A centrifugal injector was used to inject deuterium pellets (with 3--5 x 10{sup 19} atoms) at approx. equal 600 m/s into current-less, nearly shear-less plasmas in the Wendelstein 7-AS (W7-AS) stellarator. The D pellet was injected horizontally at a location where the non-circular and non-axisymmetric plasma cross section is nearly triangular. Visible-light TV pictures usually showed the pellet as a single ablating mass in the plasma, although the pellet occasionally broke in two or splintered into a cloud of small particles. The density evolution following pellet injection and the effect of pellet injection on energy confinement and fluctuations are discussed

Baldzuhn, J.

Baylor, L.

Fiedler, S.

Hirsch, M.

Kühner, G.

Lyon, J.

Weller, A.

English

MPG.PuRe

Analysis of D Pellet Injection Experiments in the W7-AS Stellarator

Lyon, J. F.

Baylor, L. R.

Max Planck Society eDoc Server

Kuehner, G.

UNT Digital Library

* a  L V  1 * “The submitted manuscript has been authored by a contractor of the US. Government under contract No. D E - A C 0 5 - 8 4 0 R 2 1 4 0 0 .  Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow $hers to do so, for U.S. Government purposes. 0 CQ 0 Analysis of D Pellet Injection Experiments in the W7-AS Stellarator* J. F. Lyon and L. R. Baylor Fusion Energy Division Oak Ridge National Laboratory Oak Ridge, Tennessee, U.S.A. and J. Baldzuhn, S. Fiedler, M. Hirsch, G. Kuhner, and A. Weller Max-Planck-Institut fur Plasmaphysik EURATOM Association 85748 Garching, Germany 43-3 ckl 0 u3 European Physical Society Meeting €33 Berchtesgaden, Germany 43-3 7 to be presented at the 9-13 June 1997 7 “Research sponsored in part by the U.S. Department of Energy, under contract number DE-AC05-960R22464 with Lockheed Martin Energy Research Corporation. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, process, or service by trade name, trademark, manufac- turer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. . Analysis of D Pellet Injection Experiments in the W7-AS Stellarator J. F. Lyon and L. R. Baylor, Oak Ridge National Laboratory, Oak Ridge, TN, 37831 U.S.A. J. Baldzuhn, S. Fiedler, M. Hirsch, G. Kiihner, and A. Weller. Max-Planck-Institut fur Plasmaphysik, EURATOM Association, 85748 Garching, Germany Pellet Path -40 I"....---; L 170 180 190 .200_ 210 220 Major Radius (cm) '4 t N 9 -.2 -.l .O .1 Teff [ml  Fig. 2. Thomson scattering profiles from before pellet injection (highest T, , lowest ne curves) to 18.5 ms afrer the pellet (second highest T, curve). A centrifugal injector was used to inject deuterium pellets (with 3-5 x 1019 atoms) at =600 m / s  into currentless, nearly shearless plasmas in the Wendelstein 7-AS (W7-AS) stellarator. The D pellet was injected horizon- tally at a location where the noncircular and nonaxisymmetric plasma cross section is near- ly triangular (Fig. 1.) Visible-light TV pic- tures usually showed the pellet as a single ablating mass in the plasma, although the pel- let occasionally broke in two or splintered into a cloud of small particles. Density Evolution Following Pellet Injection in W7-AS Figure 2 shows Thomson scattering pro- files for the electron density ne and tempera- ture Te immediately before and at times short- ly after pellet injection (at 0.4 ms, 0.5 ms, 0.65 ms, 5.7 ms, 7.6 ms, and 18.5 ms) for a neutral-beam-heated plasma. Pellet injection leads to a rapid (< 400-ps) rise in ne in the main part of the plasma and to a slow increase in the plasma edge on a longer (~20-ms) time- scale. Microwave reflectometer measure- ments of the time delay (which reflects the density gradient) and FIR interferometer measurements of the line-average density also indicate that the density rises in <500 ps (Fig. 3) and then remains unchanged. The same behavior is seen in the soft X-ray inten- sity profiles in Fig. 4 (with 1.4 ms between curves); this signal is more influenced by den- sity than by temperature because there was no m 2 . 