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 puropse of the experiment was to compare plasma performance using graphite limiters as opposed to the standard ISX-A stainless steel limiters. Heaters were installed in the graphite limiters so that the effects of operation at elevated temperatures could be evaluated

Cecchi, J. L.

Colchin, R. J.

Corso, V. L.

Dylla, H. F.

Ellis, R. A., Jr.

Isler, R. C.

Langley, R. A.

Murakami, M.

Nishi, M.

Simpkins, J. E.

UNT Digital Library

' // • /namesPORTIONS OF THl^BEPOPT AR£ L L L E g R ;c!py » pprqlt tha broadMt possibl* « • * •By acceptance of this article, thepublisher or recipient acknowledgesthe U.S. Government's right toretain a nonexclusive, royalty freelicense in and to any copyrightcovering .he article.' ffi ISX-A GRAPHITE LIMITER EXPERIMENTR. A. Langley, R. J. Colchin, S. C. Isler, M. Murakami, and J. E. Sispkir.sOak Ridge Nat".onp_L Laboratory, Oak Ridge, Tennessee 37330, USAANDJ . L. Cecchi, V. L. Corso, K. F. Dylla, ft. A. E l l i s , J r . , and M. IlishiPlasma Physics Laboratory, Princeton University, Princeton, New Jersey 085^0, USALE, ttAs part of t.ie ISX surface physics program and the TFTR materials research program, ATJSgraphite rail iimiters were installed in the ISX tokamak in order to compare them with theexisting stainless steel liniters and to investigate any deleterious effects arising fromoperation at elevated temperatures of the graphite. To facilitate the latter experiment, heaterswere installed in the graphite liniters since the power deposition on the limiter in ISX wasexpected to be considerably less than for TFTR. The graphite liniters also containedthermocouples to monitor the bulk temperature. Ho large systematic differences werr observedin the electron temperature profiles, electron density profile;- nor Zeff between successiveruns with the stainless steel (SS) limiters and graphite (G) limiters. There was, however, axonotonic decrease in Zeff for both cases from i- 5.6 in the first run following the installationof the graphite limiters to ̂  2.8 after a two week period when the experiment was terminateddue to a scheduled shutdown. Normal incidence UV spejtroscopic measurements of C, II, 0, andH radiation showed a factor of 3-k greater hydrogen light for the S3 case over the 5 case, butno systematic differences in the impurity light. Arc tracks were cbserved on the graphitelimiters upor. removal and 3EM analysis was performed so that the amount of material removedcould be estimated. In the hot graphite limiter experiment, the temperature of one of thegraphite liniters was increased on successive shots. The hydrocarbons formed, as determinedby residual gas analysis (RGA), increased monotonically with increasing limiter temperature,e.g. mass 16 (presumed to be CHj,) increased about a factor of two as the temperature wasraised from 150°C to SQO°C. CO and COg were also found to increase only slightly. The carbonimpurity light was also observed to increase by a factor of two as the limiter temperature wasraised from 150°C to 500°C. Further experiments using graphite liraiters are indicatec. beforetheir application in beam driven tokamaks.1. INTRODUCTIONGraphite limiters were installed and tested inthe ISX-A tokamak as part of the ISX-A surfacephysics program and the TFTR materials researchprogram. The purpose of the experiment was tocompare plasma performance using graphitelimiters as opposed to the standard ISX-A stain-less steel limiters. Heaters were installed inthe graphite limitirs so that the effects ofoperation at elevated temperatures could beevaluated.Graphite liaiters have