A fluid-kinetic framework for self-consistent runaway-electron simulations

The problem of self-consistently coupling kinetic runaway-electron physics to the macroscopic evolution of the plasma is addressed by dividing the electron population into a bulk and a tail. A probabilistic closure is adopted to determine the coupling between the bulk and the tail populations, preserving them both as genuine, non-negative distribution functions. Macroscopic one-fluid equations and the kinetic equation for the runaway-electron population are then derived, now displaying sink and source terms due to transfer of electrons between the bulk and the tail.