The effects of impurities on avalanche generation of runaway electron (RE) are investigated using a relativistic bounce-averaged drift-kinetic equation that combines the effects of impurities, stochastic magnetic perturbations (SMFs), and synchrotron radiation (Hoppe et al 2021 Comput. Phys. Commun.268 108098). The momentum range of RE is established through the balance between SMFs induced diffusion rate and acceleration rate (Li et al 2017 Plasma Phys. Control. Fusion59 055003). Impurity injection affects momentum-space dynamics of RE by modifications to bremsstrahlung radiation and collision-induced pitch-angle scattering and slowing-down processes, which leads to enhancement of both the sustainment and avalanche threshold electric fields ( and ) as well as reduction of the avalanche growth rate. However, the inclusion of impurity contributions to the avalanche source term leads to a rapid increase in avalanche growth rate once the electric field exceeds . Furthermore, parameter scans based on ITER and DIII-D demonstrate that higher post-TQ temperature and higher amount of injected high-Z impurities are beneficial for raising the avalanche threshold electric field. These results may provide some useful theoretical insights for strategies of suppressing RE.
A study on combined effects of stochastic magnetic fluctuations and synchrotron radiation on the production of runaway electrons