H− beam neutralization measurements in a solenoidal beam transport system

H− beam space‐charge neutralization is measured for 65‐mA, 35‐keV beams extracted from a circular‐aperture Penning surface‐plasma source, the small‐angle source. The H− beam is transported to a RFQ matchpoint by a two‐solenoid magnet system. Beam noise is typically ±4%. A four‐grid analyzer is located in a magnetic‐field‐free region between the two solenoid magnets. H− beam potentials are deduced from kinetic energy measurements of particles (electrons and positive ions) ejected radially from the beam channel by using a gridded energy analyzer. Background neutral gas density is increased by the introduction of additional Xe and Ar gases, enabling the H− beam to become overneutralized.