Progress at DIII-D toward predicting wall damage in ITER from post-disruption low-Z vs high-Z runaway electron wall impact*

Predicting whether low-resistivity ("low-Z") or high-resistivity ("high-Z") runaway electron (RE) beams will tend cause more wall damage is crucial for ITER to optimize its disruption mitigation strategy. Experiments at DIII-D have attempted to measure the kinetic energy and pitch angle of REs striking the wall during the post-disruption loss event of large (>250 kA) RE beams, both using IR data analysis and using a sacrificial limiter probe. Preliminary data suggests that the range of wall-striking RE pitch angles is 0.2 – 0.4 radians while the range of wall-striking kinetic energies is 2 – 5 MeV. These values are similar to estimated pre-loss values, consistent with full orbit simulations, which indicate that REs increase pitch angle slightly and do not change kinetic energy significantly during the loss process. 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 cooling channel failure than high-Z RE impact.