4  5 1 . 6  * 0 . 8  a 0  0 E-1.6 -2.4 - . -0 .8  .- e - 1  0 - s t  0.12 I 0.122 0.124 0.126 0.128 0.13 time (s) Fig. 3. Time delay and fluctuation level from a microwave reflectometer, MHD oscillations from a Mirnov coil, and line-average density around the time of pellet injection. X-ROV lntensitv (Lower Cornera) # 33714 Tlrn. (.) 1 ,29507 2 ,29646 3 ,29785 4 29924 5 .sW63 6 ,30203 7 ,30342 9 .30620 10 .so759 1 1  . W 9 8  a . m ~ i  -20 -io 0 10 20 Rodius (cm) Fig. 4. Soft X-ray intensity profiles from 6.5 ms before pellet injection to 8.4 ms after a pellet. filter in front of the soft X-ray diode array. Neutral lithium beam measurements of ne at radii r >12.5 cm (Fig. 5) show that the outer density rises slowly after pellet injection (on a 60-ms timescale), presumably due to recy- cling from the wall. The increase in ne de- creases with distance into the plasma. Pellet ablation calculations using the stan- dard neutral gas shielding model and the ne(r) and Te(r) profiles before injection indicate that the increment in density should be peaked. However, this is not seen in W7-AS on the timescale of any of the measurements. The change in the ne(r) profile in Fig. 2 is not diffusive. If it were, then the particle diffu- sivity would have to increase by a factor of >lo0 for a relatively short time (e400 ps, about the transit time for the pellet through the plasma). There was not a significant loss of injected particles during the fast density change: comparing the number of particles in the pellet with the increase in the number in the plasma after pellet injection indicates that >60% of the pellet is retained in the plasma for times >20 ms. This behavior, where the density profile after pellet injection assumes the shape of the preexisting profile without significant loss, is not seen in tokamaks. Either the ablation model is not correct and the deposition profile is the same as the initial density profile for different initial densities and field values, or a fast density redistribu- tion occurs in these experiments in W7-AS. 1 0 l " ' l " ' l " ' l " ' 1 " ' 1 " ' l  r (cm) 12.5 --E -13 - * -13.5 - - X - - 1 4  . - + -  -14.5 --A- -15 --0--15.5 +-.-I6 --*--16.5 .... h... 17 +18 - -19 --ht -20 - - m -  -22 0 . 2 6  0 . 2 8  0.3 0 .32  0 .34  0 .36  0.38 .zi:$i t (SI Fig. 5. Increase in the plasma density in the edge region from 30 ms before to 70 ms after a pellet. I .  Effect of Pellet Injection on Energy Confinement and Fluctuations in W7-AS Figure 2 shows a rapid drop in Te after pellet injection because the stored energy W does not change immediately. Although the ne(r) profile remains at the new (higher) value for the duration of the discharge following pellet injection, Te(r) gradually returns toward its preinjection value, indicating an increase in the energy confinement time TE. This is illus- trated in Fig. 6 where Wand TE increased by a factor of 2.7 for a factor of 1.7 increase in ne. From the W7-AS TE scaling expression [l] w = P7Ew7-As = ne0.5P0.46, the increase in ne should lead to a factor of ~ 1 . 