It has been observed in several tokamaks that the confinement of test impurities increases dramatically when operating near density limits. The characteristics of the working gas transport coefficients also change character under these conditions. These changes appear to be caused by a suppression of the anomalous transport mechanisms. This series of vugraphs investigates the role of these changes in initiating disruptions

Isler, R.C.

Rowan, W.L.

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The submitted manuacnpt h « beenauthored by a contractor of the U.SGovernment unoV contract No DE-AC05-84OR21400 Accordingly. ih« USCover nment retami a nonaxctusive.royalty-free been** 10 piirteh or reproducethe published form of this contribution oraHow others to do so. tor U S Government«Howpurposm.LONG-TIME IMPURITY CONFINEMENTAS A PRECURSOR TO DISRUPTIONSIN OHMICALLY HEATED TOKAMAKS_ . . _ , _ , CONF-880163~1-VugraphsRalph C. IslerFusion Energy Division DE88 006183ORNLandWilliam L. RowanFusion Research CenterUniversity of Texas at AustinPresented at the IAEA Technical CommitteeMeeting on Density Limits and DisruptionsAbingdon, England -26-28 January, 1988Work sponsored by the Office of Fusion Energy, U. S. De-partment of Energy, under contract No. DE-AC05-84OR21400with Martin Marietta Energy Systems Inc.D i S T R i s U T . r , ; - ; r,' :, ~ . - . • ' " . " '•"•• • " . ' J r -It has been observed in several tokamaks that theconfinement of test impurities increases dramat-ically when operating near density limits. Thecharacteristics of the working gas transport coef-ficients also change character under these condi-tions.These changes appear to be caused by a su-pression of the anomalous transport mechanisms.What role do they play in initiating disruptions?TOPICS• TEST IMPURITY CONFINEMENT NEAR DENSITYLIMITS AND INTRINSIC IMPURITY ACCUMULA-TION.• SOFT AND HARD DISRUPTIONS.• SAWTOOTHING AND LOW MHD MODE BEHAV-IOR• COMPARISON OF HYDROGEN AND DEUTERIUMDISCHARGES.• UNIVERSALITY OF OBSERVATIONS.Two Types of DisruptionsAre Observed* Soft or partial disruptions. These are preceededby impurity accumulation, central cooling, andlittle or no increase in MHD activity, Ip doesnot go to zero, but confinement is poor after thedisruption.