December 2007

  In this month, experimental operation series has been started with wall temperature of 150 degree C. The following are the results on several major topics during this month.

(1) Neoclassical tearing mode stabilization
Neoclassical tearing mode (NTM) control using electron cyclotron (EC) wave is considered most promising due to its ability of highly localized current drive. In the past JT-60U experiments, as in other devices, NTMs were overstabilized in most cases, where EC wave power was larger than the minimum required power. For efficient NTM stabilization, identification of the minimum required EC wave power is an important issue. In this campaign, stabilization of an m/n=2/1 NTM with reduced EC wave power was performed, and the minimum EC wave power was identified at Ip=0.85 MA, Bt=1.7 T, q95~3.5. Here, m and n are the poloidal and toroidal mode numbers, respectively. This experiment was done in collaboration with Max-Planck-Institute für Plasmaphysik (IPP), where experimental condition was set at IPP and sent to JT-60 under high security by using a recently developed remote experiment system. The remote experiment was very successful and the usefulness of this system was demonstrated.

(2) Effect of current profile on H-mode plasmas
In order to examine the effect of current density profile on pedestal structure and confinement in H-mode plasmas, Ip ramp up/down experiments were performed. H-modes operated at higher li with the current ramp down have shown higher energy confinement with higher density. The H-factor evaluated for the core plasma (H89core) depended strongly on li with the relation of H89core∝li 0.74 for the case without sawtooth oscillations. Center-peaked profiles of electron density and electron temperature were obtained in high li H-modes. While the peripheral current density profiles were largely modified by the current ramp, the pedestal pressure was not significantly changed.

(3) Supersonic molecular beam injection
Supersonic molecular beam injection (SMBI) was installed in collaboration with CEA-Cadarache. Injection frequency is less than 10 Hz and duration is ~2 ms/pulse. Gas flow rate was estimated to be ~1.2 Pa•m3/pulse at back ground pressure of PBK=6 bar from increase in gas pressure after SMBI pulses. SMBI speed is expected to be 2.2 km/s at the wall temperature of Twall=150C and PBK=5 bar. By improving heat resistant capability of the vacuum seal placed inside the injector head, SMBI was successfully operated at Twall=150C with PBK=6 bar and 4 Hz after one-month baking at high Twall~280C. In experiments, jumps of electron density by the SMBI pulses were observed.

(4) Development of full current drive plasma
Toward full current drive (CD) plasma development using NBCD and bootstrap, target plasma was developed by optimizing configuration and heating profile in order to avoid NTM (mainly m/n=2/1 NTM). The target plasma (Ip=0.85 MA, Bt=2.4 T, q95=4.5) with βN=2.0-2.1 (βp=1.6-1.7), fGW=0.58 and loop voltage of 0.08 V was sustained for 7 s without use of N-NB.

(5) Development of imaging bolometer system
Over a 4-year period, an imaging bolometer has been developed in collaboration with National Institute for Fusion Science. In 2007, we improved the diagnostic sensitivity, spatial resolution, time resolution, the shielding of the camera, and so on. 2 to 3-D structural evolution of radiation enhanced phenomena such as MARFE near the X-point and metal accumulation in core plasma was measured in deuterium ELMy H-mode discharges.