  Large-angle Coulomb collisions lead to an avalanching generation of runaway
electrons in a plasma. We present the first fully conservative large-angle
collision operator, derived from the relativistic Boltzmann operator. The
relation to previous models for large-angle collisions is investigated, and
their validity assessed. We present a form of the generalized collision
operator which is suitable for implementation in a numerical kinetic-equation
solver, and demonstrate the effect on the runaway-electron growth rate. Finally
we consider the reverse avalanche effect, where runaways are slowed down by
large-angle collisions, and show that the choice of operator is important if
the electric field is close to the avalanche threshold.
