Measurement of runaway electron wall impact structure and dynamics with instrumented limiters in DIII-D

Predicting the wall damage resulting from post-disruption runaway electron (RE) beam wall strikes is crucial for future large tokamaks like ITER in order to optimize their disruption mitigation strategies. Experiments at DIII-D using intentionally created RE beams striking instrumented limiters have provided many new interesting details on the structure and dynamics of RE-wall strikes. The RE beam wetted area is seen to be strongly influenced by a locked edge tearing mode, typically (2/1), that gives the REs a n = 1 structure at the wall. This n = 1 structure typically locks to a preferred toroidal phase on the wall but can change phase shot to shot or even during the loss event. The detailed local wetted area appears to be set by limiter shape, RE orbit dynamics, and also the local magnetic field perturbation caused by the incoming RE current. RE orbits are set by lost RE kinetic energy and pitch angle, which appear to be fairly close to initial confined values, with perhaps only a small increase in pitch angle. RE penetration depth into the limiter is found to be of order 1 - 2 mm. Although material failure can be explained considering only forward-moving REs, impact by backward-moving REs is also observed, as well as impact by energetic ions.