Effect of Screened Nuclei on Fast Electron Beam Dynamics

Abstract

In a plasma, particles can be accelerated to relativistic speeds by an electric eld. These relativistic particles are of importance to, for example, fusion research, where they pose a risk of damaging the walls of a reactor. In order to understand the dynamics of runaway particles, the Coulomb interaction between particles (\collisions") is a central concept. In this thesis we focus on the collisions between particles and partially ionized ions. The interaction strength depends on the incoming particle momentum: at low momentum the nucleus is screened by the bound electrons and the net ion charge will de ne the interaction strength; at high momentum the particle may penetrate the electron cloud around the nucleus and the relevant charge is then the nuclear charge. Since the collision cross section is proportional to the charge squared, this can be expected to have a signi cant impact on the runaway dynamics. In this thesis we investigate the energy dependence of screening. Starting from a quantum mechanical collision cross section, we derive the form of the Fokker{Planck collision operator appropriate for the many-body plasma system. When accurately accounting for screening, we nd that the collision rates can be signi cantly enhanced compared to the fully screened case, in particular at high momentum. Furthermore, we derive general forms of the high energy behavior of the collision frequencies, which hold regardless of the details of the model used to describe the electron cloud of the ion. Finally, we nd indications that the use of the Fokker{Planck operator might have to be improved by considering the Boltzmann operator, in order to take large-angle collisions into account.