* Hard or complete disruptions. Impurity accu-mulation may or may not be observed. MHDactivity increases before the disruption, then Ipdrops rapidly to zero.In both cases observed in the TEXT toka-mak, a transition to long-time confinement oftest impurities preceeded the disruptionHUGILL DIAGRAM FOR DEUTERIUMDISCHARGES IN THE TEXT TOKAMAK0.50.40.30.2o0o71.8•o903o1004o2.0o60.10.00RNe/BT6 •S •4 *ao1/1TYPICAL STABLE DISCHARGE SIGNALSI1.11 ••rSOFT X RAY0 50 2 00 150 200 250 S00 350 400 450 £00TIMEJ . S Tao7528B9- 1 . •50 100 !50 200 250 500 550 *ZZ 450 500Xm m -~0N31SHBN31SAlcator Impurity ScalingTZeffzhTEXT Results for Disruptive Discharges1. BT DEPENDENCE AGREES2. Iv DEPENDENCE DOES NOT AGREE3. Ne(Zeff) DEPENDENCE DOES NOT AGREEORNl-DWG 86C-2833 FEDcs<z<9c3oCO1.20.92 0.6•030(a)Sc XIX(DISRUPTION1 1- " \J INSciXVI(c)"•I iSc1XIII(d)--•tin100 150 200 250TIME (m«)300 100 160 200 2*0 300TIME (mt)n6"' A"1.01. 0 1 6.014.012.01.006.004.0020.TYPICAL HISTC-tY OF SOFT X-RAY SIGNALPRECEEDING PARTIAL DISRUPTIONSCENTRAL COOLINOEDOE COOLINOAND REDUCTION OfMETAL INFLUXDISRUPTIONACCUMULATIONINJtCTSCANDIUM0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 5CCENTlAl COOLINOtOOt COOLINOAND •EDUCTIONMETAL INFLUXDISIU'TION200TIME (IDS)-=• 2e2I 1otai^***^^(b)100 200 300TIME (ms)400 5001 *"¥;*-out Perentially rresistance stops the sawtootORNL-DWG i6C-2836 FEDc9iD42n--\ 2 6 k GvVi i ^(a)—-• 6c3" 2 -<i o1002 -1 -1v 20ikGDISRUPTIONl1 ' 1(c)160 200 260TIME (m«)2 -1 -300 100 ifO 200 260TIME (mt)300ao1/1I..e. 620 kG200 kASc XVI.20.2.0 x 101 3cm100A . « J\.200 500TIME4 0 0a2Oin,e.6.22.6 x 101 3cm 37̂ ,' I100 200 500 400 500TIMEaoi ..6.63.2 x 10lscm1 3 — 3ORNL-DWG 86C-2636 FED1.5c 1.0 -O «H 0.6(a)Sc XIX |A! H \' AVL_iDISRUPTION^^fAW/ .1 - . . . . i100 200 300 400TIME (ms)Isler et al. , Fig. 5SIGNAL(arbitrary units)SIGNAL(arbitrary uni ts)SIGNAL(arbitrary units)Aozr-oonM0»©Io100 100 «0 tOO£3r(c)150 MO J «Ii«lit<Ds1009<v10 U> l/> U» U>c* • ftI t3£tW100 m woTIME | » )400 100(b)«ou soo140A(0"MN91S3!nRELATIVE INTENSITY B$ SOFT X-HAV SIGNAL (iiUinir u lu )8nTSOTT H U T 3KHAL (akinq w^m)SOFT XRELATIVE INTENSITY RELATTVE DfTENSmrRESULTS OF TRANSPORT STUDIES OF HYDROGENAND DEUTERIUM IN THE TEXT TOKAMAKK. Gentle, B. Richards, F. Waelbroeck• WORKING GAS TRANSPORT CAN BE MODELED BY CONSTANTD AND V EXCEPT NEAR DENSITY LIMITS. D AND V SCALE AS1/nq.• D FOR DEUTERIUM IS SYSTEMATICALLY LOWER THAN FORHYDROGEN.• NEAR THE DENSITY LIMIT D AND V ARE NOT UNIFORM:* IN THE CORE, D IS SMALLER THAN EXPECTED FROM THE1/nq SCALING, V«0,* IN THE EDGE D AND V ARE BOTH HIGHER THAN EX-PECTED FROM THE 1/nq SCALING.SOFT X-RAY (PIN) SIGNALS REFLECTTHE DIFFERENCES OF IMPURITY ACCUMULATIONBETWEEN DEUTERIUM AND HYDROGENu(a)ORNL-DWGU/ 1 1 I80-3668AH1FED40 80 420TIME (ms)460OEUTEBlUM HVOROGCNfa)t n «> I< eO SFIG. 1. The line-averaged electron concentrationsaad the soft-x-ray signals during Ohmically heateddeuterium CD) and hydrogen (H) discharges.2 0TIME <mt) TIMEFIG. 