  Energetic electrons are of interest in many types of plasmas, however
previous modeling of their properties has been restricted to the use of linear
Fokker-Planck collision operators or non-relativistic formulations. Here, we
describe a fully non-linear kinetic-equation solver, capable of handling large
electric-field strengths (compared to the Dreicer field) and relativistic
temperatures. This tool allows modeling of the momentum-space dynamics of the
electrons in cases where strong departures from Maxwellian distributions may
arise. As an example, we consider electron runaway in magnetic-confinement
fusion plasmas and describe a transition to electron slide-away at field
strengths significantly lower than previously predicted.
