Optimization of runaway electron mitigation by massive material injection in ITER and SPARC

During tokamak disruptions, strong electric fields can arise which are sufficient to cause electron runaway, whereby electrons are accelerated continuously. In future large-current tokamaks, such as ITER and SPARC, significant runaway electron generation is expected, due to the runaway generation being exponentially sensitive to the pre-disruption plasma current. Should the beam of runaway electrons come in contact with the tokamak wall, its energy can be almost instantly deposited deep into the wall. During disruptions, high heat loads can also arise from heat being transported into the wall when the magnetic surfaces are broken up during the thermal quench. Additionally, if the current decay is too fast or too slow, electromechanical forces caused by eddy or halo currents can cause forces and torques on the tokamak structure. While all of these unwanted aspects of a disruption can individually be addressed by massive material injection, they pose conflicting requirements on the injected material quantity and composition. In this talk, we investigate disruptions mitigated with combined deuterium and noble gas injection in ITER and SPARC. We use multi-objective Bayesian optimization of the densities of the injected material, taking into account limits on the maximum runaway current, the transported fraction of the heat loss, and the current quench time. Regions in the injected material density space corresponding to successful mitigation are found for both machines when optimizing pure deuterium plasma scenarios. When optimizing deuterium-tritium plasma scenarios, on the other hand, simultaneous mitigation of runaway current, transported heat loss and electromechanical forces appear more challenging for ITER than for SPARC.