Session CI2A - Edge Transport.
INVITED session, Monday afternoon, November 15
Room Chatham C, SCC
Recent progress in the heating capability in the Large Helical Device (LHD) enables exploration of MHD stability in the beta range up to 4%. This high beta value was also attributed to the features of MHD instabilities in the edge region, that is, its control was found to be very important, since there is a magnetic hill in the edge region of the heliotron configuration. MHD modes excited at m/n = 1 and m/n < 1 resonant surfaces were observed to grow with the increase of the pressure gradient and the average-beta in previous experiments. Recent high-beta experiments with higher heating power of NBI, however, show that MHD activities spontaneously become stable from the inner region to the outer region when the average-beta exceeds a certain value, and then a flattening of the electron temperature, Te, profile is observed around the resonant surface. Such a flattening can be formed externally by producing an m/n = 1/1 magnetic island, and the complete stabilization of the m/n = 1/1 mode is demonstrated by the moderate island width. Furthermore, m/n > 1 resonant modes as well as the m/n = 1/1 mode are completely suppressed by the Local Island Divertor (LID), which is a divertor utilizing the m/n = 1/1 island. The LID was found to be able to control the edge plasma. More specifically, with the LID, the Te gradient inside the island separatrix becomes larger, compared with that of the standard helical divertor (HD) configuration. In addition, with the LID a factor of about 1.2 improvement of the energy confinement time was observed over the International Stellarator Scaling 95. The stabilization of peripheral modes by the LID may contribute to the improvement of global energy confinement, in addition to the particle control in the edge region.