Neutralization of an intense proton beam during transport in vacuum

Neutralization of the ion beam during ballistic transport in the target cha mber is an important issue for Heavy Ion Fusion (HIF). The Gamble Il genera tor at NRL was used to inject a 100 kA, 1 MeV, 50 ns proton beam into a vac uum (similar to 10(-4) Torr) transport region. High-sensitivity laser inter ferometry was used to measure the resulting electron density. Efficient bea m transport is only possible for these beam parameters if electrons are ext racted 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. Th e measured line-integrated electron density has the same time dependence an d magnitude as the proton beam. A numerical simulation of the experimental arrangement produced results in reasonable agreement with the measurements. (C) 2001 Elsevier Science B.V All rights reserved.