Initial results from the Tokapole-II poloidal divertor device This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1979 Nucl. Fusion 19 1509 (http://iopscience.iop.org/0029-5515/19/11/010) Download details: IP Address: 128.104.165.254 The article was downloaded on 07/02/2011 at 21:29 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience
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Initial results from the Tokapole-II poloidal divertor device
This article has been downloaded from IOPscience. Please scroll down to see the full text article.
1979 Nucl. Fusion 19 1509
(http://iopscience.iop.org/0029-5515/19/11/010)
Download details:
IP Address: 128.104.165.254
The article was downloaded on 07/02/2011 at 21:29
Please note that terms and conditions apply.
View the table of contents for this issue, or go to the journal homepage for more
Home Search Collections Journals About Contact us My IOPscience
INITIAL RESULTS FROM THE TOKAPOLE-IIPOLOIDAL DIVERTOR DEVICE
A.P. BIDDLE, R.N. DEXTER, RJ. GROEBNER, D.J. HOLLY,B. LIPSCHULTZ, M.W. PH4LLIPS, S.C. PRAGER, J.C. SPROTTUniversity of Wisconsin, Madison, Wisconsin,United States ef America
ABSTRACT. The.latest in a series of internal-ring devices, called Tokapole II, has recently begun operationat the University of Wisconsin. Its purpose is to permit the study of the production and confinement of hot,dense plasmas in either a toroidal-octupole (with or without toroidal field) or a tokamak with a four-nodepoloidal divertor. The characteristics of the device and the results of its initial operation are described here.Quantitative measurements of Impurity concentration and radiated power have been made. Poloidal divertorequilibria of square and dee shapes have been produced, and an axisymmetric instability has been observedwith the inverse dee. Electron cyclotron resonance heating is used to initiate the breakdown near the axis andto control the initial influx of impurities. A 2-MW RF source at the second harmonic of the ion cyclotronfrequency is available and has been used to double the ion temperature when operated at low power with anunoptimized antenna. Initial results of operation as a pure octupole with poloidal Ohmic heating suggest atokamak-like scaling of density (n <* Bp) and confinement time (T.<* n).
1. INTRODUCTION
Internal-ring devices (multipofes, spherators,levitrons, and surmacs) have a long and impressivehistory of contributions to the understanding ofmagnetically-confined, toroidal plasmas. Most of theexperiments have used relatively cold (few eV),low-density (109 — 1012 cm'3) plasmas, not out ofnecessity, however, but rather as an experimentalconvenience. With the progress in heating andimpurity control in tokamaks and the renewed interestin internal-ring devices as advanced fuel reactors [ 1 ]and as poloidal divertors for tokamaks, it seemedtimely to construct a new internal-ring devicespecifically to produce and confine hot (few hundredeV), dense (1013 cm"3) plasmas. The device, calledTokapole II, is basically an octupole with a relativelystrong (up to 10 kG) toroidal field. By controlling theshape and timing of the octupole and toroidal fields,the device can be operated as an octupole (with orwithout toroidal field), or as a tokamak with a four-node poloidal divertor. Poloidal divertor configura-tions with square or dee-shaped cross-sections can beproduced by adjusting the positions of the internalrings. Plasma in the divertor region can be retainedor suppressed by use of a retractable scrape-off plate.The octupole case can be studied with or withoutOhmic heating. The versatility of the device allows a
direct comparison of a wide variety of magneticconfigurations in a device of the same size, fieldstrength, wall cleanliness, etc. This paper willdescribe the device and initial measurements of theplasma characteristics, with particular emphasis on itsoperation as a tokamak with a poloidal divertor.Studies of electron cyclotron resonance breakdownand high-power RF heating at the second harmonicof the ion cyclotron frequency will be discussed.Initial results of density and energy confinementtime scaling for the pure octupole with poloidalOhmic heating will also be given.
2. DEVICE DESCRIPTION
The Tokapole II device consists of a 50-cm majorradius, 44 X 44 cm square cross-section, 3 cm thick,aluminium, toroidal vacuum vessel which is linked byan 0.15-V • s iron transformer core as shown in Fig. 1.The vacuum chamber walls have a field penetrationtime of about 15 ms, thus ensuring a high degree ofmagnetic symmetry for the field pulse lengths used.This magnetic symmetry was preserved by machiningthe vacuum tank to a high precision (0.5 mm) andpaying careful attention to the placement of thetoroidal and poloidal field primary windings in thevicinity of the toroidal and poloidal voltage gaps.
