An analytical criterion for significant runaway electron generation in activated tokamaks

A disrupting plasma in a high-performance tokamak such as ITER or SPARC may generate large runaway electron currents that, upon impact with the tokamak wall, can cause serious damage to the device. To quickly identify regions of safe operation in parameter space, it is useful to develop reduced models and analytical criteria that predict when a significant fraction of the Ohmic current is converted into a current of runaway electrons. In deuterium–tritium plasmas, the seed runaway current may have a significant contribution from – or may even be dominated by – tritium beta decay and Compton scattering. In this work, a criterion for significant runaway electron generation that includes tritium beta decay and Compton scattering sources is developed. The avalanche gain factor includes the effects of partial screening of injected noble gases. The result is an analytical model that can predict significant runaway electron generation in the next generation of activated tokamak devices. The model is validated by fluid simulations using Dream (Hoppe et al. 2021 Comput. Phys. Commun. , vol. 268, p. 108098) and is shown to delineate regions in parameter space where significant runaway electron generation may occur.