Runaway Electron Avalanche in Tokamak Geometry in a Disrupting Plasma.

Tokamak disruption mitigation continues to be a broadly pursued effort, specifically in the context of runaway electron (RE) formation and evolution. The exponential growth (avalanching) of REs during a tokamak disruption remains a large uncertainty in RE modeling. The present work investigates the impact of tokamak geometry on the efficiency of the avalanche mechanism across a broad range of disruption scenarios. It is found that the parameter ν_*,crit describing the collisionality at the critical energy to run away delineates how toroidal geometry impacts RE formation. In particular, utilizing a reduced but self-consistent description of plasma power balance, it is shown that for a high density deuterium dominated plasma ν_*,crit is robustly less than one, resulting in a substantial decrease in the efficiency of the RE avalanche compared to predictions from slab geometry. In contrast, for plasmas containing a substantial quantity of neon or argon, ν_*,crit ≥ 1, and no reduction of the avalanche is observed due to toroidal geometry. This sharp contrast in the impact of low versus high-Z material results primarily from the relatively strong radiative cooling from high-Z impurities enabling the plasma to be radiatively pinned at low temperatures with a correspondingly large electric field even for modest quantities of high-Z material. Ongoing work is focused on the evolution of a RE beam in a vertically unstable plasma.