Bombarda, F.

Cracken, G.M.

Goetz, J.A.

Graf, M.

Jablonski, D.

Kurz, C.

Labombard, B.

Lipschultz, B.

Rice, J.

Welch, B.

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PFC/JA-94-016Impurity Transport in the Divertorof the Alcator C-Mod TokamakG.M. McCracken, F. Bombarda*, M. Graf,J.A. Goetz, D. Jablonski, C. Kurz,B. Labombard, B. Lipschultz, J. Rice, B. WelchtPlasma Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139June 1994*ENEA, Euratom, Frascati, Italy.tUniversity of Maryland, College Park, MD.Submitted to Journal of Nuclear Materials.This work was supported by the U. S. Department of Energy Contract No. DE-AC02-78ET51013. Reproduction, translation, publication, use and disposal, in whole or in partby or for the United States government is permitted.Paper presented at the 11th PSI Conference, Mito, Japan, May 1994Impurity Transport in the Divertor of the Alcator C-ModTokamakG M McCracken, F Bombarda , M Graf, J A Goetz, D Jablonski, C Kurz,B Labombard, B Lipschultz, J Rice, B WelchtPlasma Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139AbstractArgon has been injected into the private flux zone of the Alcator C -Mod divertor and its transport into the confined plasma studied. Theinjected gas pulse lasts for about 200 ms, but the impurity concentrationincreases to an equilibrium level and remains there for -1s. It is clearthat the argon is acting as a recycling impurity. Nevertheless it has beenobserved that less than 0.5%of the injected atoms reach the confinedplasma and that the percentage decreases with increasing plasma density.The good impurity screening is discussed in terms of the ionisation meanfree path in the divertor.* ENEA, Euratom, Frascati, Italy.t University of Maryland, College Park, MD.11. IntroductionIn divertor tokamaks the impurity source, resulting primarily fromion-surface interactions, is in principle remote from the confined plasma.One of the anticipated advantages of this configuration is that impuritiesproduced at the divertor target plates have a high probability of beingionised and swept back to the target by the fuel species ion flow. Otherfactors may however over-ride the simple picture. Reverse flow in thescrape-off layer (SOL), as observed in 2-D fluid calculations[1], is onepossibility and the ion temperature gradient force acting away from thetarget plate is another.An attempt to assess the global effect of these factors experimentallyhas been made by introducing impurities at a known rate through thetarget plate. The experiments are similar to those which have been carriedout on limiter tokamaks [2]. The impurity gas has been injected at twopoints in the divertor region of the Alcator C-Mod tokamak, the privateflux region and the inner SOL. The transport of these impurities from thedivertor to the confined plasma has been studied spectroscopically.2. Experiment.Alcator C-Mod is a high field, high density, divertor tokamak [3,4].Typical operating conditions for the present experiments were I- p=0.8 MA,ne=6-12x10 19 m-3, BT= 5 Tesla,Te(O)=1.0-1. 8 k eV and elongation =1.6, in asingle null diverted configuration.The arrangement of the impurity injection and the diagnostics isshown in fig 1. The impurity gas puffing system consists of an array ofcapillary tubes which are connected to various poloidal positions at thedivertor target plates and the inner wall. The system has been used both2for impurity puffing and for fuelling the main plasma. Only the positionsin the private flux zone (A) and on the inner wall (B) have been used in thepresent experiments. The flow in the capillary tubes is controlled by apneumatically operated valve and the preset pressure in a plenum. Theflow rate is measured with a differential capacitance manometer(Baratron type 221) using a method described by Dylla[5]. The totalnumber of atoms injected is 2x10 19 atoms in a ~ 0.2 s pulse. The plenumoperates at relatively high pressure because of the low conductance of thecapillary tubes. With argon at 1 torr there is a typical delay of ~100 msbetween the opening of the valve and the arrival of the gas in the divertor.This delay has been calibrated as a function of pressure for argon anddeuterium. The flow has a rise time of 100 ms but the decay time of theflow rate is longer due to the flow of gas out of the capillary volume afterthe valve is shut; it has an approximately exponential tail with a timeconstant of ~200 ms.The argon concentration in the core plasma is measured using a crystalspectrometer [6] viewing Ar XVII at up to 5 chords simultaneously. Inaddition there are conventional measurements of the ne, Te, totalradiation profiles, Zeff and other spectroscopic measurements. The neand Te profiles in the SOL and along the target plates are measured witha scanning probe and an array of Langmuir probes in the tiles [7]. Typicalconditions at the divertor plate