NUCLEAR FUSION, Vol.9, No. l l (1979) 1509
BIDDLE et al.
PrimoryWindings
ContinuityWinding
FIG. 1. The Tokapole II device.
A total of 42 ports was provided to facilitate diagnosticaccess, and the ports were arranged wherever possiblein pairs on the top and bottom so as to preservesymmetry about the horizontal midplane. The per-turbations caused by the larger (19-cm-diameter)pump ports are minimized by the use of 55% trans-parent copper plugs which have the same averageresistivity as the aluminium walls.
The octupole field is produced by four,5-cm-diameter, solid-copper rings which encircle theiron core and can carry a total peak current of up to700 kA. The rings were machined to a high tolerance(0.13 mm) and polished. The rings are positionedcarefully near the corners of the vacuum vessel so thatthe three field nulls coalesce into a single octupolenull near the minor axis as shown in Fig.2.
Experiments on an earlier device [2], Tokapole I,which was operated as an octupole (four rings), aquadrupole (two rings), and a tokamak (no rings)indicated the difficulty of driving toroidal plasmacurrents in a low-order field null, as shown in Fig.3.Additionally, it is interesting to note that the toroidalcurrent is proportional to voltage in the tokamak casebut proportional to the square of the voltage in theOhmically heated multipoles.
Each ring is supported by three beryllium-copperrods which are threaded to allow a vertical adjustmentof ± 5 mm of each ring position. The total supportarea of 133 cm2 limits the confinement time of100-eV ions in the divertor region to about 1.5 ms.Retractable divertor scrape-off plates with a total areaof 535 cm2 (each side) are also available. The rings
are electrically insulated from the vacuum chamberand can be biased up to 10 kV. A 90-kJ capacitorbank, which can produce up to 125 V across thepoloidal gap, is used to excite the poloidal field.
The toroidal field winding consists of 96 turns ofAWG 4/0 welding cable (1.17 cm diameter) and issecured to the vacuum chamber by 24 sets ofaluminium channels as shown in Fig. 1. The mechanical
TO AXIS
FIG.2. Poloidal magnetic flux plot in the absence of plasmacurrents.
2
I 10 * 100SINGLE TURN VOLTAGE AT SEPARATRIX
FIG. 3. Toroidal current as a function of loop voltage forTokapole I with four rings (octupole), two rings (quadrupole),and no rings (tokamak).
1510 NUCLEAR FUSION, Vol.9, No.H (1979)
TOKAPOLE II
TABLE I. SUMMARY OF DEVICECHARACTERISTICS
Major radius: 50 cm
Minor cross-section: 44 X 44 cm2
Toroid walls: aluminium, 3 cm thick with poloidal andtoroidal insulated gaps
Vacuum volume: 600 litres
Vacuum surface area: 6 m2
Number of internal rings: 4 (copper, 5 cm diameter, supportedat 3 points)
Ports: 2 - 19 cm diam., 5 - 11.5 cm diam., 22 - 4 cm diam.,13 — 0.6 cm diam.
BT on axis: 3.7 kG (typical), 10 kG (maximum)
L/RtimeofBT: 20 ms
Ohmic voltage: 45 V (typical), 125 V (maximum)
Poloidal flux: 0.06 W (typical), 0.15 W (maximum)
Available energy (poloidal + toroidal fields): 505 kJ(5 kV capacitors)
A more detailed description of the device isavailable elsewhere [3]. Table I summarizes the maincharacteristics of the device.
strength is sufficient to permit toroidal fields of up to10 kG on axis, but the existing 415-kJ capacitor banklimits the field to 9 kG. A peak field on axis of~ 3.7 kG was used for most of the experimentsdescribed here. Both the toroidal and poloidal fieldscan be crowbarred with a decay time of ~ 20 ms.
A base vacuum of ~ 3 X 10"8 torr is achieved witha 1500-litre-s"1 turbomolecular pump and a 1200-litre s'1
10 K cryopump. All vacuum seals are viton, but careis taken to ensure that the only insulators exposed tothe plasma are ceramic. Provisions were incorporatedfor baking the entire machine to 150°C, but so far thebake has been limited to 70°C. A residual gasanalyser typically shows water to be the dominantimpurity. Discharge cleaning is accomplished bypulsing the toroidal field to about 1 kG every twoseconds and ringing the poloidal gap with a decayingsine wave of about 50 V and a frequency of about3 kHz.
