The ISX-A graphite limiter experiment

Journal of Nuclear Materials 85 & 86 (1979) 215-219 0 North-Holland Publishing Company

THE ISX-A GRAPHITE LIMITER EXPERIMENT* and 3. E. Simpkins R. A. Langley, R. J. Colchin, R. 6. Isler, M. Murskatni, Oak Ridge National Laboratory,Oak Ridge, Tennessee 31830, USA AND J. I,.Cecchi, V. L. Corso, H. F. Dylla, R. A. Ellis, Jr., and M. Nishi+ Plasma Physics Laboratory,Princeton University,Princeton,New jersey 08540, USA

ATJS graphite rail limiters were installed in the ISX tokamsk in order to compare the resulting dischargesto those obtained with the existing stainless steel limiters and to investigateoperation of the graphite limiters at elevated temperatures using heating elements inside the graphite. No large systematicdifferenceswere observed in the electron temperatureprofiles. electron density profiles, nor Zeff between successiveruns with the stainlesssteel (SS) limiters and graphite (G) limiters. There was, however, a monotonic decrease in Z,ff for both cases from Q5.6 in the first run followingthe installationof the graphite limiters to ~2.8 after a two week period when the experimentwas terminateddue to a scheduled shutdown. Normal incidenceUV spectroscopicmeasurementsof C, N, 0, and H Radiation showed a factor of 1.5-4 greater light for the SS case over the G case. Arc tracks were observed on the graphite limiters upon removal and SEM analysiswas performed so that the amount of material removed could be estimated. As the graphite limiter was heated during a series of discharges,no increasewas observed in either the plasma carbon contaminationnor RGA hydrocarbonsignals until the temperaturerose above %400°C.

1.

was attributedto thermal loads exceeding 10 kW*cm-2 with local temperatureexcursions greater than 3OOO'C. No appreciabledifference in plasma performancewas observed between graphite and molybdenumlimiters. Carbon radiation from the plasma was observed to increase but the total power radiated and the hard x-ray signal decreasedwhen graphite limiters were used. Hollow profiles were observed for both molybdenum and graphite limiters. In ohmicallyheated PLT discharges changing from stainlesssteel limiters to graphite limiters had a negligibleeffect on the gross plasma parameters such as Z,ff, 'Et and T,, although the carbon radiationtypically increasedby a factor of three while the iron radiation decreasedby a similar factor [z]. With high-powerneutral beam injection,however, decidedlybetter results were obtained with graphite limiters and Ti gettering than with stainlesssteel limiters and Ti gettering [33.

INTRODUCTION

Graphite limiters were installed and tested in the ISX-A tokamak as part of the ISX-A surface physics program and the TFTR materials research program. The purpose of the experimentwas to compare plasma performanceusing graphite limiters as opposed to the standard ISX-A stainless steel limiters. Heaters were installed in the graphite limiters so that the effects of operationat elevated temperaturescould be evaluated. Graphite limiters have been used with some limited success in tokamaks [1,2,3]. TFR used 5890 CL-PT graphite and observed that it behaved well mechanicallyup to plasma currents of 400 kA for 1000 discharges. Surface erosion and a few microfailureswere observed. This

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Research sponsoredby the Office of Fusion Energy (ETM), U. S. Departmentof Energy under contractW-i'405-eng-26 with the Union Carbide Corporationand EY-76-C-02-3073with Princeton University.

2. NXPERIMZNTALCONFIGURATION ATJ-S [4] graphite was used for this study because of its relativelygood thermal shock

'Energy Research Laboratory,Hitachi, Ltd., Ibaraki,Japan.

215

R.A. Langley et al. / The ISX-A graphite limiter experiment

216

resistanceand purity (i.e., ash level of 0.01 *. ff),availability,large size and low cost. ATJ-S is a commercialgrade graphite used mostly for rocket reentry noseconds; (see Table 1 for its physical properties). With permanentlymounted 304L stainlesssteel (SS) limiters in ISX-A, the plasma minor radius was 26 cm and major radius was 92 cm. The movable graphite limiters could be inserted 2.5 cm beyond and withdrawn 2 cm behind the SS limiters. A schematicdiagram of the configuration is shown in Fig. 1. Only three graphite limiters were used; top, outside and bottom (not shown in Fig. 1). Use of a movable inside limiter was consideredtoo formidablea task for the experiment. For graphite limiter experiments, the plasma was positionedso that it did not contact the inner SS limiter. It should be noted that when the graphite limiterswere fully insertedthe plasma diameterwas 2 cm smaller in minor radius. Graphite limiters were used alternativelywith SS and Mo limiters during the final 12 days of operationof ISX-A before the scheduledshutdown.

