Generalization of the Rosenbluth-Putvinski avalanche growth rate to the drift kinetic regime

Abstract

A new collisionless detrapping mechanism is shown to modify the rate of runaway electron (RE) generation by as much as a factor of two for strong inductive electric fields. This detrapping mechanism results from the magnetic well inherent to tokamak geometry being tilted by the strong inductive electric fields present during tokamak disruptions. The impact of this collisionless detrapping mechanism is found to be described by a single dimensionless parameter, whose identification enables the transition between distinct regimes of RE generation to be characterized. Using this composite parameter, it is shown that modifications to the avalanche growth rate by tokamak geometry across a broad range of plasma parameters collapse onto a one-dimensional curve that can be accurately fit with a two parameter function. The resulting analytical formula extends the classic Rosenbluth-Putvinski avalanche growth rate expression beyond the bounce-averaged limit into RE regimes where collisionless detrapping strongly modifies RE generation rates.