April 1999
Two weeks were devoted to NBI heating experiments accompanied by low power (~3 MW) N-NBI, and one week to RF launcher conditioning and electron heating experiments with RF. Control of the outer gap between the plasma outer edge and the first wall was introduced and was confirmed to function satisfactorily.
CORE PLASMA
Experiments on heating, current drive (CD) and steady-state high performance (SSHP) were carried out to enhance their performance with the additional use of N-NBI. Although the N-NBI power was comparatively low (~3 MW at 350-360 keV) in these weeks, the central heating by the N-NBI led to an increase in βN in a low-q (q95 ~3.8) SSHP shot with the triangularity of 0.55, reaching βN = 2.8. In an optimized shot, almost full CD ELMing SSHP was obtained at 1.5 MA with the parameters of HITER-89L = 2.47 and βN = 2.40. In this series of SSHP discharges on the basis of ELMy H-modes, the amplitude of giant ELM was found to diminish with time even at low q (q95 = 4-4.5), possibly encouraging low q discharges on ITER. Current drive experiments mainly focused on producing high Te target plasmas that will be used for the demonstration of high CD efficiency with N-NBI. The optimization has been almost completed, producing a steady-state high βp H-mode plasma with Te(0) ~ 9 keV.
With the combined injection of 2 GHz (1.5 MW) and 110 GHz (0.4 MW, 0.4 s) RF waves, electron heating experiment was performed, reaching Te(0) = 11 keV. No distinctive difference in the heating efficiency was found between the electron heating only with LHRF and with the combined heating. The 2nd harmonic electron cyclotron heating with 110 GHz RF was carried out at 0.4 MW for 0.4 s. For X-mode injection, with which better absorption of the waves is expected, Te(0) rose from 2.5 keV to 4.5 keV.
In halo current experiment, the current reduction by impurity gas puff was attempted, resulting in a significant reduction in the current by the gas puff of Ne or Ar at 0.2-1 Pam3. In contrast, He puff did not affect the halo current even with the strong gas puff of ~10 Pam3. Measured Te in the halo region suggests that a decrease in Te in the halo region is responsible for the observed halo current reduction.
DIVERTOR/SOL PLASMA
Argon seed H-mode experiment was conducted in order to produce high density ELMy H-mode plasma with enhanced radiative loss in the main plasma and divertor regions: In the long run, the target of the experiment to access RI-modes. In the experiment in April, the confinement performance of HITER-89L = 1.45 at ne/nGr = 0.7 was achieved in an Ar seed H-mode plasma, showing better confinement than in the plasmas with other gases puffed; HITER-89L = 1.3 for Ne+D2 and only HITER-89L = 1.15 for D2 only. The electron and ion temperatures in ELMy H-mode plasmas with Ar gas puff were higher than those without Ar. In the case of Ar seed (Ar+D2 gas puff), the main radiation loss during MARFE was enhanced by a factor of 2, compared with that in the Ne seed case. However, the divertor radiation loss in the Ar seed plasma was almost the same as that in the Ne seed plasma.
Neutral pressure at the outside and inside divertor increased with enhancing divertor closure. The experiment found that the detachment regime expanded up to a higher density with divertor closure and pumping effects. Also found is the importance of a feedback control of the inside and outside neutral pressure in the divertor to sustain the detachment.