Runaway electrons in fusion plasmas

In order to ensure the safe operation of future tokamak reactors, the generation of runaway electrons must be prevented or their effects mitigated. Due to the separation between the parameter regimes of present-day and future tokamaks, robust prevention and mitigation schemes for future devices must be developed with the help of validated simulation codes. This talk covers recent developments in modelling of runaways in plasmas, bringing together diverse insights into the runaway problem gained over more than two decades. We describe recently developed numerical tools for self-consistently simulating the evolution of temperature, electric field, and impurity densities, along with the generation and transport of runaway electrons in tokamak plasma initiation and termination scenarios. We show examples of modelling runaway dynamics in present-day devices, including how synthetic diagnostics can be used for benchmarking theoretical models and probing runaway dynamics. We address the expected runaway beam formation and evolution during disruption scenarios in future tokamaks, and identify operating regions in terms of the injected impurity and deuterium densities for which the runaway current is acceptable without excessive thermal energy being lost to the wall via particle impact.