TU Wien) and Drs Josef Schweinzer and Elisabeth Wolfrum (IPP Garching) for longstanding cooperation in research activities described in this paper This work has been supported by the European Commission under the contract of Association EURATOM-OEAW and was carried out within the framework of the European Fusion Development Agreement (EFDA) The views and opinions expressed therein do not necessarily reflect those of the European Commission
References [1] Janev R K 1995 Atomic and Molecular Processes in Fusion Edge Plasma[2] Atomic and Plasma-Material Interaction Data for Fusion vol 1 - 12 IAEA Vienna 1991 - 2003 [3] Crandall D H 1983 An overview of the symposium Status and direction of current research
with highly charged ions Physica Scripta T3 249 - 253 [4] Janev R K 1989 Atomic collisions in fusion plasmas J de Physique 50 C1-421 - 443 [5] Fawcett B C 1991 Role of highly-charged ions in astrophysical and laboratory plasmas
ZPhysD 21 S1 - 5 [6] Donets E D Ilyushenko V I and V Alpert 1969 Ultrahigh vacuum electron beam source of
highly stripped ions Proc 1 Conf Int sur les Sources dIons Saclay France 635 - 642 [7] Bliman S et al A high intensity ECR stripped ion source IEEE TransNuclSci 1971 NS-19
200 - 203 [8] Marrs R E Beiersdorfer P and Schneider D 1994 The electron-beam ion trap Physics Today
October 27 - 34 [9] Arnau A et al 1997 Interaction of slow multicharged ions with solid surfaces Surface Science
Reports 27 113 - 240 [10] Schenkel T et al 1999 Interaction of slow very highly charged ions with surfaces
ProgrSurfSci 61 23 - 84 [11] Aumayr F and Winter HP 2004 Potential sputtering PhilTransRSocLond 362 77 - 102 [12] Stoumlhlker T et al 2003 Test of strong-field QED in The physics of multiply and highly charged
ions (ed F J Currell) vol 1 chapter 11
39
[13] Kluge H-J 2006 Trapped and cooled Atomic physics experiments with highly charged ions in Penning traps (these proceedings)
[14] Braams C M and Stott P E 2002 Nuclear Fusion - half a century of magnetic confinement fusion research IoP Publ Bristol
[15] Kaw P K 2005 Status report on controlled thermonuclear fusion Nucl Fusion 45 A 1 - 28 [16] Lawson J D 1955 Some criteria for a useful thermonuclear reactor AERE GPR 1807 [17] wwwjetefdaorg[18] wwwiterorg[19] Philipps V Roth J and Loarte A 2003 Key issues in plasma-wall interactions for ITER a
European approach Plasma PhysControl Fusion 45 A17 - 30 [20] Samm U 2005 Plasma-Wall Interaction Status and Data Needs in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [21] Loarte A 2005 Energy deposition from ELMS in fusion devices in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [22] Garbet X et al 2004 Physics of transport in tokamaks Plasma PhysControl Fusion 46 B557 -
574[23] Challis C D 2004 The use of internal transport barriers in tokamak plasmas Plasma
PhysControl Fusion 46 B23 - 40 [24] Greenfield C M et al 2004 Advanced tokamak research in DIII-D Plasma PhysControl Fusion
46 B213 - 233 [25] Schorn R P et al 1992 Radial temperature distribution of C6+ ions in the TEXTOR edge plasma
[26] Wolfrum E et al 1993 Fast lithium-beam spectroscopy of tokamak edge plasmas RevSciInstrum 64 2285 - 2292
[27] Brandenburg R et al 1999 Fast lithium beam edge plasma spectroscopy at IPP Garching ndash Status and recent development Fusion technology 36 289 ndash 295
[28] Schweinzer J et al 1999 Database for inelastic collisions of lithium atoms with electrons protons and multiply charged ions At Data NuclData Tables 72 239 - 273
[29] Zoletnik S et al 1998 Determination of electron density fluctuation correlation functions via beam emission spectroscopy Plasma PhysControl Fusion 40 1399 - 1416
