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R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007
A summary of some recent edge physics research on TCV and JET
R. A. Pitts
Centre de Recherches en Physique des PlasmasEcole Polytechnique Fédérale de Lausanne, Switzerland
Association EURATOM-Swiss Confederationand
Leader, Task Force E, EFDA-JET
R. A. Pitts: KFKI-RMKI, Budapest 12/04/20072
OutlineTCV
• Brief overview of the machine: first wall, heating systems, edge diagnostics
• Divertor detachment• Turbulent transport• Parallel flows• Not covered: ELMs, Infra-red investigations, SOLPS5 H-mode
modellingJET
• Retarding field energy analyser• Flows and far SOL ELM ion energies• A lot more not covered!
R. A. Pitts: KFKI-RMKI, Budapest 12/04/20073
TCVTokamak à
Configuration Variable
R. A. Pitts: KFKI-RMKI, Budapest 12/04/20074
The TCV tokamak R= 0.88m; a= 0.25m BT ≤ 1.5T; Ip ≤ 1.2MA 0.9< <2.8; -0.6< <0.9
•X2: 82.7GHz•6 0.5MW, 2s•Side launch ECH,
ECCD•ncut-off = 4.21019m-3
•X3: 118GHz•3 0.5MW, 2s•Top launch ECH•ncut-off = 11.51019m-3
R. A. Pitts: KFKI-RMKI, Budapest 12/04/20075
TCV first wall All graphite machine
• Upgrade to ~90% coverage in 1998
• First wall tiled with polycrystalline graphite (~1700 individual elements)
• Cold walls (during operation)
• Regularly boronised (~220C, Glow with 10% B2D6/90% He)
• Pulse length typically ~1.2 s
• All tiles being B4C blasted this summer
R. A. Pitts, R. Chavan, J-M. Moret, Nucl. Fus. 39 (1999) 1433
R. A. Pitts: KFKI-RMKI, Budapest 12/04/20076
TCV: configurational flexibility
16 independently powered poloidal field coils• Enormous scope for flexibility in plasma shape
• Nightmare for edge physics and PSI however!
R. A. Pitts: KFKI-RMKI, Budapest 12/04/20077
TCV: Edge diagnostics• 80 tile embdedded Langmuir
probes • IR cameras• Fast reciprocating probe
(flows and turbulence)• In-vessel pressure gauges• Fast AXUV diode cameras
R. A. Pitts: KFKI-RMKI, Budapest 12/04/20078
Divertor detachment Mandatory for successful ITER (and reactor) operation
• Without (partial) divertor detachment in the separatrix region, power fluxes will be beyond the design power handling capacity
• SOLPS5 (B2.5-Eirene) solutions show that this will be possible• But has the code been sufficiently benchmarked on today’s machines
for us to have confidence?
OS
P
ISP
ITER Divertor DDD 17 (SOLPS5 runs by A. Kukushkin)
R. A. Pitts: KFKI-RMKI, Budapest 12/04/20079
Divertor detachment on TCV Studies always made in simple
ohmic discharges• Isolate physics, obtain best
possible data
• X2 ECR heating system precludes high density L-mode operation
• Studied effect of geometry on detachment – “plasma plugging”
R. A. Pitts et al., J. Nucl. Mater., 290-293 (2001) 940R. A. Pitts et al., IAEA-CN77/EXP4/23 (2000) Time (s)
en (1019m-3)
Zeff
P
PRAD,TOT
PRAD,DIV
D,divertor
Jsat(Acm-2)ISP
OSP
(kW)
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200710
“Anomalous” detachment TCV outer divertor does not
detach like in other tokamaks • Divertor densities too low• Neutral baffling insufficient• SOLPS4 simulations (with A. Loarte)
unable to reproduce observed detachment
M. Wischmeier, Phd Thesis (EPFL: TH3176 (2005))M. Wischmeier et al.,ECA 29C P-5.013 (2005)M. Wischmeier, R. A. Pitts, in preparation for Nucl. Fusion
3 year study with SOLPS5 tracked the problem down (probably)• Strong outward convective transport
main chamber recycling increased C release increased radiation “power detachment”
Jsat(Acm-2)
Te(eV)
ne(1019m-3)
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200711
Turbulent transport Expts. on TCV some of the first
to identify profile broadening with increased plasma density • Fast RCP under midplane
• ne, Te and fluctuation driven flux
• Large database in ohmic plasmas
Broad profiles at high density increased main chamber wall interactionWhy does this happen?
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200712
Intermittency In the far SOL, density
fluctuations more bursty and rare• Almost all the radial
transport in these regions carried by the blobs
• Convect plasma quickly to the wall regions
• Competes equally with parallel transport
• Consistency with known statistical distributions discovered on TCV
J. P. Graves, J. Horacek, R. A. Pitts, Plasma Phys. Control. Fusion 47 (2005) L1
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200713
Modelling the turbulence
O. E. Garcia, J. Horacek, R. A. Pitts, et al., Plasma Phys. Control. Fus. 48 (2006) L1
2-dimensional fluid turbulence simulations – ESEL code (Risø) • Centred on outer midplane
• ne, Te and vorticity evolution
• Collective motions driven by non-uniform B-field
• Linear SOL damping terms driven by SOL transport
Model parameters set by a high density TCV case • Sample turbulent fields over
long time series by an array of trial probes
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200714
Encouraging agreement with expt.
