Brau, K.

Fußmann, G.

Kallenbach, A.

Mayer, H.

English

MPG.PuRe

175 MOMENTUM CONFINEMENT STUDIES ON ASDEX A Kal!enbach H.M. Mayer, K. Brau+. G. Fussmann, and ASDEX-, NI-, ICRH- and Pellet Teams IPP Garching, EURATOM Association, Fed. Rep. of Germany 1. Introduction The toroidal plasma rolalion induced by unbalanced neutral beam injection is a measure of plasma confinement, especially ion confinement. Since rot.ation velocities are experimen-tally easier accessible than ion temperatures, and the physics of momentum transport permits Ihe negleclion of electron transport effects, the study of momentum confinement is an effective diagnostic tool. We have investigated the rotational behaviour of the ASDEX plasma under different neutral beam healing schemes using charge exchange recombination (CXR) spectroscopy on up to live lines of sight (1 J. Comparing the momentum confinement times derived from rotational profiles with the corresponding energy confinement times derived from j3pol measurements, the main differences and common features of momentum and energy confinement are discussed. 2. Ex perimental procedure The plasma bulk rotation was determined from the Doppler shift of the CXR-excited CS+ (343.4 nm) line after subtraction of a cotd, edge excited blending feature from the measured line profile. Five lines of sight in the outer plasma midplane have been used simultaneously, which intersect the northwest neutral beam line at rfa- 0.125, 0.375, 0.625, 0.875 and 1. In order 10 reduce the number of data points and to obtain a comparative figure, global momentum confinement times were deduced by integrating the momentum of the particles with respect to the whole plasma volume. To facilitate this, an exponential fit has been applied to each radial rotation (half-) profile which was extended to the inner part of the plasma under the assumption of constant angular rotation frequency of a flux surface (2J. For steady slate conditions, the global momentum confinement time can be calculated as the quolient of plasma angular momentum, L, and beam torque, r: tlb _ LJr. 3 . . Results Figure 1 shows the typical rotational behaviour of the ASDEX plasma with co and counter neutral beam injection under identical experimental <!onditions. Although the density rises during the counter NI , the plasma speeds up to a considerable higher velocity, which is an expression of impr.oved confinement of momentum. Parallel to the observed improvement of momentum confinement, the energy confinement lime increases as well (31. The temporal and spatial development of the angular rotation frequency with DO injection under similar conditions as those of Fig. 1 are plaited in Figure 2. The corresponding rotation speed has slightly raised against that achieved with HO injection due to the higher torque of the deuterium beam in comparison with the hydrogen beam at the same injection power. + on leave from MIT, Cambridg~, USA 176 In order 10 reduce the thermal load of the divertor plates and the erosion of copper, C04 is puffed into some ASDEX discharges. This measure increases considerably the duration of the counter NI discharges. As illustrated in Fig. 3, the additional CD4 puffing slows down the rotation speed and prevents the plasma !mm lurlher acceleration as observed from the counter NI discharges without methane puffing. A comparison of the temporal development of the momentum confinement limes of a standard discharge, a discharge with pellet fuelling and the discharge with CD4 puff is given in Fig. 4a, the corresponding energy confinement limes are plolted in Fig. 4b. Although the two quantities scale in the same way, the change of confinement comes out much more clearly in the momentum confinement time than in the energy confinement time. 11 should be noted, that the steepest rise in momentum confinement without CD4 puffing appears just when the sawteeth disappear in these discharges (t::1.25 s), whereas the sawteeth do not disappear with CD4 puffing and '0 stays at a lower level [4J. Pellet fuell ing does not affect the momentum confinement drastically. While to is slighty improved during the pellet fuelled discharge, at its end this improvement stops. At tl=le same time, the energy confinement begins to degrade until a disruption