3  increase in the stored energy because P = Pabsorbed is ap- proximately constant at the densities in these experiments. The additional factor of 2 im- provement in TE is due to an increase in con- finement: 7E/q3W7-AS is 0.7 before the pellet and 1.4 afterward. In Fig. 2, the confinement improvement (1.35) is less: TE/TEW7-AS was 0.8 before the pellet and 1.1 afterward. A few-ms quiescent period is sometimes seen in the MHD signals on Mirnov coils fol- lowing pellet injection when the stored energy starts to increase, as seen in Fig. 3. In all cases, the Mirnov loop signal (often associa- ted with degraded confinement) is reduced, and TE/q3W7-AS is >I after a pellet. LOW den- sity fluctuation levels, followed by a decaying high-frequency burst in several discharges, were seen with the microwave reflectometer after the rapid density rise (Fig. 3). These oscillations are correlated with the Mirnov coil signal. An example of pellet injection sup- pressing MHD activity for a longer period with a resultant increase in TE is shown in Fig. 7. Without the pellet, the MHD activity and the associated enhanced energy loss per- sist for a longer time. It is thought that the pellet rapidly depletes the fast particle popula- tion that drives the instability [2]. I . _ . .  I . . . _  I .8 ‘ NBI(MW) 1 i 0 100 200 300 400 500 Time (ms) Fig. 6. Response of the plasma to pellet injection at t = 300 ms. # 32564 -pellet d EUdt (TIS) p-p .20 .25 .30 .35 Time (s) high-P phase (2 top traces) is suppressed by pellet injection ( 2  bottom traces). Fig. 7. MHD activity that retards transition to a The soft X-ray (and MHD loop) signals in Fig. 8 show short-duration oscillations (for followed by slow (650-Hz) oscillations that 300 200 ~ 3 0 0  ps) immediately after pellet injection, zoo 100 7o damp out after a few ms and are in phase across the plasma (m = 0). The phase veloc- ity of the slow oscillations indicates a distur- 70 7o 70 bance propagating outward at 350 d s .  The initial burst is coincident with the fast change 80 6 - in n, in the bulk of the plasma seen in Fig. 2. C '  I' 80 5 E -  BO a 0 -  If the rapid change in ne(r) is due to a fast E density relaxation rather than an anomalous 80 z be evidence of an instability that causes the fast radial spreading of the high-P pellet blob. - 70 7o particle deposition profile, then this burst may 70 6o 60 The magnetic field has low shear, a relatively 100 200 ~ weak radial gradient along the pellet path, and .& coil jL....--,.&----c- 29 7.9 28 6.6 27 5.4 -,-Ji +,L*/->--. 26 4.2 ' ,-.-.--- 25 3.0 A. ,--/-------- ..- .---L WL_- .A* \.,.--.---- .-h G.,. -+%..,A. -L,' u1. ./-' ________I 24 1.9 /.------/.------- 23 .8 22 -.2 "I.?.-/ /.-b#-..---.- -_- //-..\-,/-----.------~- 21 -1.3 .-,.- ,/-~------~-- 20 2.3 ,'-\--.--..---------. _.I' -. -.,/ 19 -3.3 18 -4.2 17 -5.2 16 -6.1 15 -7.0 14 -7.8 -' +...- /---.---- 13 -8.7 --.- ,p ..-- -------- ,/--.,/"-_-.--~--~ /----------.----v~ .----bL-/ -k*,-''.--------- 12 -9.5 :-.i?.-/* .I L-----. 9 -11.9 -v- ,,, '-'. ..-------- -.+<-<.A __ -7A.-J /--.----------- *. 11 -10.3 -- -A /- k/-d- 10 -11.1 IL,-.-----̂ l--. . a -12.7 ,300 ,302 ,304 ,306 tion of the magnetic field strength are needed to understand these observations. The next steps involve use of a one-dimensional transport code to model the evolution of the density profile and experiments with a more versatile gas gun that allows injection with a larger range of pellet masses. Improved diagnostics - Thomson scattering triggered by the pellet, fast multichannel ECE and microwave and FIR interferometers, a Cotton-Mouton-effect polar- imeter, an extensive soft X-ray array that surrounds the plasma poloidally, improved MHD loop arrays - should allow better study of the evolution of n,(r) and the associated transport. References [ 13 U. Stroth et al., Nucl. Fusion 36, 1063 (1996). [2] Max-Planck-Institut fur Plasmaphysik, ZPP Annual Report 1995, p. 59. [3] A. Weller et al., 17th EPS Con$ on Controlled Fusion and Plasma Heating, Amsterdam, Part II,479 (1990). *Research sponsored in part by the Office of Fusion Energy Sciences, U. S. Department of Energy, under contract DE-AC05-960R22464 with Lockheed Martin Energy Research Corp. M97888549 I llllllll 111 11111 lllll lllll 111ll III lli 11111 1111 1111 Report Number Publ. Date (1 1) DOE 

