Measurement of impurity assimilation into the post-disruption runaway electron plateau in DIII-D and comparison with the plasma vertical loss rate

Experiments on the DIII-D tokamak studied the assimilation of high-Z impurities into the runaway electron (RE) plateau and the effect on the current channel vertical loss. It is found that the equilibrium impurity assimilation into the RE plateau can be explained by a balance between the neutral gas ionization source term and the impurity ion radial transport out to the wall, with significant (>50%) of the impurities typically residing as neutral gas outside the RE beam. Radial ion transport is found to be significantly (>10x) faster than neoclassical (collisional) transport. A clear saturation in assimilation efficiency is observed, but this saturated assimilated quantity can be increased by increasing power input into the REs or background plasma, consistent with the assimilation being ionization source term-limited. The experiments find that the RE plateau vertical loss rate is not directly proportional to the current dissipation rate, with a slow initial vertical loss and faster current dissipation, suggesting that the evolution of the current profile is playing a role.