3. DIAGNOSTICS
Since the internal rings and plasma both link theOhmic-heating transformer, the measurements of theplasma loop voltage and current are not straight-forward. Spatial toroidal electric field profiles aredirectly obtained by measuring the time-varyingpoloidal magnetic flux within toroidal loops.Axisymmetry enables this to be done with a smallrectangular loop of wire (0.3 cm width in toroidaldirection) inserted along the midcylinder to the givenradius. Spatial profiles of the toroidal current densityare obtained with a small Rogowskii coil (3 cmdiameter) or inferred from experimentally measuredmagnetic-flux plots. A large square cross-sectionRogowskii coil can be inserted into the plasma tomeasure the plasma current flowing in the centralchannel (inside the separatrix). When in place, thefour corners of this coil nearly touch the internalrings. Also, an appropriate circuit model, includingthe plasma (assumed localized near the minor axis)and the rings, allows us to infer the plasma currentby measurement of the poloidal gap voltage and theOhmic-heating primary current. Electron density ismeasured with Langmuir probes and a 40 GHz phase-shift microwave interferometer. The interferometersignal is cut off at the maximum density.
Spectroscopic diagnostics include a 1/2-mJarrell-Ash monochromator, a 1/2-m Seya-Namiokavacuum ultraviolet (VUV) spectrometer, a 1-m SeyaVUV monochromator, and a variety of filters anddetectors. The 1/2-m Seya is equipped with aphosphor-coated microchannel plate so that we canobtain a panoramic view of the spectral region from400— 1300 A in one shot. This output can bephotographed or displayed on a television screen bya gated storage vidicon. The 1-m Seya has beenabsolutely calibrated and is used to measure impurity-line-radiated power. An absolute calibration for the1/2-m Seya has been inferred from power measure-ments with the 1-m instrument.
Ion temperature (Tj) is determined from theDoppler broadening of visible lines of impurity ionsand from the He II 4686 A line. Electron temperature(Te) is determined with the aid of a computer code
NUCLEAR FUSION, Vol.9, No. 11 (1979) 1511
BIDDLE et al.
5 0 -
30
20
10
50-
40
< 30
20
10
- _v-
-
G
10k WECRH
a
———
\
BT —- — —
0 I 2Time (ms)
- 1 0
150
100
50
I 2 3 4Time (ms)
FIG. 4. Time sequence of typical discharge.
discharge. Since Tj is obtained from low-Z impurityions, this technique only approximates the centralion temperature. The peak, spatially averaged,electron density is about 1 X 1013 cm"3.
Spatial-electric-field and current density data showthat after the discharge is well formed, the Ohmicinput power to the central channel is 100 - 150 kWwhile the input power to the entire plasma is200-300 kW. With the assumption that the electrondensity profile is a cosine, the maximum energyconfinement time of the central channel is250— 300 jus and that of the full plasma is100- 150 us. The Hugill-Sheffield empirical scalinglaw for energy confinement time [5 ] predictsT£ ~ 150 JUS for the central current channel. (Theminor radius is taken to be 8 cm, the distance fromthe magnetic axis to the separatrix.) Confinementin the Tokapole configuration is evidently comparableto that of circular cross-section tokamaks.
which models the observed ionization state sequenceof O III through O VI. This technique has beendescribed elsewhere [4].
4. DISCHARGE CHARACTERISTICS
Figure 4a indicates the timing of the fields andpre-ionization for a typical tokamak discharge. A fastpiezoelectric valve is used to fill the machine withabout 3 X 10'4 torr of hydrogen. About 12.7 mslater, the toroidal field is triggered and is crowbarredwhen it reaches its peak field strength. Shortlybefore peak field, a 1-ms pulse of 10-kW, 8.8-GHzmicrowaves is used as pre-ionization. The Ohmic-heating voltage, which is sinusoidal in time with aquarter-period of 2.8 ms, is triggered during the pre-ionization pulse. The Ohmic-heating primary currentcan be crowbarred at its peak value.
