  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.
