Büchl, K.

Cayton, T.

Lang, R.

Sandmann, W.

Weber, P.

Wurden, G.

English

MPG.PuRe

1561 PELLET ABLAT ION IN T HE REVERSED FIELD PINCH AN D TOKAMAK, A COMPARISON G. A. Wurdeu oj , K Biichl, T. E. Cayton· , R. S. Lang W. Sandmann, and P. C. Weber" Max P lanck Inst itut fur Plasmaphysik, Association EURATOM-IPP D-8046 Carching, FRG I ntroduction: While modelling of pellet ablation in plasmas using the neutra.l shield-ing cloud generated by the pellet gives general agreement wi th tokamak experilllcnts,1-3 many detai ls are still not well understood . In particular, issues of ablation rate flu c-tuat ions, asymmetric cloud format ion, pellet deflection, and self consistent effects of nonthcrmal energy and particle Ruxes on the pellet (especially in auxiliary hCiltcd plasma.s) require fu r ther consideration. We compare t.he ablation of deuterium pellets with simi lar sizes and velocities in the ZT·40M reversed field pinch and ASDEX tokamttk. vVhile the two experimcnts have radically diffcrent plasmas , their effect on pellets is examined to illuminate simi~ larities and d ifferences in the pellet ablation physics. Photographs obtaincd both with still cameras and a fast gated·CCD video camera allow measurements of pellet cloud features, including intensity, cllrvature, velocity changes, cloud shape and st ria.tions. The shape of the pellet cloud ,md its light emission yield clues to processes in both the ablat ion layer and thermonuclear plcisma.',,5 Symmet.ry of the cloud is affected by the direct ion of t he electron drift a nd magnetic field lines.6 Gross track features , ranginp; from narrow, smooth and uniform, to broad and highly striated are noted, along with the correspond ing plasma conditions that produce these features. ZT -40M RFP: Injection of Deuteri um pellets (- 5x10 1!)D " atoms/pellct) at '·c-loci ties of 400-600 m/sce into the ZT-40M reversed field pinch at Los Almnos i gin!s deep penetration and large 6.11/11 refuelling. Typical pre-pellet plasma par;uuct('J"s are I", ...... 120-250 kA, ne'" 1.5_3x1013cm- 3 , Te(O) '""" 150-300 cV, and global TI': "" 150-400 J1.sec . Howcver, enhanced ablation in the outer plasma regions, strong('r from the electron drift side than the ion side, results in extreme trajectory cllI"v;tture.8 A small population of energetic electrons ("" 1-10 ke V) directed along magnet ic field lines from the electron d rift direction, would be sufficient to account for the obsverved mot ions assuming that Asymmetric ablation rates (of order 30-100%) cause rocket-like acceleration of the pellet . Images of the pellet taken every 100 JISec, with 5 I-'SCC exposures, show the pellet cloud is brightest in the p lasma edge region , with intensity decreasing through the plasma core.9 Lineouts of these images show bright centrally peaked emission clouds with typical F W HM dimensions of "" 1.5 cm. A weak comet-like tail is often visihle. The track gets narrower over the course of the pellet li fetime (",600-800 JISCC). The pellet trajectory curvature is generally reproducible, ·although succesive pellets IlUlY be sh ielded by the first, and have less deflection. 8 Highly deflected pellets undergo significant veloci ty changes. Depending on t he relative di rection of the pellet velocity and elect ron drift (magne tic field di rection ), the pellets are observed to speed up , or slow down in t he plasma. In extreme cases the pellet may be stopped, or cven have its trajectory turned around in the plasma! In the plasma edge region, where the magnetic fie ld is nearly poloidaI, the deflection is poloidal, whereas in the plas ma core region (with a p rincipally toroidal field), the deflections are more to~oidal. 1562 ASDEX Tokamak: A variety of pellet sizes arc possible in ASDEX,IO typically 4xlO l 9 ("small" ) but ranging to 1.4xIQ70D" atoms/pellet (" large"), with a typical pellet velocity (for the centrifuge injector ) of 570 m/sec. The plasma volume of 5.2 m 3 is about 5x larger than ZT-40M , and for ohmic conditions, a typical plasma is 10;'1 ..... 