Effect of Partial Screening on Runaway-Electron Dynamics

Licentiate thesis, 2018

One of the essential results of kinetic plasma physics is the runaway phenomenon: sufficiently large electric fields can accelerate a fraction of an electron population to relativistic energies. While such runaway electrons are fundamentally interesting objects of study in astrophysical settings, they are also of great practical relevance to fusion research. In the most developed fusion power production device, known as the tokamak, runaway electrons have the potential to cause severe damage to the first wall. Accordingly, runaway-electron mitigation is one of the critical issues in the design of a fusion power plant.

The most promising mitigation method to date is the injection of heavy atoms which only partially ionize and collisionally dissipate the energy of the runaway beam before it can collide with the wall. When the ions are partially ionized, their bound electrons screen out a fraction of the atomic charge, which directly affects the collisional scattering rates. However, accurate expressions for these collisional scattering rates between energetic electrons and partially ionized atoms have not been available previously, compromising modeling. In this thesis, we derive collisional scattering rates using a quantum-mechanical treatment, and study their effects on the kinetic runaway-electron dynamics. Using kinetic simulations, we find that the presence of partially ionized atoms significantly increases the dissipation rate of runaway electrons, compared to when the bound electrons completely screened the atomic nuclei. Moreover, we find that the increased scattering rates elevate the threshold electric field for runaway acceleration, but also enhance the avalanche growth rate at electric fields much larger than this threshold.

The results outlined in this thesis contribute to more accurate runaway-electron modeling and can lead to more effective mitigation schemes in the longer term. Experimental predictions of runaway mitigation however require that the kinetic model developed here be combined with the effect of spatial variation, which is a subject for future work.