Space-charge neutralization measurement of a 75-keV, 130-mA hydrogen-ion beam

A 75-keV, 130-mA dc mode proton injector is being developed at Los Alamos. The two-solenoid focusing system used in this injector relies on effective beam space-charge (s.c.) neutralization by beam ionization of the residual gas. The degree of s.c. neutralization is determined by measurement of the energy distribution of slow ions created in the beam region. A non-interceptive gridded-energy analyzer is used for the slow ion-energy measurement. Application and development of this diagnostic to a positive hydrogen-ion beam is presented. One feature of the ion-energy distribution measurement is a double-humped distribution. The higher energy component contains less than 20% of the total ion current. Measurements are reported which help validate the axisymmetric model used here to derive the degree of beam s.c. neutralization, f. Fairly weak dependencies of f on beam current and background gas densities are found, and f is typically found to vary between f=95-99% depending on the presence of the "high-energy" tail on the slow ion distribution. This degree of beam s.c. neutralization assures good transport of the hydrogen positive ion beam to the next accelerator.