320-450 kA , ne::::: 1.5_4x lO ' 3 cm- 3 , Tc(O) ~ 1.0-1.5 keY, and global Tf.; "'" 60-80 mscc. We have recently installed the gated-CCD camera on ASDEX, and used it to obtain time resolved images of the pellet clouds. In situ pellet veloci ty remains close to that from the gun, although significant toroidal velocities can occur in the la ter part of t he pellet lifetime. Velocity measurements axe complicated by uncertainties in the pellct location within the dynamically evolving pellet cloud. Figure 1 shows a CCD photo (a), looking down on the pellet track from an angle of 75-45 degrees to the vert ical , and corresponding densitometer lineouts, as well as the wide-angle spatially inl.!'grated Dotime hi story (b), of the pellet ablation. The darker cent ral region in each bright cloud, with dimensions of2 cm diameter , seen in the lineouts of Figure l(b) , is difficldt to explain based on self absorption alone. Abo, bumps or protrusions of t he clond, ill t he direct ion of pellet motion , sugges t that the light emission is dynamically evolving, and is "left behind" as the pellet moves on. Prec ise location of the pel!et is nHl de difficult by the changing shapes of the light emi t1.i ng region. Cross fie ld dimensions of the cloud are'" 1 cm radius, while along the toroidal fie ld dimension, the elongated FWHM diameter is 5-7 cm, as seen in these 2JlseC exposures. Collapse of t he old emission region surrollnding tbe pellet is strongly suggested by t ime resolved photos on ASOEX , and has also been reported in TEXT.ll This darker cent ral region Ileal' the pellet is not seen in experiments where the luminous region remains "spher ic ill" . as was always t he case in ZT-40M , where t he pellct t racks were uniformly smooth. and striations were the rare except ion rather t han t he ru le. This may have also been the case in OR1."IAK6 An assortment of time-integrated ASDEX pellet photos arC' shown in Figure 2. The plasma separatrix is at r::::::: 40 cm, although the pellet light emission first begins a t. r= 42-41 cm, near the bottom of each picture. The field of vi ew extends rndially from r= 44 cm to r= O cm, and a reference picture with ruled 10 cm marks is l:ihowlI for calibration. Two pellet.s from a multi-pellet sequence in the same ohmic disd l<lrgc (#21143) are shown Fig. 2(0.) and (b). The first is at 1.0 seconds and the fpurth at 1.3 sec with a higher density of 7l e= 4.6x101J cm- 3 . For lower electron densities, and/or higher Te , the toroidal extent of the cloud is larger. Pellet trajectories arc st raighter, or curve in the ion dri ft direction with the appli cfltion of significant Neutra l Injt, ct.ion, as shown in Fig.2(c), shot #23216 with a "large" pellet , I~=385 kA, B~=2.2 1 T Hnd 2.6 MW of co-benms. One of the largest t.oroidal deflections seen for pelle t.s in ASOEX is shown in Fig. 2(d), where the 7th and 8th pellet (30 msec apart) ill a 380 kA weak NI shot #21559 (0.35 MW co-beams) can be seen turn ing nearly 90°. A transition between a relatively smoot.h ablation track, 'l.Ild an oscillRting , or stri ated track, is commonly observed in t he edge region, and can be seen in Fig. 2(a ) (if t he reproduction is good enough!). Ourst 1 ! has associated this transition frOll1 smooth to non-symmetric (and oscillating ) light emission with the q= 2 layer in the TEXT tokamak. We see no correlation with the smooth /oscillat ing t rack boundary to the q value. In particula r, beams force the oscillations to begin even closer to the plasma boundary t han the lIsual 5-6 cm depth in ohmic dischnrges, for the same q <It t.he edge. Also, ohmic discharges with q=1.95 at t he plasma edge, still have a well defined smooth t rack zone, before the striations begin . Disc ussion: The time development of the pellet ablation rate is de pendent on the type of plasma conditions that are encountered . It is st riking to compare the typical 1563 ohmic lokamak pellet ablat ion ti me history, wi th that of a pellet ablating in the ohmic reversed field pinch. Usually in the tokamak discharge, the pellet ablates at first slowly, t hen ever more rapidly, with a sharp falloff in time as the pellet is consumed. In contrast, t he RFP pellet time history is nearly t he opposite .... t hat is, the pellet light eril~ss ion is strongest near the edge of the plasma, and then decreases as it nears t he centcr. Tokamak discharges with large runaway content, or even neutral beam heating, also show enhanced edge ablation, I , l l Unl ike runaways in tokamaks, the energetic electrons in the RFP are ev idently not of high enough energy to cause bulk heating of the pellet (and hence its explosive demise), and instead t he pellet remains intact. Striations are a common feature in the tokamak, and even stellarator discharges.6,1,IJ However, uniform, smooth bending t racks can also occur in tokamaks when pellets are fired early during