which have been used for argon injectionexperiments, vary from Te=2 0 eV, ne=1x101 9 m -3 to Te=10 eV,ne=5x10 1 9 m -3 . The target temperature stays approximately constantfor most of the injection time but falls by 20% at 500 ms.33. ResultsThe response of the plasma to an argon injection is shown in fig 2.The argon is injected at 0.4 s for about 0.2 s and its concentration in theconfined plasma monitored as a function of time using the crystalspectrometer. The impurity concentration builds up over a period of 120ms to a nearly steady-state level. The density remains approximatelyconstant, Zeff increases slightly (8Zeff = 0.3) and the total radiationincreases by about 200kW as a result of the impurity injection. There is nodetectable delay (<10 ms) between the spectroscopic measurements alongdifferent chords at minor radii varying between r=0 and r=0.18m. This isconsistent with the measured cross field diffusion rates of impuritiesinjected by laser ablation into the confined plasma [8]. No argon had beeninjected for more than 10 discharges before shot 931014022 and it is seenthat the argon signal is close to zero before the impurity injection at 0.4 s.Argon injected through capillary B on the inner wall, shot 931014023,rose to the same equilibrium value and had the same time constants as theinjection into the private flux zone, fig 2. There appears to be a delaycompared to the injection into the private flux zone, but this may be due tothe longer capillary leading to the inner wall . The time delay is within theuncertainty in the calibration. It is notable that in shot 931025023 theargon signal rises before argon injection, to about 10% of the maximumvalue in the previous shot. This is attributed to the release of argontrapped in the walls during the previous shot. By comparing with a shot inwhich there is no argon injection it is found that the argon density decaysto the background level in a few discharges.The total impurity concentration is calculated using the MIST 1- Dtransport code [9], normalising the code results to the measured value ofthe Ar XVII concentration. The concentration profile varies, depending onconditions but is normally flat or slightly hollow. In a series of injections4with identical argon injections, into discharges over a range of densities, itwas found that the central argon concentration decreased with increasingne, see fig 3. In all cases the total number of impurity atoms in the coreplasma was less than 0.5 % of the total number of atoms injected. Theincrease in radiation due to the impurity injection obtained from the MISTcalculation is -80 kW and the increase in Zeff =0.25. The increase in Zeff isconsistent with measured value, fig2, but the PRAD is significantly lowerthan the measured -200kW. This discrepancy is not understood.At a critical plasma density the plasma detaches from the divertortarget reducing the power to the target plates by a factor of 10 or more [4].Such a mode of operation would possibly be advantageous in reducing thepower loading on the divertor target in ITER [10]. However the effect ofthe detachment is to increase the impurity radiation, as shown in fig 4. Inthis case a short (20ms) gas puff was introduced at 0.1 s and the argondensity in the main plasma was increasing approximately linearly from 0.4s. However at 0.61 s the plasma detaches from the target. This isindicated by the increase in dne/dt, fig 4a, and the sudden drop in the ioncurrent to the target, fig 4b. At the same time the density of ArXVIIincreases suddenly by a factor of over 2. Similar behaviour is observed forthe intrinsic carbon impurity lines, CV and CVI. This increase is thought tobe related to increased penetration through the SOL, due to lowerelectron temperatures.4. Discussion.The low argon impurity content measured in the confined plasma isa rather encouraging result. However similar experiments using neon inDIIID showed much higher fuelling efficiency [11]. The difference couldbe ascribed either to the differences in atomic physics, eg ionisation ratecoefficients, to the higher density in the C-Mod divertor or to the5difference of the divertor configuration, eg target orthogonal to fluxsurfaces in DIIID vs steeply inclined plate in C-Mod . Clearlyexperimental measurements with a number of different impurities need tobe done in the same machine under the same plasma conditions.The spatial distribution of the argon ionisation has been calculatedusing a simple Monte Carlo code. The ne and Te profiles obtained fromthe scanning Langmuir probe [7] are mapped onto the magnetic fluxsurfaces obtained from the EFIT magnetics code [13]. It is assumed thatboth ne and Te are constant on flux surfaces which is a reasonableapproximation at the lower densities used. The argon atoms arelaunched from the gas puff position with a cosine angular distribution anda Maxwellian energy