Fluid and kinetic studies of tokamak disruptions using Bayesian optimization

When simulating runaway electron dynamics in tokamak disruptions, fluid models with lower numerical cost are often preferred to more accurate kinetic models. The aim of this work is to compare fluid and kinetic simulations of a large variety of different disruption scenarios in ITER. We consider both non-activated and activated scenarios; for the latter we derive and implement kinetic sources for the Compton scattering and tritium beta decay runaway electron generation mechanisms in our simulation tool DREAM [M. Hoppe et al 2021 Comp. Phys. Commun. 268, 108098]. To achieve a diverse set of disruption scenarios, Bayesian optimization is utilized to explore a range of massive material injection densities for deuterium and neon. The cost function is designed to distinguish between successful and unsuccessful disruption mitigation based on the runaway current, current quench time and transported fraction of the heat loss. In the non-activated scenarios, we find that fluid and kinetic disruption simulations can have significantly different runaway electron dynamics, due to an overestimation of the hot-tail generation rate by the fluid model. The primary cause of this is that the fluid hot-tail generation model neglects superthermal electron transport losses during the thermal quench. In the activated scenarios the fluid and kinetic models give similar predictions, which can be explained by the activated sources' significant influence on the RE dynamics and the seed.