Analysis of D pellet injection experiments in the W7-AS Stellarator

1629Analysis of D Pellet Injection Experiments in the W7-AS StellaratorJ. F. Lyon and L. R. Baylor, Oak Ridge National Laboratory, Oak Ridge, TN, 3 7831 USA.J. Baldzuhn, S. Fiedler, M. Hirsch, G. Kuhner. and A. Weller,Max-PIanck-Inslimtjfir Piasmaphysik, EURATOM Association, 85748 Garching, Germanyil ' ‘mm ,40-_ PelletPath“20 .nl l l 7no was 190 .300. 210 :20mtlim lMajor Radius (cm)Fig.1. Typicalflux surfaces and the pellet injec-tion path in W7-AS.I ron [1010 m"3]tor I [m1-Fig. 2. Thomson scattering profiles from beforepellet injection (highest TE . lowast n, curves) to18.5 ms after the pellet (second highest TE curve).A centrifugal injector was used to injectdeuterium pellets (with 35 x 1019 atoms) at=600 m/s into currentless, nearly shearlessplasmas in the Wendelstein 7-AS (W‘7—AS)stellarator. The D pellet was injected horizon;tally at a looation where the noncircular andnonaxisymmelric plasma cross section is near»ly triangular (Fig. 1.) Visibledlght TV pic—tures usually showed the pellet as a singleablating mass in the plasma, although the pel-let occasionally broke in two or splintered intoa cloud of small particles.Density Evolution Following PelletInjection in W7»ASFigure 2 shows Thomson scattering pr0~files for the electron density n: and tempera-ture TC immediately before and at times short-ly after pellet injection (at 0.4 ms, 0.5 ms,0.65 ms, 5.7 ms, 7.6 ms, and 18.5 ms) for aneutral—beam-heated plasma. Pellet injectionleads to a rapid (< 400—us) rise in "e in themain part of the plasma and to a slow increasein the plasma edge on a longer (>20-nis) time-scale. . Microwave reflectometer measure—ments of the time delay (which reflects thedensity gradient) and FIR interferometermeasurements of the linc~avcrage density alsoindicate that the density rises in <500 its(Fig, 3) and then remains unchanged, Thesame behavior is seen in the soft X-ray inten—sity profiles in Fig. 4 (with 1.4 ms betweencurves); this signal is more influenced by clen—sity than by temperature because there was noefleoommer5—»:iimadalayfnaD9‘ah'a'uEam-‘iu—so-reileclomelw:tlncluatlona—4—nn eulmf—b WW ‘- s.10-phaseImalr . r i .0.12 0.122 0.124 0.125 0.12: n.13time (s)Fig. 3. Time delay andfluct‘uarion level from amicrowave reflectomerer, MHD oscillationsfrom a Mimov coil, and line-average densityaround the time of pellet injection.X-«Rey Intensity [Lower Camera)tau-1.55245 u we: , 951714loin - .5313°“ 2 “5 TIM (I)s i 7"’ Mittsa .2915:g 2932‘s Isms7 m25 sum9 W20m ems11‘ it rmt N-EuE.5 _o. " _«20 —iD o to 20Radius (em)Fig. 4. Soft X—ray intensity profiles from 6.5 msbefore pellet injection to 8.4 ms after a petlet.filter in front of the soft X-ray diode array.Neutral lithium beam measurements of ne atradii r>12.5 cm (Fig. 5) show that the outerdensity rises slowly after pellet injection (on a60-1113 timescale), presumably due to recy—1630cling from the wall, The increase in 715 de»creases with distance into the plasma.Pellet ablation calculations using the stan-dard neutral gas shielding model and the n60)and Tait) profiles before injection indicate thatthe increment in density should be peaked.However, this is not seen in W7—AS on thetimescale of any of the measurements. Thechange in the er) profile in Fig. 2 is notdiffusive. If it