Charge-neutralization processes for intense relativistic electron beams in low-pressure neutral gases

Processes affecting the charge neutralization of intense relativistic electron beams injected into low-pressure neutral gases are investigated. Processes examined include electron impact ionization, electron avalanching, ion impact ionization, ion avalanching, charge exchange, precursor ionization effects, secondary-electron escape, recombination, diffusion, and attachment. Cross sections and analytic estimates are given for each process. The ion avalanche-like process is apparently new and unique to intense relativistic electron beams. Although omitted in past research, the ion ionization effects discussed here can play dominant roles in the charge neutralization process. Pressures of the order of 0.1 Torr ${\mathrm{H}}_2$ are considered specifically, but the results should apply at least qualitatively to even higher pressures and, with appropriate cross sections, to gases other than ${\mathrm{H}}_2$. The results should have immediate relevance to the problems of collective ion acceleration with drifting intense beams, ion loading for the electron ring accelerator, and intense relativistic electron beam propagation in low-pressure neutral gases.