February 1998


Neutral beam port conditioning and reduction of hydrogen / deuterium ratio after a long vent was completed by Feb. 6th. For equilibrium control under the high elongation connection of poloidal field coils, the feedback coefficients for vertical (Zp) and horizontal (Rp) position of the plasma center and height of X-point (Xp) was optimized. The scatter of Rp, Zp and Xp around the pre-programmed values is less than 2 cm.

In the tripartite workshop on "Optimization and Current Drive for Improved Tokamak Performance" held in Naka on Feb. 16-18, 1998, studies on steady state high integrated performance and sustainment of ELMy reversed shear plasmas were presented.


The ICRF and LHRF systems were operated for degassing from launchers and for conditioning. The ICRF power of up to 4 MW was coupled to the plasma and the LHRF power increased up to 2.4 MW by the vacuum conditioning.

In the tripartite workshop on "Optimization and Current Drive for Improved Tokamak Performance", progress on the NNB system, recent results on NBCD experiments, LHCD current profile optimization in reversed shear, TAE modes, and design study ECCD system were presented. In the 5th ITER Expert Group Meeting on Energetic Particles and Heating and Current Drive held in Naka on Feb. 19-20, 1998, JT-60U contributed to the physics R&Ds of NBCD, LHCD and ICRF heating in reversed shear plasmas.


In order to obtain the less peaked radiation loss at X-point and less contamination in the main plasma, radiation profile control during the MARFE phase was tried to control by using neon gas injection and pumping rate reguration. It was found the radiation profile control during MARFE phase was sensitive to deuterium gas puff rate and unsuccessful even if neon injection was applied. However reattachment from MARFE was successfully obtained by increasing divertor pumping rate.

In order to investigate how the divertor geometry (dome & W-shaped divertor targets) affects the generation of carbon impurity and the MARFE characteristics, the strike points were scanned while the X-point height was unchanged in L-mode and ELMy H-mode discharges. It was found that MARFE onset densities increased as the neutral backflow was reduced by moving strike points towards the bottom of W-shaped targets.

Divertor neutral gauges were used in divertor experiments. Separate control of the divertor neutral pressure from the main plasma density was tried. Divertor neutral pressure and main plasma density could be controlled separately in relatively low density plasmas when divertor and main puffs were in good operation. Neutral pressure was measured during MARFE. Trend of the neutral pressures in the divertor region behaved like that of H alpha signal. Neutral pressure at the inner divertor and pumping duct decreased after the onset of MARFE and neutral pressure on the outside baffle increased after the onset of MARFE.