were, then the particle diffu-sivity would have to increase by a factor of>100 for a relatively short time (<4OO its,about the transit time for the pellet through theplasma). There was not a significant loss ofinjected particles during the fast densitychange: comparing the number of particles inthe pellet with the increase in the number inthe plasma after pellet injection indicates that>60% of the pellet is retained in the plasmafor times >20 ms. This behavior, where thedensity profile after pellet injection assumesthe shape of the preexisting profile withoutsignificant loss, is not seen in tokamaks.Either the ablation model is not correct and thedeposition profile is the same as the initialdensity profile for different initial densitiesand field values, or a fast density redistribu—tion occurs in these experiments in W7-AS,o . _ . .._0.25 0.29 0.3 n.32 0.34 0.35 0.35 "9;?“1(8)Fig. 5. Increase in the plasma density in the edgeregion from 30 ms before to 70 ms after a pellet.Effect of Pellet Injection on EnergyConfinement and Fluctuations in W7-ASFigure 2 shows a rapid drop in Ta afterpellet injection because the stored energy Wdoes not change immediately. Although thenew profile remains at the new (higher) valuefor the duration of the discharge followingpellet injection. T60) gradually returns towardits preinjection value, indicating an increase inthe energy confinement time TE. This is illus—trated in Fig. 6 where W and t5 increased by afactor of 2.7 for a factor of 1.7 increase in me.From the W7‘AS ‘CE scaling expression [1]W = PTEW7'AS ac rigor-5P0“, the increase in71¢ should lead to a factor of =13 increase inthe stored energy because P = Pabsorbcd is ap-proximately constant at the densities in theseexperiments. The additional factor of 2 im—provement in 113 is due to an increase in con-finement: TE/TEW7'AS is 0.7 before the pelletand 1.4 afterward. In Fig. 2, the confinementimprovement (1.35) is less: tE/tEVW'AS was0.8 before the pellet and 1.1 afterward.1 Wm (kJ)CO101OMirnov CWMo wdB/dt‘ (TISWWNBI (MW) i01. m35-hP5. (kW/m2)'10 .iU ..........u.....n'u..,........0 10 200 300 400 500Time (ms)Fig. 6. Response of the plasma to pellet injectionat t a 300 ms.2 DII.i’”nun-u In.1631A few-ms quiescent period is sometimesseen in the MHD signals on Mimov coils fol»lowing pellet injection when the stored energystarts to increase, as seen in Fig. 3. In allcases, the Mirnov loop signal (often associa-ted with degraded confinement) is reduced,and tE/‘tEVW‘AS is >1 after a pellet. Low den—sity fluctuation levels, followed by a decayinghigh—frequency burst in several discharges,were seen with the microwave reflectometerafter the rapid density rise (Fig. 3‘). Theseoscillations are correlated with the Mirnov coilsignal. An example of pellet injection sup~pressing MHD activity for a longer periodwith a resultant increase in ’EE is shown inFig. 7. Without the pellet, the MHD activityand the associated enhanced energy loss per~sist for a longer time. It is thought that thepellet rapidly depletes the fast particle popula-tion that drives the instability [2].# 32562WallaNB]dB’dHT/s) pupit 32564Wm: (N)NE!dBldt (TIS) p-p.20 .25 .30 .35Time (s)Fig. 7. MH'D activity that retards transition to ahigh-ti phase (2 top tracer) is suppressed bypellet injection (2 bottom traces).1632P SAW—+8 ,l‘ 327709. Upper U‘SX—Cemera‘The soft X—ray (and MHD loop) signals in o m at m m blade Ricm)Fig. 8 show short-duration oscillations (for 30° .1... W“~. . . . . . 