Table 1 Physical Propertiesof ATJ-S Graphite [51 p = 1.85 g.cm-3 Tensile strength,lo6 Pa wg

4 Young's Modulus, 109 Pa

wg ag

177 106

Fracture Strain, %

vg ag

0.44 0.55

0.43 0.31

Thermal Conduction 422 vg (J*m/s*m3*%) ag 304 wg = with grain; ag = against grain

178 141

The graphite limiters were 30 cm long and machined from a billet 22.5 cm in diameter. The limiter surface which was exposed to the plasma containedmachining grooves which were 0.025 mm wide and deep and regularly spaced at 0.25 mm. Xeaters were constructedby coiling a tungsten wire on an alumina mandrel. These were placed in a machined groove in the rear side of the limiter. The heaters were used to thermally outgas the limiter and for operationat elevated temperatures. The maximum bulk temperatureof the graphite during heating was *600%, above this temperaturethe tungsten filamentapproached its melting point. In the temperatureregion higher than 55O'C an oxygen producingreaction between the alumina mandrel and tungsten was observed, therefore elevatedtemperaturetokamak operationwas limited to <550°C. Tokamak operation using MO limiters has been described previously [61. 3. ~~I~T~

i

PROCEDUREARD RESULTS

After installationof the graphite and MO

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2o"c llCOO 522-Y%?430 507

1

I

0

I METER

Fig. 1. ISX experimentalset up. The axis of toroidal symmetry is to the right. For clarity the bottom graphite limiter is not shown. limiters, 4 hours of dischargecleaningwas run before outgassingthe graphite limiters to ~400°C. RGA results indicatedthat the major contanimantsevolvedwere B20, CO and C3H6 with the followingrelativemagnitudes1.0, 0.7 and 0.055. After cooling to %25cC, tokamak operationwas begun using SS limiters followed by operationusing graphite limiters. Measurements of various plasma parametersare shown in Fig. 2 for the graphite limiter (G limiter in) and the SS limiter (G limiter out). Spectroscopicmeasurementsmade with a normal-incidencespectrometerviewing the plasma away from the limiter are listed in Table II. The smaller values of the emission rate normalizedto the average electron density 7iefor the graphite limiter compared to the SS limiter may reflect a decrease in the fractionof recyclingof hydrogen and

R.A. Langley et al. / The ISX-A graphite limiter experiment

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Table 2 Comparisonof Plasma Parsmeters Stainless Steel I(M) V(volts) ne(1013cm-3)
Graphite

120.0 2.0 2.15 5.6

120.0 1.6 1.25 5.1

47.0 9.4 14.5 14.1 0.55

28.3 5.8 4.5 5.8 0.25

EMssion Rate/Ti, (gR/lOl3cm-3) 1034 1-o VI 629 1-o v 977 X-c III 1216 R-H I pw/pGH

-1 gR = giga RAYLEIGH = lo15 photons cmm2.S

4.0

0.8

OS

0.4

0.2

0

owoo TIME

(msd

01

0

I

I

I

I

I

I

I

400

200

300

400

300

boo

700

LIMITER

Comparisonsof plasma parameters for Fig. 2. dischargeswith graphite (G) limiter inserted and retracted (stainlesssteel limiter). Ordinate:?ie (loll cm-3): average a) electron density in plasma; Pw(kW): power radiated to wall; I(kA): plasma current and V(lO-1 volts): loop voltage b & c) Pw(kW): power radiated to wafl PoR(kW): input Ohmic heating power impuritiesfrom the vessel wall (as opposed to the limiter) due to the smaller plasma minor radius with the graphite limiter in. On the fifth operationalday after graphite in temperatureto 600'~ during tokamak operation. Both spectroscopicand RGA results for this series of dischargesare shown in Fig. 3. There is a good correlationfor onset of increase and magnitude of increase between the C IV radiation and evolution of CH4 and CO as a function of limiter temperature;the increase

TEMPERATURE

1%)

Fig. 3. Impuritypartial pressure and carbon radiationas a function of limiter temperature. occurringabout 400%. The temperaturewhere the formationof CH4 increasesagrees well with previous laboratoryexperimentalmeasurements [7,8]. During this run the time between discharges was increased so that the base pressure (predominantlyH2 but with some CH4 and CO contamination)at the start of each discharge was approximatelythe same. The increase in C IV radiation observed above 450% may be the result of an increasedmethane partial pressure contributionto the base pressure before a discharge despite the fact that the total pressure was constant before each discharge. The major erosion mechanism observed for the graphite limiterswas arcing. The arcs consisted of a series of craters which appear to have originatedon the edge of the machined

R.A. Langley et al. / The ISX-A graphite limiter experiment

218

Fig. 4.

