Direct measurement of electrons co-moving in vacuum with a 100-kA, MeV-proton beam

Abstract

Summary form only given. Propagation of intense ion beams in vacuum can be shown to be physically impossible without a significant degree of charge- and current-neutralization from co-moving electrons. High-sensitivity laser interferometry was used to measure the electron density co-moving in vacuum with an intense proton beam (100 kA, 1 MeV, 50 ns) from the Gamble II generator. This measurement is non-perturbing and sufficiently quantitative to allow benchmarking of codes (particularly IPROP) used to model beam-gas interaction and ion-beam transport. Very high phase sensitivity is required for this measurement. For example, a 100-kA, 1-MeV, 10-cm-radius proton beam with uniform current density has a line-integrated proton density equal to n/sub p/L=3/spl times/10/sup 13/ cm/sup -2/. An equal electron line-density, n/sub e/L=n/sub p/L, (expected for transport in vacuum) will be detected as a phase shift of the 1.064 /spl mu/m laser beam of only 0.05/spl deg/, or an optical path change of 1.4/spl times/10/sup -4/ waves (about the size of a hydrogen atom!). The line-integrated electron density, measured across a diameter of the transport chamber at 43 cm from the input aperture, has the same time dependence and magnitude as the proton density (n/sub e/L/spl sim/n/sub p/L), where n/sub p/L is estimated from ion beam diagnostics. The measurements will be compared with theoretical predictions from the IPROP code.