Figure 4b displays a plasma current (Ip) trace. Thisyields a peak current of about 40 kA, while theRogowskii loop gives a peak Ip of about 20 kA, whichindicates that about half of the total current flowsinside the separatrix. The line-averaged Te, as deter-mined by the method described above, is also shownin Fig.4b. Te is low during the formation of thecentral current channel and rises to 100 eV or moreonce the current channel has been formed. Thisspectroscopic measurement of Te agrees with theconductivity Te, if Zeff = 3. The ion temperature isabout 15 eV and is relatively constant during the
5. IMPURITY MEASUREMENTS
In an effort to understand the energy lossmechanisms, we have made qualitative and quantita-tive impurity line radiation measurements. Most ofthe radiation has been shown to arise from oxygen,carbon, and nitrogen (associated with vacuum leaks)with lesser amounts from copper and aluminium.Typical impurity concentrations are shown in Table II.
TABLE II. TYPICAL IMPURITYCONCENTRATIONS
Impurity Density
Oxygen
Nitrogen
Carbon
3 X l 0 n c m ' 3
2X1011 cm"3
5Xl010cm"3
The densities of the metal impurities have not beendetermined. From the spectroscopic impuritydensity measurements, Zeff is estimated to be- 2 . 5 - 3 . 5 .
The radiated power from the full plasma in the4 0 0 - 1300 A region is 35 - 70 kW. Measurementswith VUV-transmitting broadband filters and anappropriate detector show that the power radiated at
1512 NUCLEAR FUSION, Vo!.9, No.ll-(1979)
TOKAPOLE II
shorter wavelengths is insignificant. With the exceptionof C IV, no known important resonance lines lie atwavelengths longer than 1300 A. Thus, the dataindicate that line radiation is not more than about15 - 30% of the Ohmic input power and that lineradiation is not the dominant energy loss mechanism.
Spatial observations show that there are markedpoloidal asymmetries in the emission of impurityradiation. In particular, low-ionization states ofimpurities often radiate intensely near one or moreof the internal rings. Evidently, the rings are animportant impurity source, but it is not yet clearwhether or not they are the primary impurity source.It is likely that impurities diffusing from the ringstend to form shells in the vicinity of the separatrix.In some cases, chordal observations of the soft-X-rayradiation (40 - 210 A) have been consistent with theassumption that the highly stripped impurities arelocalized in a shell near the separatrix.
6. EQUILIBRIUM AND STABILITY OF APOLOIDAL DIVERTOR CONFIGURATION
Divertor configurations are becoming moreimportant as solutions to the impurity problems nowbesetting tokamaks. One such configuration is thepoloidal divertor where poloidal flux is divertedaxisymmetrically and plasma outside the separatrixis scraped off. The poloidal divertor configurationwhich is by nature non-circular is also advantageouswith respect to q-limited and (3-limited MHD-modes.However, the non-circular nature of this configurationmakes it unstable to axisymmetric displacements(circular plasmas are neutrally stable). The importanceof these axisymmetric modes is apparent and hasgiven rise to a fairly large amount of linear theoryusing the energy principle — mostly for idealizeddisplacements of ideal analytic equilibria and fornumerically calculated equilibria using Princeton'sstability code (PEST) [6 - 8]. Recently, the non-linear evolution of the instability has been followedby Jardin [9] by the numerical integration of thetwo-dimensional, axisymmetric, time-dependent,ideal-MHD equation in toroidal geometry.Axisymmetric displacements of dee [10, 11] andelliptical [12, 13] plasmas have been deduced in a fewprevious experiments from magnetic probes externalto the plasma. Plasma shapes and positions have beeninferred from equilibrium computer codes usingexternal experimental signals as input.
We have made the first direct experimentalobservation of the stability to axisymmetric modes ofsquare and inverse-dee shaped equilibria in a 4-nullpoloidal-divertor configuration [ 14]. Equilibrium isverified by mapping the flux plot as a function oftime, and its stability is determined by studying theevolution of these flux plots. We have found thatinverse-dee- and square-shaped equilibria exist and thatinverse-dee-shaped equilibria are more unstable thanthe square equilibria. Growth times for the inversedee are ~ 1000 poloidal Alfven times implying thatpassive stabilization due to the rings and walls has occurred.
