Multi-faceted asymmetric radiation from the edge formation in DIII-D high-confinement mode discharges with continuous gas puffing

Abstract

A set of continuously gas fueled DIII-D [Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1986) p. 159] high confinement mode (H-mode) shots at three neutral beam injection powers and two values of magnetic field (q95) have been analyzed to investigate the evolution of plasma edge conditions leading to the formation of X-point MARFEs (multifaceted asymmetric radiation from edge), which are followed by an H–L transition that constitutes a practical density limit for this class of shots in DIII-D. The MARFE initiation is found to be caused by a combination of a sharply increasing concentration of recycling neutrals in the plasma edge, decreasing edge inverse temperature and density gradient scale lengths, and increasing edge density. The line-average density at which MARFEs precipitates increases with increasing injection power and with decreasing q95. It is not clear whether the formation of the X-point MARFE causes or merely precedes the H–L transition.