O. E. Garcia, R. A. Pitts, J. Horacek et al., PSI 2006, Heifei & Plasma Phys. Control. Fus. 48 (2006) L1
Code matches turbulent statistics • Relative fluctuation level
• Higher moments of PDF (Skewness, Flatness)
• Detailed “structure” of blobs – sharp front and trailing wake
Conditionally averaged density
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200715
Implications for main wall fluxes
J. Horacek, O. E. Garcia, R. A. Pitts et al., IAEA EXP4/21 (2006)
Good agreement between expt. & simulation provides extremely strong evidence for interchange motions as the origin of anomalous SOL transport• Flux-gradient paradigm: = Deffn/r not adequate to describe TCV data.
• Convection: = nVeff does better across region of broad SOL profile
• TCV results show that the scaling of wall flux with density seen elsewhere is due to turbulent interchange motions
PDF of turbulent flux at wall radius
2e
turb
walln
2ewall nn
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200716
B
BxB
ErxB, pxB
Ballooning
Pfirsch-SchlüterDivertor sink
ExB
Determine transport of impurities from source to destination in a tokamak – material migration – T-retention
FWD B
SOL Flows
B
BxBREV B
Poloidal
Parallel
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200717
#26092 #27585 #27582 #27588
Studying flows on TCV
BxBxBB BxBxBB
Mach probe
Use configurational flexibility of TCV to study flows in simplest possible diverted, ohmic plasmas• Emphasis on direction of B, configuration and density (|B| = 1.43 T)
• B and Ip always reversed together to preserve helicity
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200718
FWD-B/REV-B, Ip = 260 kA, density scan
wall
Strong field direction and density dependence near outer midplane• Flows always co-current
• Direction consistent with Pfirsch-Schlüter flow
• Slight, field independent negative offset
R. A. Pitts et al., PSI 2006
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200719
Ballooning drive?
+10 cm0 cm-10 cm en = 4.2 x 1019m-3
OU
TE
R d
iverto
r
Change of M|| with location above and below plasma midplane is consistent with a ballooning drive for the field independent flow offset• Not unambiguous owing to
presence of lower divertor sink – new results this week(!!) show that is a real “ballooning” drive
BxBxBB
R. A. Pitts et al., PSI 2006
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200720
260 kA density scan in FWD/REV-B:
OU
TE
R
diverto
r
INN
ER
div
erto
r
en (1019m-3) 1.7 2.5 4.2 7.36.3
REV B
OU
TE
R
diverto
r
INN
ER
div
erto
r
en (1019m-3) 1.7 2.5 4.2 7.36.3
REV B
FWD B
Choose radial band in the main SOL:8 < r-rsep < 12 mm
Take mean exptl. M|| and plot versus density
Compare with predicted Pfirsch-Schlüter flow
2B
Ben
pE
c er
s
2qcosMPS
||
Field dependent componentO
UT
ER
d
ivertor
INN
ER
div
erto
r
OU
TE
R
diverto
r
INN
ER
div
erto
r
R. A. Pitts et al., PSI 2006
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200721
Interchange driven flows Assume radial transport by
interchange motions in outer midplane vicinity – estimate parallel flow due to transients • Time averaged Mach No.
due to transport driven flow: M|| ~ 0.5fp>p
• fp>p = t(p > p)/t
• Duration of time series, t
• t(p > p) time over which p exceeds p by factor
R. A. Pitts et al., PSI 2006W. Fundamenski, R. A. Pitts et al., accepted for publication in Nuclear Fusion
Reasonable agreement with experiment• TCV results show that parallel flows can be explained by combination of
classical (drift driven) and transport (turbulence driven) components
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200722
JET
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200723
JET DOC-L Discharges
Retarding field energy analyserRFA
Designed and built at CRPP as part of enhancement project (JW0-ED-3.7) for reciprocating probe head upgrades • Previous attemps to make
such a device function had always failed on JET
Provides radial profile of SOL Ti
• Almost never measured, especially in large tokamaks
• Can also yield plasma potential and local Mach flow
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200724
-180 V-150 V 0 V
Vs
RFA Principle
Usual application is to sweep ion retarding grid to generate I-V characteristic and extract Ti, Vsheath agreement with experiment
• Negative slit bias allows simultaneous extraction of parallel ion flux Mach flows can be measured with a bi-directional device
• Long cable lengths (> 100 m on JET!) and small signals (A) prevent fast grid sweeping, but ELMs can be measured – see later
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200725
JET DOC-L Discharges
The JET RFA
Complex design – probes on JET subject to much greater constraints than elsewhere• Bi-directional – 2 RFA cavities
looking along B
• All boron-nitride design, like all JET probes
• 30 m wide entrance slits
