Acceleration experiments for an intense H− ion beam

Intense H− beams have been extracted from a large multicusp plasma source operated with cesium seeding. The H− beams were accelerated up to 100 keV by a single‐stage or a two‐stage electrode system. Spatial profiles of the beams are measured calorimetrically and the beam divergence angle is obtained from half of the e‐folding width. A minimum beam divergence angle of 5 mrads is achieved at a H− current density of 30 mA/cm2 with a beam energy of 100 keV. The ratio of acceleration current to H− current increases abruptly when a H− current saturates in the space charge limited region. This enhancement is mainly due to secondary electrons caused by the intersection of H− beams with an extraction grid. When the operating gas pressure decreases, the ratio of the acceleration current to the H− current decreases. This is related to a stripping loss of H− ions in the electrodes. A beam divergence angle reaches a minimum when a ratio of Vacc to Vext is set at an optimum value of 1.6 in the single‐stage acceleration...