JT-60U MONTHLY SUMMARYJune 2006
The following are the results on several major topics during this period.
A carbon tile came off the divertor dome during the experiment on June 16. The experiments planned in the rest of June and July were canceled for repair work.
(1) Bootstrap current overdrive
The OH coil ('F coil') current was fixed during the flat-top of a high poloidal beta (βp) reversed shear discharge at Bt = 3.7-4 T without co-tangential NB injection, in order to demonstrate the bootstrap current overdrive. Feedback control of the stored energy was also applied. As a result, 520 kA of the plasma current was maintained for about 2 s quite steadily (without any collapse) with the one-turn voltage nearly zero and almost constant βp. The result suggests that the plasma was sustained with an almost 100% bootstrap current fraction. For comparison, one perpendicular NB unit was replaced by one co-tangential unit in a similar discharge. Owing to co-NBCD, Ip continued to increase for 1 s and then was maintained constant for 2 s.
(2) Impact of electron heating on ITBs
Impact of the ECRF injection on an ion temperature (Ti) ITB in a high βp H-mode plasma (Ip = 1 MA and Bt = 3.75 T) has been studied. Measurement of change in the Ti profile with a sampling rate much faster than before was attempted by using the newly installed charge exchange recombination spectroscopy (CXRS) diagnostics. It was confirmed even with faster time resolution, central Ti was degraded with almost no delay to the ECRF injection deposited off-axis. It was also confirmed that the degradation effect of ECRF is independent on the toroidal launching angle, that is the amount of current drive by ECRF.
(3) H-mode characteristics by the installation of ferritic steel tiles
The H-mode pedestal characteristics were investigated at fixed absorbed power in a large volume (Vp = 75m3) configuration, with a low recycling level at which the reference data, without FSTs, was obtained. The plasma current and the toroidal field were mainly Ip = 1.2 MA and Bt = 2.6 T (q95 ~4.1). At the same absorbed power, the pedestal pressure and energy confinement were clearly higher than those without FSTs. The temperature profile was increased throughout the whole radial range of plasma.
(4) QH-mode study
The effect of toroidal rotation on production of QH-mode was investigated with the ferritic steel installation, which enabled us to reduce the ripple loss of fast ions. QH-mode was successfully reproduced during the counter-NB and the balanced-NB injections with PNBinj ~12-16 MW. QH-mode with co-NB injection was observed only with lower NB heating power (PNBinj ~8 MW). Transitions of QH-mode to ELMy H-mode or ELMy H-mode to QH-mode were observed at the similar toroidal rotation velocity at the pedestal, suggesting there is no hysteresis. ELMs appeared with the increase in NB power during the counter-NB injection although the toroidal rotation velocity at the pedestal did not change essentially. Edge harmonic oscillations were observed only in the edge channel of ECE and BES diagnostics during the QH-mode phase.
(5) Stabilization of neoclassical tearing mode (NTM) with electron cyclotron current drive (ECCD)
Complete stabilization of an NTM with the poloidal mode number m=2 and the toroidal mode number n=1 has been demonstrated by ECCD at the q=2 surface. Dependence of the stabilization effect on ECCD location has been also investigated in detail. It has been shown that the NTM can be completely stabilized with the misalignment of ECCD location within about half of the island width and that the NTM is destabilized with the misalignment of about the full island width.