2. Comparisons of the emission rates (1 (Igaxay-letph * in15 pbotons/cm2 si from several Iron linesduring Ohmically heated deuterium tad hydrogen dis-charges.334SOFT X-RAY SIGNALS INDICATE THAT THE IMPURITY CONTENTINCREASES DURING DEUTERIUM DISCHARGES BUT SOON REACHESAN ALMOST STEADY STATE IN HYDROGEN DISCHARGES.ORNL-DWG 81-3407 FEDHYDROGENMHDSOFT X-RAYDEUTERIUMMHDSOFT X-RAYLoI I I I I I j _ i II40 80 120TIME (ms)160 200403o;.ms) 20101.0e1.0.27H<2•0Xnesl.6qL- < .32WITH SAWTEETH •WITHOUT SAWTEETHHIGH DENSITYAVERAGE #••oo0o° 8o° oo1.5 2.0Zeff2.5FIGURE 10NON-GETTEREDZ.//=2.8GETTEREDSi1ial|1113: y132716S. l*>rty1711r.000--3J0J3- 32S20 ft 1.000so IOD isoTine insirIOC ISO JDD EDTIHE IMS)SHOT:|32724- 3272S-•/n://XVI/360un1.̂.000 SH0T:32826- 3272S XVI 360.00D100 110TlfC IH5)82-i-cSIo/T:/3S71» 32711imFTl<0xvniIIIIan91.I000a 3BiJ -\i\\WIj/Ii1rI0U ISOTlflC ( « )15-r, <•"'°O.I0.0SHOT:Irf/1/I32811-IV32815/l i/milly91.Itft!L\v000\TWC («S iSO IB) ISO JDJ aT1»C insiIam m001 MR A** OR•• -OJS« si41I11fJ.i/13-11CCI/- -I.IXIV NI 3UIIooo'iec »« wa--tRLi| 11/I1•JlMzZin» w toset >:c>-{3>n< MI;:::::::±ttjii|r/3UUTiT1fti ...J±:. ..Ti•"T •_ - .ocn :rr «»ti.Confinement Times andTransport CoefficientsNear Density LimitsNeoclassical Impurity Transpi\^" "Observation of Lone unnement Times in the ISK.H. Burrell et al. . NF.. 'ltJf f«>und that both impurity »times and impurity containment times "are5with neoclassical predictionsEMISSION lATE (Gft) fMISSlON I A T E COt) EMISSION tATE (Ot)o o o o o P O — » M M6 i» 6 i» b U• • • r iL ^ X ^ ^ J ^ ^EMISSION tATE (Gt) EMISSION tATE (Ot) EMISSION tATE (Gt)0RNL-DWG 81-17059 FCO800600£ 400200I- T t ( r ) AND n f ( r ) AT 450msIN DEUTERIUM DISCHARGE?WITH TITANIUM INJECTIONI I Is-20 -10 0 10RAOIUS (em)201.2_ 1.0o"b 0.6e© 0.40.2014001200> 1000o 800*~ 600400200INFERRED TITANIUMAT 150\ N~~ Ti XX\ '• x/TIX>•CONCENTRATIONmsXIVVfA\Tixlv-j\5 10RAOIUS (cm)1.2Z en0.8 tO0.6 UJ?E0.4 g |0.2 I 80— 81 1 1THE ELECTRON TEMPERA-- TURE DECREASES AS THEDENSITY RISES tfi r \ i• T#(0) —ont(0)'"* r4 - .O400 150 200TIME (ms)250TEMPORAL EVOLUTION OF TITANIUM EMISSIONAFTER TRANSPORT EQUILIBRIUM IS ESTABLISHEDORNL-OWG 84-47057 FED» IEXPERIMENTTHEORY: CONSTANT160 430 200TIME (fflt)T ITi XVII • Ti XYIIJ479.79^*179.91 A160 480 200TIME (ms)EMPIRICAL DESCRIPTIONS OF IMPURITY TRANSPORTIF D = CONSTANTTHE TIME DEPENDENT SOLUTIONS AREWHERE 5 = ^ £ , A = i , AND M = THE CONFLUENT HYPERGEOMETRIC FUNC-TION.THE STEADY STATE SOLUTION (T'r = 0) ISTHIS RESULT IMPLIES THAT THE INTRINSIC IMPURITY LEVEL IS STRONGLYPEAKED AT p = 0 if % IS LARGE.Nt IS