As part of the programme to study the equilibriumand stability of poloidal-divertor configurations, anaxisymmetric magnetohydrodynamic equilibrium codewas written to handle internal ring devices withcurrent-carrying plasmas such as the Tokapole II.This code solves the Grad-Shafranov equation for thepoloidal flux, i/>, given pressure and poloidal currentas functions of \p and subject to the proper boundaryconditions outside the plasma. The plasma is assumedto be confined in the central region of the torus in adivertor configuration where the boundary of theplasma is one of the separatrices. Since the plasma/vacuum interface must be solved as part of theproblem, this is a free-boundary-type of problem.Since these problems are non-linear, it is necessary touse iteration techniques to find the solution. Thecurrent in the rings and the plasma is inducedinductively so the proper representation of the internalrings is to take their surfaces to be Dirichlet boundaries.Hence, the currents in the rings will be coupled to theplasma providing some degree of wall stabilization.
A sample calculation for one of the standard ringconfigurations and for typical Tokapole II parametersis shown in Fig. 5. The general tendency of the plasmais to lean against the outside rings with the resultingimage currents (in the rings and wall) providing avertical field to keep the plasma centrally located.
Since internal probes may be used as a diagnostic,Tokapole II provides the opportunity to verify firsthand the shape and subsequent development of itsvarious equilibrium configurations. Figure 6 shows onesuch experimentally determined flux plot for roughlythe same parameters and ring placement as thesample computer calculation. The values of poloidalflux in this plot were found by integrating measure-ments of the poloidal magnetic field taken on a2-cm grid. As can be seen, the agreement is fairlygood. Preliminary results have shown that when theplasma outside the separatrix is scraped off, theaxisymmetric stability of the equilibrium is unaffected.
NUCLIiAR FUSION, Vol.9, No. l l (1979) 1513
BIDDLE et aL
FIG.5. Numerically calculated poloidal flux plot with aplasma current.
FIG. 6. Experimentally measured poloidal flux plot in theregion near the magnetic axis.
7. ELECTRON CYCLOTRON RESONANCEBREAKDOWN
Ten kW of 8.8 GHz ECRH is applied before theOhmic heating begins, as shown in Fig.4. Since at thistime the magnetic field is purely toroidal and isroughly constant in time, the ECRH resonance zoneis a vertical cylinder. By varying the strength of thetoroidal field, the resonance zone can be positionedat different radii. By thus starting the discharge withECRH, impurity radiation can be reduced by up toabout 50% compared to an Ohmic discharge withoutECRH pre-ionization.
Langmuir probe traces (Fig.7) taken with nopoloidal (Ohmic heating) field show that ECRH resultsin a plasma profile which peaks at the resonance zoneas expected. This is also verified by visible-lightphotographs of the ECRH plasma. However, whensome Ohmic heating is added, Langmuir probemeasurements show little difference with and withoutthe pre-ionization (Fig. 8). Photographs of the plasmaalso show no change due to the pre-ionization oncethe Ohmic-heating power has been applied.
However, the impurity radiation in the VUV, whicharises principally from oxygen, carbon, and nitrogen,does show a marked decrease for the first two milli-seconds of the discharge when ECRH pre-ionizationis used. For the remainder of the discharge, theVUV radiation returned to the level observed in thecase with no pre-ionization. The effect of the ECRHpre-ionization on impurity radiation is a function ofthe position of the ECRH resonance location.
ECRH in a toroidal field.
on axis = 2.32 kG
- 2 0 -10 0 10
DISTANCE FROM MINOR AXIS (cm)
FIG. 7. Spatial profiles of ion saturation current for the ECRHpre-ionization plasma for various resonance zone positions.Arrow indicate?electron cyclotron resonance zone.
FIG.8. Spatial profiles of ion saturation current just beforeand at various times after the initiation of the Ohmic dischargewith and without ECRH pre-ionization.
T0KAP0LE II
RESONANCE ZONE LOCATION ( cm from minor axis)-20 -10 0 10 20
O 6 0 -
80 -
4 0 -
3 3 0 -
co 2 0 -Uieo
10 -
1
•
•
•
•
• •
•
1—• • •
^ LINES
•
• * <
• J
INDICATE
• •
• •
T
RING
• •
• *
f -
P0SITI0N
• •
• •
• •
1•
t
y
•
2.0 2.9 3.0 3.5
Bj on oxit (kO)
4.0
FIG.9. Percentage decrease in VUV radiation as a function ofthe ECRH resonance zone location.
Figure "9 shows the percent decrease in VUV radiationwhen ECRH pre-ionization was used, relative to thecase with no pre-ionization, as the resonance zonewas swept across the machine by varying the toroidalfield strength. The effect of the ECRH is to decreasethe impurity radiation as much as 50% compared tothe no-pre-ionization case. Surprisingly, the effect ismost pronounced when the resonance zone intersectsthe rings.