been used with somelimited success in tokamaks [1,2,3]. TFR used5390 CL-PT graphite and observed that it be-haved well mechanically up to plasma currentsof U00 kA for 1000 discharges. Surface erosionand a few nicrofailures were observed. Thiswas attributed to thermal loads exceedingnesearch sponsored by the Office of FusionEnergy (EM), U. S. Department of Energy undercontract W-7U05-eng-26 with the Union CarbideCorporation and EY-76-C-O2-3073 with PrincetonUniversity.10 kW-cm~2 with local temperature excursionsgrater than 3000°0. No appreciable differ-ence in plasma performance was cbserved betweengraphite and molybdenum limiters. Carbonradiation from the plasma was observed to in-crease but the total power radiated and thehard x-ray signal decreased when graphitelimiters were used. Hollow profiles were ob-served for both molybdenum and graphite lim-iters. In ohmically heated PLT dischar^schanging from stainless steel limiters tographite limiters had a negligible effect onthe gross plasma parameters such as Z e f f, T 3,and Te, although the carbon radiation typicallyincreased by a factor of three while the ironradiation decreased by a similar factor [2].With high-power neutral beam injection, how-ever, decidedLy better results were obtainedwith graphite limiters and Ti gettering thanwith stainless steel ximiters ar.d Ti gettering[3].2. EXPERIMENTAL COSPIGURATIONATJ-S [1*] graphite was used for this studybecause of its relatively good thermal shockresistance and purity, i.e., ash level of 0.01wt. %, availability, large size and low cost.ATJ-S is a commercial grade graphite used mostlyfor rocket reentry nosecones; (see Table 1 forits physical properties).Vvith stainless steel (SS) iimiters in ISX-A,which were permanently mounted, the plasma minorradius was 26 ca and major radius was 92 cm-The graphite limiters which were movable couldbe inserted 2.5 cm beyond and withdrawn 2 cmjehLnd the S3 limiters. A schematic diagram ofthe configuration is shown in Fig. 1. Onlythree graphite limiters were used; top, outsideand bottom (not shown m Fig. 1). Use of a mov-able inside limiter was considered too formida-ble a task for the short time available for theexperiment. For graphite limiter experiments,the plasma was positio.-.ed so tnat it did notcontact the inner SS limiter. It should benoted that when the graphite limiters were fullyinserted the plasma iiameter was 2 cm smaller inairier radius. Graphite limiters were used al-ternatively with SS and Mo limiters during thefinal 12 days of operation of ISX-A before thescheduled shutdown.ORNL/CWG/FED 78-272l.ble 1Physical Properties of AxJ-3c = 1.85 g-csT3Tensile strength, psiYoung's Modulus, 10° psiFracture Strain, *Thermal Conduction(3TU-in/hr-ft3. °F)agwgagwgagwgagGraphite20°C515042501.751.050.1*1*0.5590065015]1100°C610050000.1*30.31380300wg = with grain; ag = against grainIte graphite Ii.iiters were 30 cm long f-id ma-chined from a billet 22.5 cm in diameter. Thelimiter surface which was exposed to the plasnacontained machining grooves which were 0 025 mmwide and deep and regularly spaced at Q.«25 ma.Heaters were constructed by coiling a tungstenvire on an alumina mandrel. These were placedin a machined groove in the rear side of, thelimiter. The heaters vere used to thermallyoutgas the limiter and for operation at elevatedtemperatures. The maximum bulk temperature ofthe graphite during heating was "-600°C, abov-ethis temperature the tungsten filament approach-ed its melting point. In the temperature regionhigher than 55O°C an oxygen producing reactionbetween the alumna mandrel and tungsten was ob-served therefore elevated temperature tokaioakoperation was limited to <55O°C. Tokanax oper-ation using Mo limiters has been describedpreviously [6].3. EXPERIMEHTAI, PROCEDURE Aim RESULTSAfter installation of the graphite and Mo\ LSTAINLESS_ J = = = 3 _ — « < y 9AR LlMlSTEELTERSMOVABLESTAINLESS STEELOR MOLYBDENUMCERAMIC BREAK•* J E D {iMETERFig. 