Photographof graphite limiter showing arc tracks on a Stainlesssteel sample as measured by ASS is shown for both oxygen and carbon/carbide. The oxygen contaminantlevel remained low, even lower than baseline data with stainlesssteel limiters. The carbon/carbidecontaminantlevel increasedabout a factor of 2.5 after each bake out of the graphite limitersbut decreasedwith subsequentdischarge cleaning. This indicates that H2 dischargecleaning is effective in reducing carbon in the ISX-A tokamak. Z,ff increased from near 1.0 before installationof the graphite limiters to about 5 immediatelyafter installation. During the 12 operationaldays succeedinggraphite limiter installationZeef steadilydecreasedto values near 2. Indications are that with continuedoperationeven lower Zeff's could probably have been obtained.

Fig.

5.

SEM replica photographshowing arc

craters.

4.

SMRY

grooves noted previously, The craters were about 0.05 mm in diameter and about the same depth, as measured using SEM sterographie techniques. Shown in Fig. 4 is a photographof the tracks and Fig. 5 is a SEM photographof a sequenceof craters. The arc tracks range in length from a few mm's to a few cm's but average about 3 cm. The directionof motion of the arcs appears to be retrograde [9,10]. There was an average of more than one arc per limiter per dischargewith an estimated 5 x 1017 atoms released per arc. This amount of eroded material is more than enough to account for the carbon observed in the plasma.

In conclusion,the only significantdifferences observed in gross plasma parametersbetween dischargeswith graphite limiters and those with stainlesssteel limiterswere lower UV emissionrates for both hydrogen and impurity radiationas well as lower total power to the wall for the graphite limiter discharges. This may have resulted in part from less recycling from the wall due to the smaller plasma minor radius when the graphite limiters were used. Although the Z,ff did not differ appreciably for consecutivegraphite and stainlesssteel runs, it did decreasemonotonicallyfrom %5.6 to ‘~2.8 during the 12 days of experimentation.

A surface analysis station with an associated sample transfer section was used to measure the amount of impuritiesdepositedon samples exposed to the plasma in the limiter shadow region Il.11* This techniqueyields semiquantitative results on impuritylevels in the tokamak and on the effectivenessof various cleaningprocedures. A summary of these results is given in Fig. 6 along with Zeff results determinedby laser Thomson scattering. At the top of the figure is given a sequentialdescriptionof the operatingconditions. The contaminantcoverage

The major erosion mechanism of the graphite limiterswas arcing, with the amount of eroded material being more than enough to account for the observed carbon contamination. The machining groves in the limiter surfacemay have contributedto the arcing problem and should certainlybe removed in future experiments. Neither the RGA hydrocarbonsignals nor the plasma carbon radiation showed an increase as the graphite limiterswere heated until the

temperaturerose above %kOO'C. This tempesa-

R.A. Langley et al. 1 The ISX-A graphite limiter experiment

219

AES DATA CARBIDE/CARBON OXYGEN 2 et, STAINLESS STEEL LIMITER GRAPHITE LIMITER

6

-

J”z

T

/’

5

1’

4

Itf -

3

/

-k. $

2 1

I 1

2

3

4

5

7

6

8

9

10

11

TIME (days1

Fig. 6.

Time history of graphite limiter experiment.

ture dependence is similar to that observed for chemical sputtering of graphite [7,81. The increase in the plasma carbon contamination may be a result of increased partial pressure of While the methane prior to a discharge. graphite chemical sputtering may become less severe at temperatures much greater than those used here, operation with the limiters in the range of 4OO'C to 600% does not seem feasible. In devices with much larger energy input than ISX-A, such as ISX-B and TFTR, the limita?rs will probably operate at temperatures above 600%; therefore future graphite limiter experiments should address the question of graphite chemical activity at temperatures >~oo'c. REFEREHCES TFR Group, EUR-CEA-FC-852 Report (1976). K. Bol, et. al., Seventh Inter. Conf. on Plasma Physics and Controlled Nuclear Fusion Research, Innsbruck, Austria (1976) IAEA-CN-37-C-3. H. Eubank, et. al., Seventh Inter. Conf. on Plasma Physics and Controlled Nuclear Fusion Research, Innsbruck, Austria (1978) IAEA-CN-37-C-3. Trademark of Union Carbide Corporation. Private communication, W. P. Eatherly (ORNL) (1978). M. Murakami, et. al., Seventh Inter. Conf. on Plasma Physics and Controlled Nuclear Fusion Research, Innsbruck, Austria (1978)

[71 [81 [9]

[lo]

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IAEA-CN-37-N-4. S. K. Erents, C. M. Braganza and G. M. McCracken, J. Nucl. Mater. 63 (1976) 399. R. A. Langley, R. S. Blewer and J. Roth, J. Nucl. Mater. 76 (1978) 313. D. H. J. GoodallTT. W. Conlon, C. Sofield and G. M. McCracken, J. Nucl. Mater. -76 (1978) 492. P. Mioduszewski, R. E. Clausing, L. Heatherly to be published, J. Nucl. Mater. (1979) (this conf.) L. C. Emerson, R. E. Clausing and L. Heatherly, J. Nucl. Mater. 'j'&(1978) 472.