The intensity of the Cu I radiation line is also muchenhanced when the pre-ionization resonance zoneintersects the copper rings. This effect persists forthe duration of the discharge. Gross plasma para-meters, such as the plasma current, are affected muchless by the pre-ionization (usually 10% or less). Thismay be in part due to the rings producing a centralcurrent channel with a stable equilibrium, accomplish-ing the same result as the ECRH pre-ionization. Thusthe effect of ECRH pre-ionization might be moredramatic in a tokamak without a poloidal divertor.
8. ION CYCLOTRON RESONANCE HEATING
Tokapole II is providing a unique opportunityfor the study of several phases of high-power ICRFheating in a plasma which has a rectangular cross-section and nearly uniform density within the currentchannel. The 29-cm plasma cross-section is surroundedby a 7.5-cm blanket of reduced density outside themagnetic limiters. As a result, toroidal eigenmodesin the vicinity of the ion cyclotron frequency are fewenough to be unambiguously identified and mappedby RF-field probe measurements. The relativelylarge evanescent region allows detailed studies of wavecoupling when the antenna is separated from thedense core of the plasma. Because of the approxi-mately square plasma cross-section, a rectangularrather than cylindrical plasma-filled waveguide modelwas adopted. Figure 10 shows the two theoreticallyaccessible modes, and Fig. 11 shows RF probe measure-ments of the v = 1 mode, including the largeevanescent region.
NUCLEAR FUSION, Vol.19, No.11 (1979) 1515
BIDDLE et al.
FAST WAVE DISPERSION RELATIONi.o
. 9 -
.8 -
. 7 -
.6 -
. 5 -
.4 -
. 3 -
.2 -
^V n
- ^
-
-
=4
V\n
= 5
\
\
n = toroidalmode #
= 6 v- Transversemode #
density = IOlscm
, . - O
n
k\
-
= 7
V1
0 .1 .2 .3 .4 .5 -6 .7
FIG. 10. Dispersion relation for the fast magnetosonic wave atto = 2coci in Tokapole II.
a 19-cm-diameter ceramic window. For theseexperiments, one of the copper plugs must be removedfrom the large ports. As the antenna is also theinductor of the tank circuit, the oscillator frequencytends to track changes in plasma reactance, therebybroadening the modes. Operation at 2 ojci in ahydrogen plasma has doubled the ion temperature asmeasured by Doppler broadening of the 4686 A lineof doped helium (Fig. 12). This represents ~ 10%coupling to the ions, with the balance of the powertentatively identified with plasma edge and tank walldeposition. Impurity line radiation is seen to scalewith applied power. However, no disruption of theplasma has resulted from the impurity influx. In fact,a characteristic of the Tokapole II device is thatmajor disruptions which result in complete loss of theplasma do not occur for any of the plasmas whichhave been examined. This may be due to the stablemultipole region which surrounds the toroidal currentchannel.
Upgrading the launching structure to one whoseseparation from the plasma is continuously variableand increasing the toroidal field are expected todramatically increase coupling efficiencies as well asallow heating in an H-D plasma with greatly reducededge heating. The relatively low poloidal flux isexpected to limit maximum ion temperatures,however.
WAVE bz VS VERTICAL SPACING
n = IOl3cm"3
f = 10.7 MH2
BT=3,8kG
-20
i 1-16 -12 - 8 - 4 0 4 8 12 16 20
DISTANCE FROM MACHINE CENTRE (cm)
FIG. II. Measured value of the spatial variation of the RFmagnetic field showing the radial structure of the toroidaleigenmode.
These studies are being done with a 2-MW self-excited RF-source with a 1.2-ms square pulse lengthand a temporary antenna located physically outsidethe vacuum tank and which views the plasma through
,-. 30>
Id 25
20
8 l0
ION TEMPERATURE v»T I ME
A with RFO without RF
Doppler Broadeningof He I I 4686 I
\h\h\\
RF ON
2 3
TIME (ms)
FIG. 12. Ion temperature versus time with and without ICRHas determined by Doppler broadening of the He II4686 A line.
