Ionization physics of electron beams in air

Summary form only given. For beam transport through a background gas, the physics of ionization and recombination processes can, through charge and current neutralization, have a strong effect on transport efficiency and beam control. We explore ionization processes and transport efficiencies of intense electron beams in moderate-pressure (0.5 to 50 Torr) air through simulations with the hybrid code LSP, experiments on Gamble II, and simple analytical models. LSP self-consistently solves the electromagnetic fields and pushes particles with an implicit algorithm while locally tracking beam impact ionization, electron avalanche processes, and recombination and attachment. We use LSP to numerically determine the scaling of secondary electron density, background conductivity, and net-current fraction as functions of neutral pressure, pre-ionization level, beam current density and beam divergence. Furthermore, we present a simple analytical model applying the gas physics of LSP in reduced geometry.