Active control of runaway electron impact phase with non-axisymmetric magnetic fields in DIII-D

We demonstrated that the toroidal phase of post-disruption runaway electron (RE) final loss (FL) impact can be controlled by non axisymmetric (3D) magnetic fields (RMP). This method could reduce or at least spread first wall damage, as repeated RE strikes at the same toroidal phase could lead to increased likelihood of first wall integrity failure. In the experiments, high-current (500 kA) purged RE plateaus (post-disruption plasmas with current carried completely by REs) were terminated on the DIII-D center post (CP). The toroidal envelope of the CP impact structure, with clear peaking at one toroidal angle, was observed with a toroidal array of hard x-ray (HXR) detectors. Without applied n = 1 RMP fields, this CP RE-wall impact phase was found to peak at a single toroidal angle. With applied n = 1 RMP fields, the CP impact phase was found to lock to the applied perturbation toroidal phase about 50% of the time; with other shots appearing to strike at random phase. Fast imaging data of the FL event shows a collapsing internal structure with a clear distinction between the two types of FL events; for the locked FL events a clear m = 2 structure is observed, while for FL events that strike at random phases an m = 3 structure is observed. Modeling efforts are underway to understand how future optimized coil designs and coil phasing could more reliably drive growth of the locked (2/1) mode rather than the uncontrolled phase (3/2) mode.