Büchl, K.

Gruber, O.

Haas, G.

Kaufmann, M.

Lang, R.

Sandmann, W.

Vlases, G.

English

MPG.PuRe

- 78 - AOMol]20M Quasi Stationary 02 Pelle t Injection i nto ASDEX Divertor O~s G. Vlases* , M.Kaufmann, O.Gruber. G.Haas; Pe llet Injection .Team : R.S.Lang, K.BUchl, W.Sandmann; ASOEX Team Hax-Planck-Institut fUr Plasmaphysik , EURATOM Associat i on, 0-8046 Gar ching , Fed. Rep. o f Germany Intr oduction We report on deuterium divertor discharges in ASDEX where the par-tic le invent ory is sustained entirely , after an initial gas fuel -ling phase, by the injection of a series of up to 80 frozen 02 pellets, 1 mm i n diamete r and 1.2 mm l ong (NO= 4.5 x 1019 ), at intervals rangi ng from 20 to 50 ms , and speeds to 720 m/ $, by means of a centri fuge /1/ . Investigations performed on Ohmic, NI, and I CRH heated discharges have shown that the most impor tant pa-rameter characterizing the d i scharges is the rat i o of t he pe l let penetr ation depth ,A, to t he fall - off length f or neutral parti -cles in a gas fue lled case (typically 5 - 8 cm in ASDEXI . For shallow penetration, only minor di fference s , relative to gas fuelled shot s , are observed. In contra s t, with deep penetration ( e . g . ~2 a/2) the density profile is mor e peaked, and the tempe ra -ture profil e f l attened , -when compared with gas fuell ed discharge s at the same line integrated density, fie- The discussion here con-centrates on these characteris t ic "deep fue lling discharge s". Be -cause of the limited pel l e t s ize and ve l ocit i es pr esent l y avail-able, these deep fuel ling discharges are Ohmically hea ted with Te ~ 1 keY . In t he f ollowing par agraphs we discuss energy and particle con finement, aspects of recycling, and density limits. Enerqy balance Figures 1 and 2 show the time dependence of several plasma parame -ters for a r epresentat i ve deep penetration (2 0 cm), ohmica lly hea -ted , pellet fue lled discharge (11 14669 , q '" 3 .4, B -= 2.2 T). In this case the density was first increased to ne : 1 x 10 19 m- 3 ' at 500 ms by a gas valve, which was then turned off. Pellets were in -jected at 35 ms interva l s , beginning at 522 ms , causing the cycle -time - averaged density (nelc to increase to 3 x 1019 m-3 . When a pellet enters, the temperature dr ops r apidly throughout the plasma / 2/ ; the density peaks continuousl y within a given pellet cycle , and is more peaked for later cycles i n the series. The neutron fluence drops and recovers, as Ti r ecovers. We have analyzed individual pellet cycles, using both a transport code /3/ based on measur ed time- dependent· Te , ne ' and Pr ad pro-files , and global magnetic measurements, and cycle-averaged agree -ment is obtained. The results of such analyse's for the 12th pellet cycle of shot 14669 are given in fig . 3 . It can be seen that Bp increases throughout the cycle, and drops discontinuously at the time of injection of pell et H 13, indicating a non-adiabatic in-jection, with a loss o f about 2 kJ plasma energy . Such a l oss at * University of Washington , Seattle . Nork supported by US OOE! OFE. - 79 -each injection event constitutes ~time-averaged l oss of up to 100 kW which lowers the value of (tE)cycl = (1/t. t)/ '1:":;dt, from the 55 ms measured within the cycle , to about 45 ms averaged over the cycle + injection, in approximate agreement with the long-time- averaged (several cyc l e) global magnetlcs value. This should be compared with ~E = 74 ms obtained in a gas fuelled comparison shot (N 14671) with the same time-averaged ne(t) (see fig. 1). The local transport analysis shows that within err or bars,J:e and Xi are unchanged between the gas fuelled and pellet fuelled dis-charges:~e corresponds to the normal scaling observed in ASDEX / 4/, while Xi is one times neoclassical. The analysis further reveals that three mechanisms are responsible for the reduction in total energy confinement seen in these deep fuelled, peaked discharges: (1) There is an enhanced convection of energy from the inner to outer plasma regions due to the cycle-averaged net-outward particle flux rp, which must equal the pellet fuelling rate. For the cycle discussed here, the convective power is about 150 kW at r : a/2. (2) There is increased radiation, con-sistent with the peaking of the density profile (Prad increases from 106 kW (N 14671) to 150 kW (N 14669). (3) The non-globally-adiabatic pellet injection constitutes a time-averaged loss rate of about 100 kW for the quasi-steady phase of N 14669. Particle Balance For the deep ' penetration case described above, the e£uivalent par-ticle supply rate is about 60 % of that required for