Research at DIII-D toward understanding wall damage from post-disruption low-Z and high-Z runaway electron wall impact

Predicting the wall damage resulting from low-resistivity ("low-Z") or high-resistivity ("high-Z") post-disruption runaway electron (RE) beam wall strikes is crucial for future large tokamaks in order to optimize their disruption mitigation strategies. Experiments at DIII-D have attempted to measure the kinetic energy and pitch angle of post-disruption REs, both pre-loss and also during the wall loss event of large (>250 kA) RE beams. The data suggests that the range of wall-striking RE pitch angles is 0.2 – 0.4 radians while the mean wall-striking kinetic energies is 2 – 5 MeV. The data indicate that there can be a significant (~2x) drop in mean kinetic energy and significant (~2x) increase in RE pitch angle during the final loss process during low-Z RE wall strikes, but no significant change during high-Z RE wall strikes. Post-mortem damage analysis of a sacrificial limiter probe also arrives at RE energies and pitch angles which are consistent with pre-loss values. The RE wetted area during the loss process appears to be significantly (~5x) larger for low-Z RE beams when compared with high-Z RE beams. This trend can be qualitatively captured by RE full orbit simulations when scaling to measured MHD instability amplitudes. Using present best guesses for the ITER RE parameter ranges, basic 1D heat deposition simulations suggest that low-Z RE impact will have a lower probability of causing severe wall damage than high-Z RE impact.