-
Author(s):
A. Ando, Y. Takeiri, O. Kaneko, Y. Oka, K. Tsumori, E. Asano, T. Kawamoto, R. Akiyama and T. Kuroda
-
Title:
Experiments of an Intense H- Ion Beam Acceleration
Date of publication:
Aug. 1995
-
Key words:
Negative hydrogen ion source, NBI, High current density, Beam divergence, Beam deflection, Beam intersection with grids, Stripping loss of H^-.
-
Abstract:
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/cm^2 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 V_acc to V_ext is set at a optimum value of I .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 I .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.
-
|
