J. Plasma Fusign Res. SERIES, Vol.3 (2000) 169-t75 Plasma Edge Physics and lsland Divertor Studies

Core, edge and scrape-off-layer (SOL) plasma behavior is studied principally under conditions of an tu = 519 boundary island configuration, which will be the primary modus operandi for the upcoming W7-AS divertor campaign. Present results are gained with ten inboard sector limiters. High densities, both in the core for central confinement and at the edge for promotion of good divertor operation, are necessary for optimal operation in future steady-state devices. On W7-AS operation at such densities, at low input power P6"u, < 0.4 MW, invariably leads to a transition to the ELM-free H-mode, accompanied by low edge densities and augmented core radiation leading to radiation collapse. The threshold density n"th' necessary to attain the H-mode increases with P1,"u,, such that at Pn61 2 MW the H-mode is completely suppressed and peak densities at the separatrix and divertor exceeding 9 x 10le m-3 and 1.5 x 1020 m-3, respectively, are transiently realized. Operation with ECRH at intermediate powers can produce ELMing discharges with the potential of attaining steady-state. The efficacy of newly-installed control coils designed to manipulate the island geometry is tested. Their expected influence on core plasma and boundary-island properties is verified in proof-of-principle experiments. Further technical achievements relevant to future stationary plasma operation on next generation devices are: a) the successful test of a polarimetric diagnostic to measure the line integrated density, based on the Cotton-Mouton effect, and b) demonstration of effective ICRH discharge cleaning in the presence of a magnetic field. Keywords