Bosch, H.

Coster, D.

Dux, R.

Haas, G.

Kallenbach, A.

Lackner, K.

Neuhauser, J.

Pena Hempel, S.

Schneider, R.

English

MPG.PuRe

1425Helium Exhaust and Transport in ASDEX UpgradeH.—S. Bosch, D. Coster, R. Dux, G. Haas, A. Kallenbach, S. de Pena Hempel,K. Lackner, J . Neuhauser, R. Schneider and the ASDEX Upgrade TeamMax—Planck—Institut {1'11 Plasmaphysik, EURATOM Association, D-85748 Garching1 Introduction 1 .One of the important requirements for steady state operation of a fusion reactor is suffi— ‘cient exhaust of the fusion—produced helium from the core plasma. This, however, involvestwo separate but linked processes, namely radial transport on closed flux surfaces in thecore plasma and scrapeoff layer (sol) transport into the divertor, including high divertorretention to optimize the pumping efficieny by increasing the helium density in front ofthe pump duct.2. Core helium transportThe global helium exhaust time in ASDEX Upgrade has been shown to decrease stronglywith increasing divertor neutral gas density[1], and this is attributed to improved divertorretention due to neutrals recirculating from the divertor chamber. Figure 1 compares thehelium transport coeflicients as deduced from CXRS~measure1nents for two CDH—modedischarges (with neon cooling and detached divertor plasma) with different values of theneutral gas flux density (as a measure of theneutral density) in the divertor chamber. In 35 , j ,the discharge with the higher neutral flux den- 3 _ diffusion coefficient .sity (#6131, (350 = 5.4 X 1022 m‘zs"1), the 2.5~ DHQ [me/Si ‘global helium confinement time, normalised to 2 _. : ° _the energy confinement time (ME: = "rm/TE) ;is 25 % smaller than in the discharge 15- ' #613§ ' _with lower neutral gas flux density (#6136,¢0 = 4.0 >< 1022 m‘zs'l), namely 6.4 comparedwith 8.4.The core transport coefficients, hovvever, arepractically identical, as shown in figure 1, asthe slightly higher D3,, in #6136 is in principleofl'set by the Vin also being larger. The quan—titative similiarity of core transport in thesedischarges was shown with the STRAHL code,including a simple scrape-off layer and diver- . . . a a 3 s 3 a o o ,3 otor model [2] To reproduce the experimentally 012 0.4 0.6 08measured decay times of the helium density, Ppo,different sol and pumping time constants have Figure 1‘, Helium transport coefficients forto be used for bOth d1scharges, expressing the two CDHumodc discharges with difi‘erent di-better sol transport in #6131. when us1ng thetransport coefficients from figure 1 for the core.Using these core coefficients, and identical timeconstants for sol and divertor, both discharges show identical helium decay times. Thisproves that the improvement in helium exhaust is solely determined by the physics in thescrape-oft layer and divertor.C in.1 ;o o- c a a a. a. 09 oooo o.320 0') _. (D _L' inward drift velocityv " [m/s] 0#813}?o. o- ..u;:.,o°#°6131oummemmuwoI1111C7vertor neutral flur densities.14263. Scrape—off layer transport and divertor retentionIt has been shown previously in ASDEX Upgrade, that the improvement in divertorcompression with increased neutral gas density in the divertor chamber is due to thisgas density, or the corresponding particle flux out of the divertor onto the divertor leg(internal recirculation), and not due to the externally fed deuterium flux in the mainchamber [3] That means, it is not the particle flux in the scrape—off layer that flushesthe impurities into the divertor, but the internally recirculating fluxes in the lower partof the scrape—off layer, namely below the divertor baffle ring which is installed in ASDEXUpgrade at about the height of the X—point.This physics picture was confirmed by 2d—modelling of the scrape—off layer and divertorplasma with the SOLPsucode package [1,4]. Not only do these simulations show the 'behaviour seen in experiment, namely the divertor compression increasing With divertorneutral flux density, and the compression of 7neon being higher than that of helium, but 1,4_ ‘ ' ' ' #7955.'7986.7'986,799~I3 “they also show that in higher density plas- 1 2 ' 1MA,-2.5 T. smw . _mas the helium {and neon) neutrals from 5‘the divertor plate are quickly ionised, and Z 1 ' §\§ Helm“ ’in ASDE‘X Upgrade they are transported 3: 0‘8- i -radially by an almost “diffusive” process g 05. RV \ \ -towards the outer edge of the scrape~off ‘3 04" New, _layer and into the divertor chamber. In 02» _this outer edge the impurity compressionis determined by the deuterium flux to— V ' ' . ' ' 'wards the target plate, which is built up ‘ i i Neon ‘by the neutrals streaming towards the di—vertor plasma, as was shown with the modelling [1]These results which have been observedin CDH—mode plasmas [3], are differentfrom similiar experiments in DIII—D H~mode plasmas [5], Where an improvement 1 . I . 1 ‘ . 