Modeling of helium transport and exhaust in the LHD edge

Abstract

Experimental results from LHD show a reduction of helium concentration in the plasma with the introduction of a magnetic island on the m/n = 1/1 resonant surface in the plasma edge. Simulations of the plasma with and without the island are carried out with the coupled code EMC3-EIRENE and compared to charge exchange recombination spectroscopy measurements of ionized core helium, and visible spectroscopy measurements of edge neutral helium. The numerical simulations indicate that the experimental parameters lie in a high density regime where the impurity transport is dominated by the outward directed friction force. The EMC3-EIRENE simulations capture the reduction in helium transport well and indicate that: (1) the reduction in core helium is a result of increased outward transport caused by the magnetic island and an increased opening of the edge-surface layer to the divertor plates; (2) the dominant source of neutral helium is best modeled by recycled helium at the targets; and (3) ionized helium density profiles are best matched in the simulations when there is a large core helium source in addition to a smaller edge source.