Investigation of an intense H- ion beam produced by a volume source

An H/sup -/ ion beam from a volume source was accelerated to 20 KeV and transported 86 cm in a region where the pressure was varied in the range 10/sup -6/ to 7*10/sup -4/ Torr. The accelerator was operated under low and high pumping conditions. The diagnostics used were a combined Faraday cup/calorimeter and a pinhole profile analyzer. It was found that the exponential dependence of the unstripped H/sup -/ current on pressure in the transport region was not valid under high pumping. This is explained by the presence of a significant neutral fraction with beam energy at the accelerator exit. A theoretical analysis is proposed for determining not only the correct value of the negative ion current, but also the values of the positive ion and neutral beam components. The profile measurements established that at low pressure in the transport region the H/sup -/ beam expands due to its space charge and that the electrons produced by stripping concentrate on a ring at the periphery of the negative ion beam. The neutral component of the beam was also determined by using the pinhole analyzer as a secondary electron detector.<>