Study of continuous wave 33 keV H^- beam transport through the low energy beam transport section

The transport of a continuous wave 33 keV negative ion beam through the low energy beam transport section, designed for beam injection into the tandem accelerator, was studied. The continuous wave H^- beam, produced by the Penning surface-plasma source with hollow cathodes and cesium addition, was separated and focused by the 90° bending magnet and then transported through the 0.8 m long transport tube, equipped with beam electrical and optical diagnostics. The beam current was measured by a water-cooled Faraday cup at the transport tube exit. Beam sizes and profiles were measured by CCD cameras looking into the beam from the back and sides. The additions of hydrogen, argon, and xenon to the transport tube and to the bending magnet chamber in the pressure range of 10^-6 to 10^-4 Torr were tested. The influence of gas addition on the beam space charge compensation and beam transport was studied. A mechanism for the H^- beam focusing due to space charge overcompensation and beam compression by the positive charge, produced with positive ion accumulation in the beam, is discussed.