In solid targets irradiated by short pulse high intensity lasers, fast electrons have collision times longer than the laser pulse duration and mean free paths much larger than the radius of the laser spot. In these conditions, fast electron transport is dominated by electric and magnetic field. Although the fast electrons are collisionless, collisions of background electrons determine the ability of the background plasma to carry the return current which balances the fast electron current. Hence collisions are important even in this regime. A successful numerical simulation has to be able to model a plasma in which some electrons are collisionless and others are strongly collisional. An expansion of the electron distribution in spherical harmonics in momentum space is well suited to this, and we describe the formulation of the Vlasov–Fokker–Planck equation in terms of spherical harmonics and its solution in our KALOS code. We review the physics that must be modelled in a numerical simulation of fast electron transport and then describe KALOS.
Hybrid simulations of fast electron propagation including magnetized transport and non-local effects in the background plasma