Current neutralization of intense MeV proton beams transported in low-pressure gas

This paper reports on the first experiments designed to study ion-beam-induced gas ionization and subsequent conductivity growth using intense proton beams transported through various gases in the 1-Torr pressure regime. Net-current fractions of 2 to 8% are measured outside the beam channel. Ionization is confined predominantly to the beam channel with ionization fractions of a few percent. Analysis suggests that net currents are larger inside the beam channel and that fast electrons and their secondaries carry a significant fraction of the return current in a halo outside the beam.