Modeling of an "inside out" thermal quench by core deposition of impurities

MHD simulations support several promising features of shell pellet injection, which aims to deliver a radiating payload directly to the core while minimally perturbing the edge plasma. In practice, the shell pellet method, recently demonstrated on DIII-D, encloses the payload in a low-Z shell that slowly ablates then breaks open in the center of the plasma. Disruption mitigation for ITER must simultaneously meet several criteria for thermal and mechanical loads and runaway electron avoidance, some of which are in tension with one another. For instance, prevention of a runaway electron avalanche may be possible if complete flux surface destruction during the thermal quench de-confines the primary runaway electron population before the secondary avalanche can ensue. But the same flux surface destruction that allows runaway electrons to follow open field lines to the wall may just as easily conduct electron heat to the wall. In the simulations, the high-Z payload is assumed to be delivered directly to the core. The strong core cooling produces an “inside-out” thermal quench that propagates toward the edge, and results in an annular current profile and an increase in total current due to dropping inductance during the thermal quench. Importantly, the flux surfaces also break up from the inside out, with the outermost surfaces remaining intact until the end of the thermal quench, resulting in a very high radiated energy fraction. Nonetheless, once the last closed flux surface is broken, a very rapid loss of test particle runaway electrons is observed.