Disruption runaway electron generation and mitigation in the Spherical Tokamak for Energy Production (STEP)
Generation of Runaway Electrons (REs) during plasma disruptions, and their impact on plasma facing components, is of great concern for ITER and future reactors based on the tokamak concept. Current STEP (Spherical Tokamak for Energy Production) concept design flat top operating point features a plasma current higher than 20 MA and thus any plasma disruption is expected to be in the seed-insensitive regime of avalanche multiplication, i.e., any runaway seed would quickly generate a large runaway beam during the current quench. This is confirmed by modelling runaway electron generation in unmitigated disruptions using the code DREAM. Hot-tail generation of runaways is found to be the dominant primary generation mechanism, and the avalanche multiplication factor is confirmed to be extremely high. Varying assumptions for the prescribed thermal quench phase (duration, final electron temperature) in a reasonable physical range, as well as the wall time, the plasma wall distance, and shaping effects, we find that all STEP unmitigated disruptions generate large runaway electron beams (from 10 MA up to full conversion). The possibility of RE avoidance is first studied by idealized, i.e. radially uniform, impurity injection of a mixture of argon and D2, with ad-hoc particle transport arising from the stochasticity of the magnetic field during the thermal quench (TQ). Unfortunately, no such injection scenario allows runaways to be avoided while respecting the other constraints of disruption mitigation. The current STEP disruption mitigation system (DMS) concept design has then been tested with DREAM, scanning 2-stage Shattered Pellet Injections consisting of pure deuterium pellets followed by mixed pellets of argon and deuterium. Such a scheme is found to reduce the generation of REs by the hottail mechanism, reducing the final RE current to about 13 MA (instead of 16 MA with a single pure argon injection), but isn’t sufficient to avoid the generation of a large RE beam. The range of impurity densities achieved is significantly reduced with SPI compared to the idealised impurity injections, and the final RE current is rather insensitive to the number of pellets injected, motivating a re-optimisation of STEP DMS. Results are much more sensitive to the stochastic particle transport during the thermal quench (both amplitude and duration), which should be better constrained by future 3D non-linear MHD modelling of STEP mitigated disruptions. Estimations of required CQ losses (from a RE mitigation coil) will also be quickly discussed.