200 “J WWW" 29 7.9~300 its) immediately after pellet injection, m MAN 2,, 55followed by slow (650—Hz) oscillations that 2°” hardw,nm-_._.w 27 5-t‘ ion rkI/fl wv as 4.2damp out after a few ms and are in phase ,0 “w [WW/W 25 30across the plasma (m = 0). The phase veloc- 7“ fl*‘en//W*W‘ M 24 "9. . . . . . 70 A Hmflmw/N“ “ M23 .6my of the slow osctllations indicates a distur- m :2 Jx fl—«w,mswm-w~ ,2 2u. -—- my»), 1 "bance propagating outward at 350 m/s. The 50 fl WKJWWHW 2‘ 4,:i . . . . . , su '5 _. ,r’km-fw‘ 20 4.3initial burst is coincrdent With the fast change ED 3 4“:/J~\/~—km~—w—~ ,9 4,5in ng in the bulk of the plasma seen in Fig. 2, 90 E» “MW/”\"W "1 -° 2. . , . ED ‘1 Wm, »"\>’ f 17 —s.2If the rapid change in netr) is due to a fast so .3 , ’/,,J—W""""" ,5 _E,- —«. sq, kmdensity reiaxation rather than an anomalous 3° é ww/jk“ WWW *5 ~10. _ . ‘ 70 m... J KKWWP— 14 77,3particle deposrtion profile, then this burst may ,0 8“”;m‘eflfMn-fl ,3 4‘,be evidence of an instability that causes the 7° Hafiz" W ”WW; *2 4-5. . . so Mvfl—W’WW l1 -m.:5fast radial spreading of the highufl pellet blob. i Wso WW,» wv ‘lD —n,1The magnetic field has low shear, a relatively ‘00 mml-muwweer S "“3, , 20D 1,“ w m a 42.1weak radial gradient along the pellet path, and m 302 ( ) .abr. 305Time sstronger field regions some distance toroidallyFig. 8. Damped soft X-ray intensity oscillationson either side, which may allow a radial mix— following pellet Injection. »ing to occur. The process may be differentthan in tokamaks because of the low B X VB radial drift in W7—AS. In earlier experiments inW7-AS [3], the density profile became more peaked, and soft X—ray tomographic reconstruc—tion showed small—amplitude (=2—mm) internal oscillations in the location of the peak of the softX-ray emission following pellet injection. In that case, the soft pay oscillations showed a dif~ferent (m = 1) mode structure. Experiments with higher shear and the opposite toroidal varia-tion of the magnetic field strength are needed to understand these observations.The next steps involve use of a one—dimensional transport code to mode] the evolution of thedensity profile and experiments with a more versatile gas gun that allows injection with a largerrange of pellet masses, Improved diagnostics — ThomSOn scattering triggered by the pellet,fast multichannel ECB and microwave and FIR interferometers, a Cotton-Mouton-effect polar-irneter, an extensive Soft X-ray array that surrounds the plasma poloidally, improved MHD looparrays — should allow better study of the evolution of 2250) and the associated transport.References[l] U. Stroth et al., Nucl. Fusion 36, 1063 (1996).[2] Max—Planck~1nstitut fiir Plasmaphysik, [PP Annual Report 1995, p. 59.[3] A. Weller et 211., 17th EPS Conf. on Controlled Fusion and Plasma Heating, Amsterdam,Part II, 479 (1990).*Research sponsored in part by the Office of Fusion Energy Sciences, U. S, Department ofEnergy, under contract DEsAC05 9601122464 with Lockheed Martin Energy Research Corp.

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Analysis of D Pellet Injection Experiments in the W7-AS Stellarator

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