Abstract
A 1D radial diffusion model is developed to study the observed rapid expulsion of argon from the runaway electron plateau in the DIII-D tokamak following secondary massive low-Z (D2 or He) gas injection. The expulsion of argon is found to be caused by further cooling of the background plasma due to the added neutrals, accompanied by recombination of argon ions and the greatly increased outward radial transport rate of argon (now dominantly in neutral form) out of the runaway electron beam. After argon expulsion, power loss out of the runaway electron plateau is found to be dominated by neutral transport to the wall (rather than by radiation); this result resolves the power balance discrepancy highlighted in previous work on argon expulsion out of the runaway electron plateau.