the currenL rise phase.s A model of a neutral cloud being al-ternately formed , and then left behind as the pellet edges out of the dense cloud shielding influence, lends itself naturally to an oscill atory explanation 'of the pellet ablation rate, An instability in t he neutral shielding cloud can evidently be triggered by as yet unspecified plasma conditions. The association of a dip in t he light intensity in the ablatant cloud , with the presence of t he instability, remains to be tested. Plas-masjpellets without st riations may hold the key to the instability trigger conditions. Wc note, that for sufficiently short energy confinement t ime ( ie, t he tokamak edge, or todays RFP plasmas) compared to the time the pellet spends in these regions , smooth ablation of the pellet should be expected, since the energy will "fiU in" locally faster than being depleted by the pellet . This may explain why in ZT-40M, st ri ntions were almost never vi sible, since the pellet cloud basically sampled the entire plasma energy flux during its lifetime, due to the short global energy confinement time of t he pinch. Also, conditions which bypass the neutral shielding cloud (ie, energetic fiuxes of particles), should also have smoother pellet ablatant tracks. This work is supported by the U. S. DOE and the EURATOM Association . • Los Alamos National Laboratory, Los Alamos, NM 87545, USA t Present address: JPP, Carching , FRG IS. L. Milora, Journal of F\lsion Energy 1, 15 ( 198l). 2M. Kaufmann, K. Lackncr, L. Lcngye1, and W. Schneider, Nuclear Fusion 26 , 171 ( 1986). 3W. A. Hou1berg, S. L. Milora , and S. E. Attenberger , Nuclear Fusion 28 , 595 ( 1988). 4D. H. McNeill, C. J. Greene, and D. D. Schuresko, Phys. Rev. Lett. 55 , 1398 (1985) . sTFR Group, Nuclear Fusion 27 , 1975 (1987). 6C. A. Foster, R. J. Colchin, S. L. Milora, K. Kim , and R. J. Thr nbill , Nuclear Fusion 17, 1067 (1977). 7G. A. Wurde n, P. G. Weber, R. G. Watt , et al. , Nuclear Fusion 27 , 857 (1987). dG . A. Wu rden, P. G. Weber, R. G. Watt, C. P. Munson, T . E. Cayton, and K. Buchl, in 1711. School of Plasma Physicj, Course and Workshop 071 Ph y.~ ics of Mirrn rs, RFPs and Compact Ton, ed ited by S. Ortolani and E. Sindoni. Vo\. 1, p. 411 (Socie t.a Italiana Di Fisica, Varenna, Haly, 1987). 9C. A. Wurden, P. G. Weber, C . P. Munson, et aI., in lAEA Tu h. Corn. Meet ing on Pellet/njce/ion and Toro idal Confjnemc Ilt, p. (G ut isillg , Bavaria, FRG, 1988) [Oct . 24-26, 1988). IO W. Amenda and R. S. Lang, J . Phys. E: Sci. Inslr. 19,910 (1986). 11 R. D. Durst, Tech nical Report FRCR 302, University of Tex as (1988) [Ph.D. Thesis) . 12K . Ouchl, G. C. V1ases, W. Sandman n, and R. Lang, Nuclear Fusion 27 , 1939 ( 1987) . IlK . P. Bu ch1, WVlIA Team, NI Team, and ECRII Team, Technical Report IPP t /238, (1986) . , .... #:: I , , , + 'to CM t 1564 "0 [mmll a) Toroidal S cale b) Figure 1: Deuterium pellct (4x1019 DO atoms) al 570 m/scc. ablating in an I ~ = 380 kA diverted ASDEX ohmic shot #26082. (a) Multiply-gated eCD pic ture (2,lsec ex posure every SOpsec) . (b) Lineouts of the elongated (along field lines) cigar-shaped emission clouds show di mi nished emission in the central region of the brightest douds, and Dnemission on the dual time/penetration depth axis. Figure 2: Top-view, tilll( integrated ASD£X pellet photos. (il l Qhlllie 1.=350 kA , D,,=2A9 T , 11.=3A:d01!lcm - 3 , "small~ pelle ls. (a) first pellet and (b) fou rt h pellct. The striation (oscillation) fr equency is about 7& kill . (cl 2.61\'IW Neutral injeclion , cm vat urc in ion drift lli t ec t~oll . (d1 St ro l ~gesL pellet cu rvalure seen in ASDEX. (e) Reference photo, with nl.l.ilal callbra-l ion rou, marks every 10 Clll . The diameter of the rou is 28.5 mm . T he r= <10 cm position is at th e inner edgc of the black mark , jU;'i t abovc t he inver ted "3~ on the rod. Due 10 lenses, th('" photos arc invertcJ lert- right , so t he plasma cur rent and toroidal field d irections appear as if oriented from right to le rt , and drifting electrons illlpact the pelle t doud preferen -t ially from the left. side. The hori7.0nlal cu rved arc is light fr om t ile 10wI'r di \"erlor th roat. 

Pellet ablation in the reversed field pinch and tokamak: a comparision

https://pure.mpg.de/pubman/item/item_2227072_2/component/file_3374529/EPS_paper.pdf

Pellet ablation in the reversed field pinch and tokamak: a comparision

Abstract

Similar works

Full text

Available Versions

MPG.PuRe