Investigation of the angular scattering model on the electron runaway condition

The Monte-Carlo electron code, MCSwarm [1], is being developed as a test bed for algorithms to be included in particle-in-cell codes like Quicksilver [2] and EMPHASIS [3] to self-consistently model the transport of intense beams through a background gas. In this paper, the MCSwarm code is used to study the electron kinetics as the secondary electron distribution approaches equilibrium. Electron runaway is observed in MCSwarm for moderately high electric fields. The runaway condition is strongly dependent on the angular scattering law as well as on the secondary electron energy distribution created by beam impact. The scattering model in MCSwarm is based on a doubly-differential cross section calculated from the first-Born approximation which is sampled so that sampled distributions preserve the total cross-section and the momentum-transfer cross section.[4] The method of preserving low-order moments statistically hides the underlying distribution and sampling method but is sufficiently accurate provided there are a large number of collisions over the scale-length of interest. The secondary electron energies are sampled using the results of Opal.[5] The MCSwarm code is used to study the sensitivity of the angular distribution and the secondary electron energy distribution from beam impact on the runaway condition.