[30] Falter H D et al 2000 Helium doped hydrogen or deuterium beam as cost effective and simple tool for plasma spectroscopy RevSciInstrum 71 3723 - 3727
[31] Menhart S et al 2001 Explorative studies for the development of fast He beam plasma diagnostics JournNuclMater 290-293 673 - 677
[32] Galutschek E et al 2003 Development of fast helium beam emission spectroscopy for tokamak plasma density and temperature diagnostics Proc contr papers to 30 EPS Conf on Contr Fusion and Plasma Phys (StPetersburg 7 ndash 11 July 2003)
[33] Proschek M 2001 Towards fast He beam edge plasma diagnostics PhD thesis TU Wien [34] Schupfer N et al 2006 Effect of particle-induced electron emission (PIEE) on the plasma sheath
voltage Plasma PhysControl Fusion 48 1093 - 1103 [35] Cernusca S et al 2003 Edge plasma-relevant ion-surface collision processes Int J Mass
Spectrom 223-224 21 - 36
40
[13] Kluge H-J 2006 Trapped and cooled Atomic physics experiments with highly charged ions in Penning traps (these proceedings)
[14] Braams C M and Stott P E 2002 Nuclear Fusion - half a century of magnetic confinement fusion research IoP Publ Bristol
[15] Kaw P K 2005 Status report on controlled thermonuclear fusion Nucl Fusion 45 A 1 - 28 [16] Lawson J D 1955 Some criteria for a useful thermonuclear reactor AERE GPR 1807 [17] wwwjetefdaorg[18] wwwiterorg[19] Philipps V Roth J and Loarte A 2003 Key issues in plasma-wall interactions for ITER a
European approach Plasma PhysControl Fusion 45 A17 - 30 [20] Samm U 2005 Plasma-Wall Interaction Status and Data Needs in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [21] Loarte A 2005 Energy deposition from ELMS in fusion devices in Nuclear Fusion Research-
Understanding Plasma-Surface Interaction (eds R E H Clark and D H Reiter) [22] Garbet X et al 2004 Physics of transport in tokamaks Plasma PhysControl Fusion 46 B557 -
574[23] Challis C D 2004 The use of internal transport barriers in tokamak plasmas Plasma
PhysControl Fusion 46 B23 - 40 [24] Greenfield C M et al 2004 Advanced tokamak research in DIII-D Plasma PhysControl Fusion
46 B213 - 233 [25] Schorn R P et al 1992 Radial temperature distribution of C6+ ions in the TEXTOR edge plasma
[26] Wolfrum E et al 1993 Fast lithium-beam spectroscopy of tokamak edge plasmas RevSciInstrum 64 2285 - 2292
[27] Brandenburg R et al 1999 Fast lithium beam edge plasma spectroscopy at IPP Garching ndash Status and recent development Fusion technology 36 289 ndash 295
[28] Schweinzer J et al 1999 Database for inelastic collisions of lithium atoms with electrons protons and multiply charged ions At Data NuclData Tables 72 239 - 273
[29] Zoletnik S et al 1998 Determination of electron density fluctuation correlation functions via beam emission spectroscopy Plasma PhysControl Fusion 40 1399 - 1416
[30] Falter H D et al 2000 Helium doped hydrogen or deuterium beam as cost effective and simple tool for plasma spectroscopy RevSciInstrum 71 3723 - 3727
[31] Menhart S et al 2001 Explorative studies for the development of fast He beam plasma diagnostics JournNuclMater 290-293 673 - 677
[32] Galutschek E et al 2003 Development of fast helium beam emission spectroscopy for tokamak plasma density and temperature diagnostics Proc contr papers to 30 EPS Conf on Contr Fusion and Plasma Phys (StPetersburg 7 ndash 11 July 2003)
[33] Proschek M 2001 Towards fast He beam edge plasma diagnostics PhD thesis TU Wien [34] Schupfer N et al 2006 Effect of particle-induced electron emission (PIEE) on the plasma sheath
voltage Plasma PhysControl Fusion 48 1093 - 1103 [35] Cernusca S et al 2003 Edge plasma-relevant ion-surface collision processes Int J Mass