1. ISX experimental set up. The axis sftoroidal symmetry is to the right. Forclarity the bottom carbon limiter is r.ot shewn.limiters, U hours of discharge cleaning was r-j:.-,before outgassing the graphite iimiters to".*GO°C. EGA results indicated that the nâ o-."contaninants evolved were HoG, CO and C-rii vit.ithe following relative magnitudes 1.0, 6.7 s.r.i0.055. After cooling to \25°C, tokamaKoperation was begun using SS iimiters fo2.1ovedby operation using graphite liniters. Meas-urements of various plasma parameters areshown in Fig. 2 for the graphite liniter (0limiter in) and the SS limiter (G liniter out).Spectroscopic measurements made with anormal-incidence spectrometer viewing theDlasma away from the limiter ai'e listed ir.Taule II. The smaller values of the emissionrate .̂ c'rmalized to the average electrondensity-n^ for the graphite limits- comparedto the SS limiter may reflect a decrease inthe fraction of recycling of hydrogen andimpurities from the vessel wa.'.l fas opposed toORNL/OWC/FED<00fi I T T ]\\ »,«o"cm3l.C UMITER OUTft 'S \Table 2Comparison of Plasma Parameters1.U0 80.60.40.2p1p1 1f C LIMITER /"1 OUT If i CARBON UMITERIN1 1 11--—(00 <50TIMF(m««t\?if. 2. Comparisons of plasma parametersfor discharges with carbon limiter insertedand retracted (stainless steel limiter).the limiter) due to the scalier plasma minorracius with the graphite limiter in.Cm the fifth operational Say after graphiteliriiter installation, one limiter was rammedin temperature to 600°C during tokamak opera-tion. Both spectroscopic sac. RGA results forthis series of discharges are shown in Fig. 3.There is a good correlation for onset of in-crease and magnitude of increa.se between theC IV radiation and. evolution of CHi, and CO asa function of limiter temperature; toe increaseoccurring about l+50°C. The temperature wheretie formation of CK^ increases agrees well withprevious laboratory experimental measurements[7,3]. During this run the time between dis-charges was increased so that, the base pres-sure (predominantly Hg but with some CHij andK.-Ul)7(volts)ne(l0l3om-3)^ o f f >Emission Rate/:Te(gR/10l3on-3)103li S-0V1629 A-OV977 A-Clll1216 X-HlPWALLStainless Steel120.02 . 02.155 • 6H ? . O9-'-*1U.51 U . 10.55i^rashite120.01.6• 1.255.128.35.3I*.55.80.25= giga RAYLETCH = ID1? photons cm"2-S"1ORNL/DWO/fEt) T9-101T5 o.ebUI 0.6trffi 0.41- o 419 A*• 1S47A_-II ICIV•CIVI II I 1RGA SPECTSW TAKEN -15 n c AFTERDISCHARGEO O .& 16 (CH4)o is <co)1 1TOKAMAK_--2.0 5UJ0 100 200 100 400 500 600 700LIMITER TEMPERATURE COFig. 3- Impurity partial pressure and carbonradiation as a function of limiter temperature.CO contamination) at the start of each dis-charge was approximately the stune. The in-crease in C IV radiation observed above l*50°Cmay be the result of an increased methane par-tial pressure contribution to the base pres-sure before a discharge despite the fact thatthe total pressure vas constant before eachdischarge.The major erosion mechanism observed for thegraphite liiniters was arcing. The arcs con-sisted of a series of craters which appear tohave originated en the edge of the machinedgrooves noted previously. The craters wereabout 0.05 mm In diameter and about the samedepth, as measured using SEM sterographietechniques. Shown in Fig. it is a photograph ofthe tracks and Fig. 5 is a SEM photograph of asequence of craters. The arc tracks range inFig. 