A. Komori

T. Morisaki

National Institute for Fusion Science (NIFS-Repository)

§ 12. Edge Plasma Behavior During Auxiliary Heating Komori, A., Morisaki, T. The temporal evolution of radial profiles of the electron density ne and its fluctuations ne during NBI plus ICRF heating has been mea-sured in the CHS edge plasma with a thermal neutral lithium beam probe. The plasma was initially produced by ECRH using the second harmonic resonance with Bt of 1.4 T and a magnetic axis posi-tion Rax of 91.1 em in the vacuum. The mi-crowave power was 150 kW at 53.2 GHz. The line-averaged electron density fie was obtained to be (5 - 6) x 1013cm-3 by the tangential NBI with 1 MW at 40 keV. The ICRF with 150 - 500 k W at 22 MHz was applied to the NBI sustained plasma. Figure 1 shows ne in the edge plasma, mea-sured with the beam probe, together with fie, Wdia. and the time sequence of the ICRF, NBI and gas puffing. It is clear that ne after turn-ing on the ICRF oscillates with a very low fre-quency [Fig. 2(a)] till rv 92 msec, and then, its fluctuation amplitude is reduced. This very-low frquency oscillation is always observed just after turning on the ICRF, and usually contin-ues for one or two periods. A decrease in ne I ne near the last closed flux surface (LCFS, ~ == 0 em) is also found during the NBI plus ICRF heating after the very-low frequency oscillation is stabilized, accompanied by the steepening of density gradient and a little inward shift of the plasma boundary, as shown in Fig. 2(b ). An important point is that the density gra-dient near the LCFS (~ < 0.7 em) becomes steeper and the density decreases in the region of ~ > 0. 7 em even if the ICRF is switched off. The density fluctuation level iie/ne is also found to be kept low near the LCFS [Fig. 2( c)]. On the basis of these experimental results, it is concluded that the vague edge plasma can be removed by applying the ICRF to the NBI sustained plasma, and that, once the vague edge plasma is removed, it cannot be produced again even if the ICRF is turned off. ~ 2 ~~~~--~~~~~~~~ ·a ::l ~ ~ 0 ~~~~--~~~~~~~= M 0 ....... 3~· ~ ~"1.5~ 0 so 100 1 so t (msec) Fig. 1. A beam probe signal, together with fie , wdia. and the tilne sequence. c::" - 0.5 IC::" (c) - 84.5msec ---85.5 -- - 86.5 -------87.5 - 88.5 -84.5mscc --- 92 ---110 ------- 120 -84.5 msec --- 92 ---110 .. ------- 120 . ~ --, , .,- ...... ..:.r-' -::::: 0.__...___,.____.-.l.___._--'-____._---L.___.__.L.---'-___j -1 0 2 3 4 5 l; (em) Fig. 2. Radial profiles of (a) and (b) ne and (c) ne 1 ne. (a) shows the edge plasma behavior just after the ICRF is turned on, when there is a very low frequency oscillation. (b) represents the behavior after the very-low frequency oscillation is stabilized. 181 

Edge Plasma Behavior During Auxiliary Heating

Abstract

Similar works

Full text

Available Versions

National Institute for Fusion Science (NIFS-Repository)