1in divertor compression was found with 0 2 4 6 8 10 12 14increasing net particle throughput in the effectivg pumping speed 502 [ms/5]scrapeuoff layer. The influence of ELMs hasbeen discussed as a possible reason for thedifference, and figure '2 shows the resultsof experiments in ASDEX Upgrade withtype—I ELM H~rnode plasmas (1,, = 1 MA,13, 2 2.5 T, PNBI = 5 MW). For afixed divertor neutral flux density ¢o,diu,the pumping speed was varied (by closingvalves to the turbomolecular pumps on ashot to shot basis), and the external gasflux was varied accordingly to keep 450mconstant (feedback-controlled).These experiments again show, that in ASDEX Upgrade the internal recirculation of5 HeliumcompressionfactoriFigune 2: Exhaust rates (top) and compres-sion factors (bottom) for He and Ne as a func-tion of the pumping speed, The ertemal dearterium flux into the main chamber was varied tokeep the divertor neutmlfiux density in all dis-charges constant. While the exhaust rates de:crease with increasing pumping speed, and netparticle throughput, the divertor compression isindependent of these two correlated parameters.1427neutrals from the divertor region is responsible for the divertor compression of neon andhelium. This is understandable from the above mentioned 2d modelling which shows thatthis internal flux in the region below the X~point is much stronger than the net particleinflux, and therefore one can expect it to be the leading force. It also shows, however,that the geometry of the ASDEX Upgrade divertor plays an important role. The battlering is located about 14 cm above the outer divertor plate, i.e. the length of the divertorleg where the neutrals can again be ionised and where they can enhance the plasma flow,is rather large, and covers the whole region below the X-point.In DHI—D the baffle ring is very close to the divertor plate, and the particle fluxes into ‘the divertor chamber as well as the reflux of neutrals are strongly reduced. Therefore thescrapeoff layer transport of impurities as discussed above-for ASDEX Upgrade will notwork, and the smaller effect of friction drivenflow might become the leading force, This has 1,4to be confirmed by 2d modelling, but could eas- z[m] .ily explain the differences between the two ex~periments.As discussed above, the divertor geometry inASDEX Upgrade plays an important role fordivertor compression of impurities. This alsobecomes evident when the magnetic configura-tion is varied, as seen in figure 3. Very simileiar plasmas have been performed with different 0.0 --plasma shape. The separatrix of the standardequilibrium with a low triangularity is shownas solid line in figure 3, the separatrix or" themedium triangularity plasma as a dotted line,These higher triangularity plasmas Show gener—ally better energy and particle confinement, butthe two discharges shown here are almost iden—tical in global parameters, as shown in figure 4.The separatrix density and the average sol den— 4.4sity in #8197 are only slightly smaller than in 0.8#7492, as it is the case for the main chamberneutral gas flux density ¢O,M€dplcne- Also the Figure 3: 'Magnetic configurations forplasma parameters at the target plates are sim— the standard “‘55 7'“ ASDEX Upgradeiliar, but there is a large change in the divertor (”#74921 “W fine) and ‘2 higher triangu-neutral flux density dodgy, and in the deuterium larity plasma (#3196: dot line}compression Czefiodw/eiomdpgam. Modelling ofshort Ne puffs in both discharges with STRAHL [6] shows that the Ne—compression in#8197 is almost negligible, and the reduction compared with #7492 is much larger thanexpected for the lower (350.01% [7]. The shift of the X-point to smaller radii opens a gapbetween the scrape-off layer and the baffle ring, and thereby destroys the compression ofdeuterium as well as of the impurities. ‘ 'R[n1]14284. ConclusionsIt has been shown that exhaust of noble gas impurities (helium as well as neon) in ASDEXUpgrade is determined by transport in the sol and by the divertor compression. This canbe influenced by the neutral gas in the rather open divertor chamber (in divertor 1}.Variations of the mag--netic configuration, Where ”Wm. MW MIt 7 1 rem-sthe X~point is shifted to TV ~3 0 ‘smaller radii and the re - J! '4 . .. ., _ ,1gionbetweensolandhaf— E'l-Hl‘H'H’V‘ I1,,l.,.i...1.,.l..l LuL. 4 . Fred ltfi E; IS -3lie ring 15 opened, deteri— ‘1""‘T”"T"“ “W 50 ms 10 m1&3:.1Alums:l l‘ ‘plCluluorate strongly the dive!“ - l’- ‘‘ - ‘l ,. . m” .. u1 ‘ J ;‘|mI ‘i’\’.._,,,m.’..,.lu,‘ 1, ~ ml WWW!"as well as of deuterium. w m synergism 1922mr254From B2—EIRENE mod I {ii-Mm W, ~ailing of the divertorcompression of heliumand neon, it is clear thatthe geometry of divertorI played an importantrole for this mechanism, Wwhere internally recircu« ' 'lating deuterium fluxes _: .determine the divertor “HE’Hl'Hha' ”‘ "compression, lime lSlFigure 5’: Plasma parameters for the discharges shown in figure 3 .l||||11|lllrlll|llltime [5]Therefore differences inHe tranSpm-t are ex~pected for the modifieddivertor II of ASDEX Upgrade which will start operation soon. The new divertor willhave vertical target plates which are LYRA-shaped to distribute the heat fluxes morehomogeneously. Additionally, the private flux region is equipped with a. dome—shapedbaffle to prevent large influxes of neutrals to the main plasma. through the X~point and 1to increase the neutral density in the divertor chamber for a given plasma flow towardsthe divertor. 2d—m0deiling of this divertor geometry predicts a better He compressioncompared with divertor I. mainly due to the vertical orientation of the target plates.References[1] Bosch, H.-S. Particle exhaust'in radiative divertor experiments Jour. Nucl. Mat . in press.‘[‘2 ] Dux, R. et at, Plasma Phys. Controlled Fusion 38 (1996) 989.[3] Bosch, H,~S, et al.1 Phys. Rev. Lett. 76 [1996) 2499.[4] Caster, D. P. et al., BQVEirene modelling of ASDEX Upgrade, Jour. Nncl. Mat. in press.[5] Schafler. M. et ah, Direct measurement of divertor neon enrichment in DIII—D, Jour. Nucl.Man, in press.[6] Kallenbaeh, A et al., this conference.[7] Bosch, H.—S. et al., Particle exhaust studies in ASDEX Upgrade, submitted to PPCF.

Helium Exhaust and Transport in ASDEX Upgrade

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Helium Exhaust and Transport in ASDEX Upgrade

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