1516 NUCLEAR FUSION, Vol.19, No. l l (1979)
TOKAPOLETH
9. PURE OCTUPOLE OPERATION
Multipoles are of considerable interest as advancedfuel reactors because a stable equilibrium can beobtained without the need for a toroidal field.Furthermore, the poloidal magnetic field has a lowaverage value, and so the synchrotron radiation whichwould result at the high temperatures needed toburn the advanced fuels (several hundred keV) can bereduced to a tolerable level. However, very little isknown about the scaling of energy confinement ofhot, dense plasmas in a pure multipole configuration.The Tokapole II has provided an opportunity to extendthe extensive measurements at low temperaturesand densities into more reactor-like regimes.
To do this, the timing sequence of the magneticfields is changed somewhat from that shown in Fig.4.The discharge is initiated in a manner similar to thatused for the tokamak studies, but after a hot, denseplasma is created, the toroidal field is abruptly(~ 1 ms) reduced to zero. This abrupt decrease intoroidal field drives current poloidally and producesadditional Ohmic heating. Such poloidal Ohmicheating may have distinct advantages over the toroidalOhmic heating used in tokamaks and stellarators.The resistivity is greatly enhanced (typically ahundredfold) by the strong magnetic mirrors.Additionally, the Krushkal-Shafranov limit presumablydoes not preclude arbitrarily large values of Ohmicheating.
After the toroidal field has reached zero, one is leftwith a plasma with parameters similar to thosepreviously described but in a pure octupole field withno Ohmic heating. The subsequent decay of theplasma can then be studied. The first such experi-ments used a specially shaped Langmuir probe whichextends across the bridge region (the narrow regionbetween a ring and the wall shown in Fig.2) so as tovolume-average the plasma density.
Figure 13 shows the density measured as describedat the end of the poloidal Ohmic heating pulse as afunction of the total current in the rings (which is ameasure of the poloidal field strength). An approxi-mate linear scaling is observed (n «= Bp) whichresembles the scaling of density with toroidal fieldwhich is observed in tokamaks. The ion saturationcurrent to the probe is observed to subsequently decayin an approximately exponential manner with adecay time T which is plotted as a function of initialdensity in Fig. 14. Again, note the approximate lineardependence (r <*• n) at the higher densities which ischaracteristic of many tokamak experiments. Since
0 100 20C 300 400 500
T O T A L HOOP CURRENT ( k A )
FIG. 13. Density as a function of poloidal magnetic field forthe pure octupole with poloidal Ohmic heating.
0 1 2 3
AVERAGE DENSITY (IOl2cm"3)
FIG. 14. Confinement time versus density for the pureoctupole configuration.
the ion saturation current is proportional to n /Qthe measured r is a mean between the particle andenergy confinement times. These results suggest thata multipole with poloidal Ohmic heating enjoys thesame favourable scaling as tokamaks.
10. SUMMARY AND CONCLUSIONS
The Tokapole II device is a small, low-field tokamakwith a four-node poloidal divertor and the additionalflexibility of operating as a pure octupole. Toroidalfield and Ohmic heating can be used or not as desired.
NUCLEAR FUSION,-Vol. 19, No. l l (1979) 15F7
BIDDLE et al.
The general discharge characteristics agree favourablywith those of similar tokamaks. Spectroscopicmeasurements show that impurity line radiation cannotaccount for more than ~ 20% of the losses. Divertorconfigurations of square and inverse-dee shapes canbe produced, and the inverse dee is observed to developan axisymmetric instability in which the magneticaxis moves toward one of the divertor rings on a timescale of ~ 1000 poloidal Alfven times. The squarecase appears to be stable. One novel feature of theexperiment is the use of ECRH pre-ionization whichhelps to initiate the Ohmic discharge away from thewalls, resulting in a 50% reduction in impurityradiation. A 2-MW ICRH source is available, and hasbeen used to double the ion temperature, but withpoor coupling efficiency resulting from the lack of aproper antenna. When operated as a pure octupole,the density and confinement time obey tokamak-likescaling. Future work will include doubling thetoroidal field strength and discharge length, increasingthe ICRH power coupled to the plasma, studying thestability of the various poloidal divertor configurationsat high beta and low q, and determining the scalingand limitations of poloidal Ohmic heating.
ACKNOWLEDGEMENTS
We are grateful to K. Miller, T. Osborne,M. Sengstacke, D. Shepard, and D. Witherspoon forassistance in assembly of the apparatus and somediagnostic measurements. Engineering and technicalsupport was provided by T. Lovell and R. Vallem.This work was supported by the US Department ofEnergy.
