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 V_acc to V_ext is set at an optimum value of 1.6 in the single-stage acceleration. This ratio is almost the same as that in the double-stage acceleration, where the optimum ratio of E_accl/E_ext is 1.5. In the optimum E _accl/E_ext ratio the divergence angle is not affected by V_acc2. The divergence angle can be reduced by changing V _acc2 even if the ratio of E_accl/E_ext is not optimized. The beam steering effect by permanent magnets buried in an extraction grid is observed in nine beamlets experiments. A simple calculation of a single particle trajectory gives a good approximation of the beam deflection angle.