the gas fuelled comparison shot IN 14671) . The divertor plasma and neutral densities remain consistently below' that of the 'comparison shot, reaching values of about 80 % and 50 % near the end of the 1 .2 second injection phase. The flux o f partfcles leaving the plasma seems to fall rapid l y after a first very short burst, resulting from the non-adiabatic injection, and starts to recover about 20 ms after injection.This suggests that an enhanced inward convection may be present early. in the cycle. The effective particle confinement time, r eff : N/N, is about 130 ms at the beginning of the cycle and 100 ms at the end, during the quasi-stationary phase of shot H 14669. The time averaged true particle confinement times , estimated from the re-cycling flux and particle supply rate, are about 120 ms and 45 InS for the pe llet and gas fuelled shots, respectively . Plasma boundary and divertor The plasma boundary density shows a sharp peak (At ~ 5 ms ) imme-diately after injection, near the separatrix, which then is follow-ed by a pronounced decrease of the density fall - off l ength, which recovers later in the cycle. The plasma density in the dive.t;tor likewise shows a sharp spike followed by a pronounced decay, and finally recovery (fig. 2): These observations are in qualitative , agreement with the ASDEX edge-layer theory described in ref. /5/. Density limit A density limit in Ohmic pellet shots can be attained by increasing the ~e at the time injection begins, and/or by increasing the pellet injection frequency. The r .esul ts from a limited number of such discharges~are.· (1) Peak densities 20 - 40 % higher than -in - 80 -gas fuell ed discharges at the same q(a) are obtained before dis-charges i s accompanied by very strong central peaking of density and radiation (neo ~ .8 x 10 20 at q = 3.4, Ip = 310 kA; Prad(O) = .35 W/cm1 ), and collapse of the central temperature. In contrast to the situation in gas fuelled density limit discharges /6 , 7/ , strong and asymmetric radiation from the boundary is not observeq and strong disruptions do not occur . Sometimes the dis -charge recove r s to its pr edis turbance state . The observations are in line with the interpretation of the usua l density limit as a boundar y density limit. With pellet refue lling it is possible to decouple the central density from its boundary limit. The density limit in pellet fuelled NI discharges could only 'be reached by suppl emental gas puffing . As expected, no characteristic diffe -r ences from gas fuelled shots were found. Conclusions Deep penetration pellet fuelled ohmic discharges in ASDEX reach higher density limits and require a lower particle supply rate than gas fuelled discharges . The peaked profiles , along with s lightly non-adiabatic injection events, however, lead to a degra-dation of energy confinerrent, caused by enhanced convection and radiation . ACknoWled~nt \</e wish to Drs. K.I.ackner and F.Wagner for he l pful discussions . /2/ /3/ /4/ /5/ /6/ /7/ R. S . Lang , J . Phys. E: Scient. Inst . (to be published) G.Vlases, K.Biichl, D. Campbel l, "Proc. 11th Europ . Conf. on Contr. Fus. Plasma Physics",Vol. 1 , p . 127 (Aachen , 1983) R.Hawryluk, in "Physic s of Plasmas Close to Thermonuclear Conditions", Vol. 1 , Varenna (1 979 ) (EU R-FU-BRU/XII/476/80 ) O.Gruber , "Proc. Invited Papers, Int. Conf. on Plasma Physics" , Lausanne (1 984 ), Vol . 1 , p. 67 Y.Shimornura et al . , Nucl . Fus. 23 , 869 (1983) H.Niedermeyer et al., these proceedings , paper N 195 J. Neuhauser et al., these proceedings, paper N 158. • • !·0!4 ,." =.\;c---;;,o .• o--~o~.;---;o.~ ,Iml o~~~~~--~~--~~~~ __ ~~ C 200 LOO 600 800 1JOO I [ms ~ FiS. · 1: Density vs. time for a pe llet (14669) and a gas fuelle d comparison (14671) shot. Gas valve c losed at 500 ms f or pellet shot. Inset shows ne {r) for both at 91 4 ms . Neulron fl ux 150 [10' s·' 12 - 81 -100~'~~~~~~=C~~~~~~~~~ re lev 500 H.la.u.) ." '" ,,. ~60 t I ms I 1000 Fig . 2 : Representative diagnostic traces . shot 1 4669. ~~light spikes indicate pellet injection times . 80 .310 70 -.300 c 0 ~ _60 ~290 ;. ] , g 0 ':50 1280 B" E -- 0 el, O ::'.270 0 w "'-\-' 30 .260 0 0 .250 900 910 920 930 tp In I , [ms] F ig. 3 : Time dependence of Bp.t'E , P~'l..\"ithin 12th pellet cycle , sho t 14669. Note: ~E is 48 ms when non- adiabatic injection is included . For gas comparison shot : Bp = .300. PJl.= 30,,) k\v .t'E = .74 ms . 

Quasi Stationary D2 Pellet Injection into ASDEX Divertor Discharges

http://hdl.handle.net/21.11116/0000-0008-FAC1-4

Quasi Stationary D2 Pellet Injection into ASDEX Divertor Discharges

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