UNIFORM AS A FUNCTION OF p IF CONVECTION IS NEGLIGIBLE COM-PARED TO DIFFUSION.10*10©~ 10-210- 410i i i i i i iX,\-6-25 -20 -15 -10 -j i5 0 5 10 15 20 25S • OV/20TITANIUM DISTRIBUTION AT 150 MSSOUD - MODEUNG OF SPECTRAL INTENSITIESDASH - GAUSSIAN DISTRIBUTIONwa11.00.8 -0.6 -0.4 -0.2 -0.00.0 0.2 0.4 0.6 0.8 1.0ANALYSIS OF TITANIUM INJECTION EXPERIMENTSON ISX-B WITH DEUTERIUM AS THE WORKING GASTAKE 5 = 6.4 TO GIVE AN ACCEPTABLE MATCH BETWEEN THE PROFILEDETERMINED FROM SPECTROSCOPY AND THE GAUSSUN ttf (-Vo = 0.48£>ASSUME THE RISE TIME OF THE Ti XDC SIGNAL (0.020 S) EQUALS r,.-T * 0.06 for S = 6.4 (FROM GRAPH OF A,)D = l.tXlO*em*/SVo = 070 em/S (FROM GRAPH OF Ao)T0 = 3.1 S !AS EXPECTED FROM THESE NUMBERS, TYPICAL ISX-B OH DISCHARGES INDEUTERIUM EVIDENCE ACCUMULATION OF INTRINSIC IMPURITIES.THE COMBINATION OF TEXT AND ISX-B DATA SHOWS THAT THE IMPU-RITY CONFINEMENT TIME CAN CHANGE BY A FACTOR OF 100 FOR CHANGESOF LESS THAN A FACTOR OF 2 IN BT, I,, or JV,.25"oC D "tn-•mmOEFFICIEN2000NE0CLR55. C0 1000100011•Ma« • • •« " •_£MMB ••BwmmcM >• M l • a n• M• S IM *•• M M• M «ess:• M « • >/M H\mmim/• 1\M M *•MM>\J/f/\-• —r10 15 20RfiDIUS (CM)25 30Evidence of Long ConfinementTimes Near Density Limitsin ASDEX, T-10, and TEXTOR• i3oooXoOO•?;o•3.0o360oSOenoCMo* Oss:o>CM1o% • •TEXTORCastracane et al.480 kAb)360 kA ~300-200-100-xzFe• = Si«= Moo= Tio SV> " "tttX3KXo2JD 2flrie(x1013cm*3)220 kA15 23 23 25 27 2SSUMMARY• ANOMALOUS DIFFUSION OF IMPURITIES (ANDPOSSIBLY THE WORKING GAS) IS SUPRESSEDIN THE INTERIOR OF TOKAMK PLASMAS NEARDENSITY LIMITS. INWARD CONVECTION THENRESULTS IN RETENTION OF INJECTED TEST IM-PURITIES.• THIS CHANGE TO CONVECTION DOMINATEDTRANSPORT OFTEN RESULTS OFTEN RESULTSIN THE ACCUMULATION OF INTRINSIC IMPURI-TIES, AT LEAST ABOVE q«3. IF SO, A THERMALCOLLAPSE AT THE CENTER INITIATES A PAR-TIAL DISRUPTION.• SHARP INCREASES IN rz ARE ALSO OBSERVEDBEFORE MAJOR DISRUPTIONS. THE ROLE OF EN-HANCED IMPURITY CONFINEMENT IN INITIAT-ING SUCH DISRUPTIONS IS UNCERTAIN.**• FURTHER DOCUMENTATION OF rz IN DISRUP-TIVE DISCHARGES COULD BE ENLIGHTENINGFOR UNDERSTANDING DENSITY LIMITS. ALSO,CHANGES OF IMPURITY TRANSPORT NEARDENSITY LIMITS IN AUXILLIARY HEATED DIS-CHARGES HAVE NOT BEEN STUDIED.DISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither (he United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United Slates Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited Stales Government or any agency thereof.

Long-time impurity confinement as a precursor to disruptions in ohmically heated tokamaks

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