Studies of the propagation of pulsed relativistic electron beams through initially un‐ionized background gases are reported. At very low pressures, pronounced beam front erosion occurs owing to the slow buildup of radial force neutralization of the beam electrons. Numerical simulation and an approximate analytical model are used to describe this beam‐loss process. The results of experiments with two different electron accelerators (ν/γ ∼ 0.05 and 1) are shown to be in good quantitative agreement with the mathematical descriptions over the pressure range of ∼ 1 μ to ∼ 1 Torr in a large variety of gases. Propagation characteristics are found to scale in pressure from gas to gas inversely with the high‐energy ionization cross sections of the gases. The study shows that direct ionization alone is sufficient to explain the experimental results. The theoretical calculations indicate that the low‐pressure propagation characteristics of the lower ν/γ beam are sensitive indicators of the beam''s transverse energy. The calculations also indicate that this is not the case for the higher ν/γ beam.
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