Study of fuelling control for confinement experiments in Heliotron J

The optimization of gas-fuelling scenario has been studied to examine the plasma confinement in Heliotron J. Gas fuelling effects on plasma profile are discussed by comparing two fuelling methods, a short pulse H2-beam fuelling with a supersonic molecular-beam injection (SMBI) technique and a high-intensity gas-puff fuelling (HIGP) with a conventional gas puff technique. The maximum plasma stored energy Wp after SMBI is about 20% higher than that after HIGP in this NBI-only sustained plasma experiment, where the line-averaged density is almost the same (∼3x1019 m-3) for both cases. The core electron and ion temperatures for the SMBI case are higher than those in the HIGP case at the timing of maximum Wp. A peaked density profile is observed after SMBI, while it is flat or a slightly hollow for the case of HIGP. These observations point out the importance of SMBI fuelling for plasma density control to obtain better plasma performance. The effects of fuelling on plasma fluctuations and effectiveness of fuelling control for triggering of the L-H transitions are also reported. (author)