H. Funaba

N. Ohyabu

National Institute for Fusion Science (NIFS-Repository)

§6. Effects of Neon Injection on Plasma Confinement in LHD Funaba, H., Ohyabu, N. When neon gas was injected into the LHD plas-mas locally from the strong field side at the plasma start-up phase, it was observed that the increase in the stored energy, Wp , became steep. Figure 1 shows time evolution of some plasma parameters, (a) Wp , (b) averaged electron density, Tie, and (c) radiation power, Prad, for neon-puffed discharges. Two dis-charges are plotted here. The full curves (# 15389) represent a discharge with the early neon injection and the broken curves (#15371) represent the case of the later neon injection. The magnetic axis strength was 2.75 T and its position was R = 3.60 m. The ECH, ICRF and NBI pulses are also plotted. The ne signal began to increase from about 0.25 s after the injection time. It is considered that neon gas may be puffed for 1 rv 2 seconds. The increment in ne after the neon puff was 1 x 1019 m - 3. In the Prad signal of the #15389 shot, a steep peak was observed at t ~ 0.5 s. After this peak of Prad, Tie also showed a peak at t ~ 0.56 s. Then Wp became maximum at t ~ 0.70 s. The stored energy of #15389 was larger than that of #15371 by about 1.2, which is similar to the ratio of Tie at that time. Figure 2 shows the electron temperature, Te , pro-files of similar discharges to those of Fig.1 at the timing of the radiation peak (t = 0.48 s (.)) and after Prad became almost steady (t = 0.58s (0)). At t = 0.48 s, in the region of the major radius, R ~ 4.3 m, Te was lower than 100 eV. But the Te pro-file recovered to the extent of R ~ 4.63 mat t = 0.58 s. The electron density, n e , profiles of the two discharges in Fig. 1 are shown in Fig. 3. The closed circles rep-resent the earlier puff case and open circles represent the later case. In the earlier-puff case, ne was high at the central region. Moreover, a gradient existed in the region of p ~ 0.7. At t = 0.70s, when Wp be-came maximum, the profile in outer region (p ~ 0.85) were almost same. But the ne profile of the earlier-puff case was peaked in the central region. As Te in the peripheral region decreased, the neutral parti-cles easily penetrate into the main plasma, then ne increased and high Wp was observed. Therefore the discharges which showed the high radiation peak at plasma start-up phase had steep increment in Wp. Here, we discussed mainly about at the start-up phase of the discharges. The amount of puffed neon gas of the two discharges in Fig. 1 were almost same. 10 The amount of neon is estimated by the comparing the radiated power with the results of impurity trans-port calculations. An example showed that the ratio of neon to the central ne may be about 6%. This value is maximum because impurities other than neon were not included. The plasma confinement seemed to be improved transiently. But the ne and Wp signals in the latter half of these discharges were close to each other. So the effect of neon on the global plasma con-finement is considered to be small. The local trans-port analysis of neon-injected plasmas is now studied. (a )500 .400 ~oo 0.. 200 ~ 'i' ~ 1.0 "U ~ 0.5 (J.O 15371 15389 0.00 0.50 1.00 , .50 2.00 2.50 time 151 Fig. 1. Time evolution of some plasma parameters for neon-puffed discharges. (a) stored energy (b) averaged electron density (c) radiation power 4.0 3.0 '> Q) .=, 2.0 11.' t-1.0 3.00 3.50 4.00 R [mJ 4.50 Fig. 2. The electron temperature spatial profiles at two different times after the neon injection. 2.0 :;;-' 1.5 •'E •••••••••••• . , . ~ 1.0 • ~ . OJ 000010 c: 0.5 0000000000000 .00 • 0.0 u...J..I..J.W..u..u..J.W..u..u..J.W..J....J.J...<..J..J..J..I • ••••• •••••••••• ooooe~ 00000000000 I. • 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 P p Fig. 3. The electron density profiles at t = 0.50 s and 0.70 s. Two discharges are compared. (. : the earlier puff case, 0 : the later puff case) 

Effects of Neon Injection on Plasma Confinement in LHD

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