4, Photograph of graphite limiter shoving arc tracks.;>V* ̂ *jf **• -Fig. 5- SET-! replica photograph showing arccraters.length li-om a few mm's ^o a few em's but aver-age about. 3 cm. The direction of motion of shearcs appears to be retrograde [9.10]. There wasan average of more than one arc per limiter perdischarge with an estimated 5 x 1017 atoms re-leased per arc. This amount of eroded materialis mere than enough to account for the carbonobserved in the plasua.A surface analysis station with an associatedsample transfer section was used to measure theamount of impurities deposited on samples ex-posed to the 'plasma in the limiter shadow region[11j. This technique yields semiquantitativeresults on impurity levels in the tokamak and onthe effectiveness of various clearing proce-dures. A summary of these results is given inFig. 6 along with Zeff results determined bylaser Thomson scattering. At the top of thefigure is given a sequential description of theoperating conditions. The contaminant coverageon a stainless steel sample as measured by AESis shown for both oxygen and carbon/carbide.The oxygen contaminant level remained low, evenlower than baseline data with stainless steellimiters. The carbon/carbide contaminant levelincreased abcut a factor of 2.5 after each bakeout of the graphite limiters but decreased vithsubsequent discharge cleaning. This indicatesthat Kg discharge cleaning is effective inreducing carbon in the ISX-A tokamak. Zef<. in-creased from near 1.0 before installation of thegraphite limi^ers to about 5 immediately afterinstallation. During the 12 operational dayssucceeding graphite liaiiter installation Zeffsteadily decreased to values near 2. Indi-cations are that with continued operation evenlower Zeff's oouli probably have been obtained.REFERENCES[I] TFE Group, SlfH-CSA-FC-852 Report (1976).[2] K. Bol, et al., Princeton Plasma PhysicsLaboratory Report PPFL-lt92 (1973).[3] H. Eubank, et al., Princeton Plasma PhysicsLaboratory Report PPFL-lu91 (1978).[U] Trademark of Union Carbide Corporation.[5] Private communication, Walter Satherly(1978),[6] M. Murakami, e"C al. , Seventh j.nter. Conf.on Plasma Physics and Controlled NuclearFusiDn Research, Innsbruck, Austria (1973)IAEA-CN-37-N-1*.[7] S. K. Erents, Z. M. Braganza and G. M.McCraken, J. IIucl. Mater. 63. (1976) 399-[8] R. A. Langley, R. S. Blewer and J. Roth,J. Nucl. Mater. 7£ (1978) 313.[9] D. E. J. Goodall, T. W. Ccnlon, C. Sofieldand 0. M. McCracken, J. Mucl. Mater. J_6_(1978) U92.[10] P. Wioduszewski, R. E. Clausing, L.Hea'iherly to be published, J. Nucl. Mater.(1979) (this conf.)[II] L. C. Emerson, R. S. Clasuing and L.Heatherly, J. Nucl. Mater. 76 (1973) 4T2.CONTAMINANT COVfHAi^fc ON ST\ l . \ _iS STEEl S«MAS MEASURED bY AfcbTTTTTTooNi• ///Io . /1°o / •i- IIBASELINE DATA FOR TOKAMAKWlTHSS DMI7EHSG LIMtTER INSTALLATIONBAKEOUT G UMlTtRS TC 400' CTOKAMAK DISCHARGES/SS LiMiTtfA\G LiMHt R IDISCHARGE CLEANTOKAMAK UlbCH/tHGES !SS UMfTfHiTi GETTER I AES SAMfl E NOT tXPOi tOICAKEOUT G LIUlTthS TO 400 CTOKAMAK OlSCMAHl.tS ISS I IMITf HiTi GEl I t H lAtS SAMf". L NO I t>K)b t HiTOKAMAK DISCHARGESl i C t T l E H 1AESSAMPDISCHARGE C l t ANTOKAMAK DJSCHAHuDISCHARGE CLEANTOKAMAK OlSCHARl.n ft, o •Cirm Onl l > t < £— m • " • ' • ^ oSxj;i -?? H 3)- s sESFSj .gli.b A^D M.. t i M i U .TiT4OT EAPCSEDI—/ss L;.MJ7ERS \[G LIMITtilS 10 600 C//SS I (Ml !L Hs\—

ISX-A Graphite Limiter Experiment

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ISX-A Graphite Limiter Experiment

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