occurs. Another example lor the sensitivity of momentum confinement against changes of discharge parameters is given in Fig. 5. Here, at \",1.0 s, the number of active beamlines has been reduced from 3 to 1 and, additionally, CD4 puffing has been reduced. As a result, tE increases moderately, but 'C. is drastically improved, in contrast to the normally observed independence of t. from the applied torque. The discharge is able to maintain the high angular momentum even with one third of the primary input torque over many of the (old) momentum confinement ,times. The opposite behaviour is obtained, when additional ICRF heating is applied to the plasma with counter NI. As to be seen in Fig. 6, with ICRH the usually strong improvement of momentum confinement with. counter injection fails to appear and to remains at a comparatively low level in the vicinity of tE. An interesting feature appears near the end of the discharge: A minor disruption causes the plasma to lose a part 01 its mass and the energy confinement time is pushed near zero. But while tE recovers in part after the event, the rotation of the plasma does not reappear although the torque of the neutral beams is still applied. In fact, such exceptional situations are the only occasions where momentum confinement times significantly smaller than the corresponding energy confinement , limes were found in ASOEX, in contrast to the re sults obtained with limiter machines, where '0 has typically half the value of 'E [5] . 4. Conclusion The global momentum confinement time derived from rotational profiles of the plasma has been found to be a sensitive deteCtor for changes in plasma confinement, with variations arising in the energy confinement time genf:!rally coming out more pronounced. A number of features in the rotational behaviour have been found, which still have to be explained: The strong increase of '0 during etr.-NI , its degradation by ICRH and the strong influence of C04 puffing on the confinement of momentum. Since at the moment no theory of plasma viscosity exists -which is able to describe the transport of momentum in detail, a simple analysis in terms 01 momentum diffusion seems to be appropriate. Comparing the' momentum 177 confinement behaviour with that of the energy or particle confinement times, new insight may be gained concerning the overall plasma development. References ( 1 I D.E. Robel1S, H.M. Mayer el al. , Proc. 15th Europ. ConI. on Conlr. Fusion and Plasma Phys.. Dubrovnik , (1988) Pari r. 15-18 r 2 J K.H. Burre!!, R.J. Groebner. H. SI. John. R.P. Seraydarian Nuc!. Fus. 28, (1988) 3· 15 (3 J O. Gruber el aI., this conference (4) R. Nolle et aI. , this conference ( 5) S.O. Seal! et al., presented on 12th Conf. on Plasma Phys. and Conlr. Nucl. Fusion Research, Nice (1988) 4l a) s 25120 r.35 ~ 10;25 15 1. 15 .. ~ ,.5 E 10 ~ ~ • !:. 5 > 0 12 14 Ib IB 20 22 lime (s) Fig. 1 Rotational behaviour with co and clr. 1:10 ->0+ injection. a) co injection. 4l ±~ b) S 25711 15 .. E ~ 10 " •!:. 5 > 0 12 14 15 time (5) b) counter injection. " , 0 ,b , . .~ , , t\{f\e lS) 16 " ~ " , • ~ • 178 " Fig.3 Temporal development of the rotation at diffe-rent plasma radii when CD4 is pulled into the ctr-NI discharge. .. E ~ • "-12 '~ 10 J ~ 35 8 r=25 6 ~ r. 15  , ~ ,.5 > - - - -- gas pull 2 0 LI 100 E 75 -e- 50 " 25 ---0--1.1 1. 2 1.3 1.4 1.5 1.6 time (5) ! - - - - ---- -- ---- - ~ --1.2 1.3 I., 1.5 1.6 time (5) 60,-----------------------, 25662 - - -0- - 25655 pell. 25664 CD4 " , b 1.2 1.3 1.4 time (s) 1.5 1.6 Fig. 4 a) Comparison of counter injection b) corresponding energy confinement limes. momentum confinement times w-Hh standard I fuelling. pellet fuelling and additional C04 puffing. 300,---------~--------, S 25559 ;: IDTCS~2~5~'3~'C-------------~---C' -E ' ~ 50 "[ 4' VI 200 --0- T if! E I:-' 100 NI ..... -~------- --- "l "" -------- ---09 1.1 time (s) e 0-.. E-.., 1.3 Fig. 5 Improvement of momentum confinement by the reduction of ctr.-NI and C04 puffing. '" 3) r Xl , 10 0 1.2 I.' 16 time(s) --, , , , , , 1.8 ICRH M 2.0 Fig. 6 Momentum confinement with ICRH and counter injection 0°->0+. 

Momentum Confinement Studies on ASDEX

http://hdl.handle.net/21.11116/0000-0009-9DAC-5

Momentum Confinement Studies on ASDEX

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