distribution (T=O.1 eV). Using published ionisationrate coefficients [14] the position of maximum ionisation probability iscalculated for each trajectory. The results for the two launching positionsused experimentally are shown in fig 6. It is observed that all ionisationsoccur in the private flux zone for launch position A and in the SOL forposition B.5. ConclusionsThe present results indicate good screening for argon impuritiesunder the conditions in the Alcator C-Mod divertor, both for impuritiesinjected into the private flux zone and at the inner wall. The screeningimproves as the density increases and seems to be related to the highprobability of ionisation far from the separatrix. The screening isconsiderably reduced in shots where the plasma detaches from the target.On this basis the screening for gases with lower ionisation cross-sectionsis also likely to be less effective.Further experiments are desirable to investigate how impurity6screening varies with different impurities and with divertor conditions.More detailed calculations using fluid or Monte Carlo codes are alsodesirable in order to obtain a better understanding of the experimentaldata.Acknowledgements: The authors are grateful to the whole Alcator teamand in particular to the operations group, for the excellent operation ofthe machine. This work was carried out under U S Department of EnergyContract No. DE-AC02-78ET510130EReferences[1] P C Stangeby and G M McCracken, Nuclear Fusion 30(1990)1225[2] G M McCracken, U Samm, P C Stangeby et al, Nuclear Fusion,33(1993)1409[3] I H Hutchinson Physics ofFluids B, (1994) to be published[4]B Lipschultz, JGoetz, B Labombard et al, J Nucl Mater, this conference[5] H F Dylla, J Vac Sci Tech.,20 (1982)119[6] J E Rice and E S Marmar, Rev Sci Instrum., 61 (1990) 2753[7 ] B Labombard et al J Nucl Mater, this conference[8] M Graf et al Rev Sci Instrum. to be published[91 R A Hulse, Nuclear Technology/Fusion, 3 (1983) 259[10] G Janeschitz and K Borass, Nucl Fus (1994) to be published[11] D Hill, Proc 9th APS Topical Conf. on Atomic Processes in Plasmas,Dallas, TX, Sept. 1993 Ed. W Rowan AIP Conference series No[12] P C Stangeby, D Elder Nuclear Fusion[13] L L Lao, H St John, R D Stambaugh et al, Nuclear Fusion 25(1985)1611[141 M ALennon, K L Bell, H B. Gilbody et al, Atomic and Molecular Datafor Fusion Part II, Culham Laboratory Report, CLM-R 270, 19867Figure captions1. Schematic of diagnostics and the divertor gas fuelling system for argoninjection experiments. The position of all the gas puffing positions areindicated with arrows. However only A and B were used in the presentexperiments.2. Comparison of the time response of the spectrometer chords atdifferent minor radius for two discharges , 931014022, puffing into theprivate flux region, position A, fig 1, and 931014023, puffing at the innerwall, positionB, fig 1. Variation of the radiated power(a), and the Zeff, (b)are also shown. The central density is 1x10 20 m-33. Variation with density, ne, of the total number of argon atoms in theconfined plasma for a series of discharges with identical argon puffs of2x1019 atoms.4. Argon concentration as a function of time during a shot (931021018)which detached at - 0.61 s. In this case argon was injected into the mainchamber in a 30 ms pulse at 0.1 s, at a lower (uncalibrated) flow rate thanthe earlier discharges.5. Distribution of the calculated positions of first ionisation of thermalargon atoms launched with a cosine angular distribution and amaxwellian energy distribution (0.1 eV) into the private flux region of theC-Mod divertor, shot 931014022 and at the inner wall, shot 931014023.Flux surfaces are plotted from EFIT at intervals of 2.5 mm, measured atthe outer midplane8-NININNChannel crystal5 spectrometerViewing chordsIN432Gas pufflocationsused Fast scanningProbeA'Flush-Mount'Probes(16 toroidaltripleis)Figure 19800600 (a) total radiated powerV 40020003.02.5N 2.0 (b) Zeff (visible bremsstrahlung)1.51.0600- 600 - (c) channel 1400------ argon puff200 --- -------0 -------- r-----600-400 93(d) channel 2931014022200 -- - - 9310140230 -- -- - --- ---- - -'CO 1507 100 (e) channel 3-o 504040 - (f) channel 530 -20 -10 -~-00.30 0.40 0.50 0.60time (s) Pi "re10 ,a104 6 8Electron Density, ne, (101 m 3 )Figure 311108(09- 0Cr,0C0464-hI I I I202-2.5:2.0 .line-averagedelectron densityE 1.50N 1.0.50.00.6probe currrentS0.4 -0.2 -0.0200150o 100total radiated power5006E 4E - central argon densityC09 200.2 0.4 0.6 0.8 1.0time(s)Figure 4120.1eV, lonisation Position 9310140220.40 0.50 0.60R (meters)Ne(a)=2.7e19 Te(a)=33 eV LomdoN =10.4 mmFigure 513'I,+-0.20-0.30EN-0.40-0.500.70Argon

Impurity transport in the divertor of the Alcator C-Mod tokamak

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Impurity transport in the divertor of the Alcator C-Mod tokamak

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