Compensation of beamlet repulsion in a large negative ion source with a multi aperture accelerator

Abstract

Excess heat loads to accelerator grids limit extension of pulse length in operation of the large negative ion sources with multi aperture accelerator. Part of the heat loads is caused by interception of deflected beamlets due to their space charge repulsion. In this paper, a beamlet steering technique using aperture offset was examined for compensation of the beamlet deflections utilizing a three dimensional beam analysis simulating the D− negative ion source of JT‐60 U. The beamlet deflection was analyzed in detail using fifty beamlets, which were extracted from apertures arranged in a lattice pattern of 10×5. The simulation showed successful compensation of the beamlet deflection by aperture offsets defined according to the thin lens theory. Even if the beam energy was changed, the necessary aperture offset would not be changed maintaining the perveance and a ratio of extraction and acceleration voltage. In JT‐60 U, it was shown that the aperture offset of less than 1.0 mm would be enough to compensate the repulsion of all beamlets. When the magnetic field was applied for suppression of co‐extracted electrons, necessary aperture offset was estimated to be ±0.5 mm for 500 keV D− ion beam in JT‐60 U, in addition to the offset for the space charge repulsion. This result showed good agreements with the previous experimental results and design study of the JT‐60 U N‐NBI.