Experiments of an intense H- ion beam acceleration

An intense H- beam of a single beamlet is extracted from a large multicusp plasma source operated with cesium seeding. The H- beams are accelerated up to 100keV by a single-stage or a two-stage electrode system. Spatial profiles of the beams are measured calorimetrically and a minimum beam divergence angle of 5mrad is achieved at a H- current density of 30mA/cm2 with a beam energy of 100keV. A ratio of an acceleration current to a 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 an 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 becomes minimum when a ratio of Vacc to Vext is set at a 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 Eacc1/Eext is 1.5. In the optimum Eacc1/Eext ratio the divergence angle is not affected by Vacc2. The divergence angle can be reduced by changing Vacc2 even if the ratio of Eacc1/Eext 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. (author)