Neutralization of an intense proton beam during transport in vacuum

Neutralization of the ion beam during ballistic transport in the target chamber is an important issue for Heavy Ion Fusion (HIF). The Gamble II generator at NRL was used to inject a 100 kA, 1 MeV, 50 ns proton beam into a vacuum (∼10−4 Torr) transport region. High-sensitivity laser interferometry was used to measure the resulting electron density. Efficient beam transport is only possible for these beam parameters if electrons are extracted from the walls to provide a significant degree of charge and current neutralization. Complete neutralization could be obtained with an electron population co-moving with the ions and at the same density as the beam. The measured line-integrated electron density has the same time dependence and magnitude as the proton beam. A numerical simulation of the experimental arrangement produced results in reasonable agreement with the measurements.