• 2 mm grid separation
• Theoretical ion transmission ~0.2
March 2003
40 m
m
Slit plate
Grids
Collector
They don’t always last long either• Probe drive accident on last day
of operation in Campaign C14
R. A. Pitts et al., Rev. Sci. Instr. 74 (2003) 4644
After March 2004
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200726
Excellent flow data from top LFS
Probe samples at the near zero point for Pfirsch-Schlüter flow – and yet, large flows – not understood nor reproduced by edge codes (SOLPS5, EDGE2D)• BB strong parallel flow towards inner divertor at RCP
• BB flow stagnates at RCP
• Mean flow offset towards inner divertor – consistent with transport driven flow as in TCV – verified also with ESEL on JET
S. K. Erents, R. A. Pitts et al., PPCF. 46 (2004) 1757
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200727
SOL flow confirmed by Ti data
Large change in ion-side/electron side Ti ratio with field reversal• BB: Ti,i-side/Ti,e-side > 1
• BB : Ti,i-side/Ti,e-side ~ 1
• Due to the strong perturbing effect of the probe itself
• Ions depleted on the “downstream” side strong electric fields develop ion f(v||) modified
R. A. Pitts et al., ECA Vol. 27A, P-2.84R. A. Pitts et al., J. Nucl. Mater. 337-339 (2005) 146
• JET RFA provided first ever demonstration of this theoretically expected effect – quantitative agreement with theory for FWD B
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200728
D
JET #62218
t = 19.05 s, ELM-free t = 19.06 s, Type I ELM
Time (s)
H-mode Edge MHD instabilities periodic bursts of particles and energy into the SOL. Type I ELMing H-mode is currently the baseline ITER scenario
Edge Localised modes
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200729
Far SOL ELMs on the RFA Delicate edge probes can
never be used close to separatrix in high power discharges• But measurements in the far
SOL just as important (determine wall interaction)
• Use RFA to detect the ELM transient near the limiter radius
• Use constant grid bias and catch ELM convected ions able to surmount the potental barrier (~400 V)
• Only a few measurements in H-mode Hydrogen plasmasTime (s)
dsep at probe (mm)
jslit (Acm-2)
Icoll (A)
H (outer) /1015
Vslit (V)
Vgrid1 (V)
Vgrid2 (V)
Wdia (kJ)
#63214
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200730
Individual ELMsdsep ~134 mm
H
Wdia (kJ)
jslit (Acm-2)
Icoll (A)
dsep ~86 mm dsep ~74 mm dsep ~73 mm
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200731
Far SOL ELM ion energies
Clear filaments in each ELM• Net apparent flow to
inboard side ELM enters SOL mainly on outboard side
• Multiple filaments and clear trend for lower energies in successive filaments suggest picture of ELM as a train of toroidally rotating, feld aligned structures
r - rsep ~ 80 mm at the probe
Current of ions with energy > 400 eV
R. A. Pitts et al., Nucl. Fusion 46 (2006) 82
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200732
• Solves dynamical particle and energy two-fluid equations in the ELM filament frame subject to parallel losses determined by sheath boundary conditions
• Normalisation parameter: n,0 = L||/cs
• Characteristic parallel loss time evaluated at the initial conditions of the transient
• Filament origin location
• Te, Ti, ne at ELM origin
• ELM radial speed, vELM
• Parallel connect. length, L||
Input
Output
• Te, Ti, ne in the ELM filament at
any radial distance
Compute ion collector current with simple model of RFA function compare with expt.
Modelling the ELM transient
W. Fundamenski & R. A. Pitts PPCF 48 (2006) 109
New transient model of ELM parallel losses
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200733
Model consistent with RFA data Good agreement with i-side
fluxes• Assume ELM starts anywhere
from pedestal top to separatrix but with “mid-pedestal” Ti, Te, n
• Semi-adiabatic broadening
• vrELM = 600 ms-1 (from previous
JET scaling)
• Predicts Ti,RFA/Ti,ped = 0.30.5
• Te,RFA/Te,ped = 0.130.25
• ne,RFA/ne,ped = 0.30.4
Filament cools faster than it dilutes, electrons cool more rapidly than ions
W. Fundamenski & R. A. Pitts PPCF 48 (2006) 109
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200734
ELMs can interact strongly with main chamber surfaces
New KL8 visible (Photron) camera at JET picks up ELM induced main wall interactions during recent dedicated ELM ablation session (1 MJ ELMs!)
Images courtesy of CIEMAT team and C. Silva
R. A. Pitts: KFKI-RMKI, Budapest 12/04/200735
~260 eV
~600 eV
~1100 eV
Model prediction for ITER
Model implies significant ELM wall erosion in ITER and beyond, even for high Z-wall
ELM starts mid-pedestal
D+ W: 0.5% yield(1% for T+)
D+ W threshold(209 eV D+, 136 eV T+)
Confirmed experimentally on JET (RFA)
Ion impact energy = 3Te + 2Ti R. A. Pitts et al., PPCF 47 (2005) B303W. Fundamenski & R. A. Pitts PSI 2006