Description
We present a validated framework and physics basis for low-Z benign termination of runaway electrons (RE). This strategy uses hydrogenic injection as a first step, followed by a low q-edge MHD instability to deconfine and disperse the RE beam. Our multi-machine experimental campaign (AUG, COMPASS, DIII-D, JET, TCV) confirmed this robust two-step process. We established a minimum required neutral pressure for ITER (0.2-0.8 Pa) and identified an upper limit of ∼1 Pa, where the termination efficacy begins to reduce. Experimental H and D secondary gas injection comparisons above 1 Pa revealed an unexpected offset in background plasma electron density (ne) dependence on mode growth.
Multi-code modeling was done with SOLPS-ITER, JOREK and M3D-C1. The SOLPS-ITER simulations, modelling the power balance of the REs and background plasma, reproduced the experimental non-monotonic ne trend with neutral pressure. The resulting profiles were ported to JOREK and M3D-C1 to model the MHD collapse phase. Results showed that lower ne and higher resistivity accelerate the m/n=2/1 mode growth, in line with experimental observations. The instability was shown to grow fast enough to induce strong magnetic stochasticity and significant RE transport within 0.1ms in the ASDEX simulations with M3D-C1.
This work provides the essential operational boundary map and multi-code physics validation necessary to implement this scheme on ITER, addressing one of the most critical challenges for the realization of fusion energy.