Three-Dimensional Analysis of Beamlet Deflection in a MeV Accelerator for ITER NBI

Abstract

At Japan Atomic Energy Agency (JAEA), a MeV accelerator has been developed to demonstrate acceleration of H⁻ ion beams at the ITER-relevant power density. After long pulse beam acceleration tests of up to 10 s, molten areas were observed around the apertures of the grids due to excess heat load on the grids. To identify the cause of the melting, a three-dimensional (3D) beam analysis was performed. The stripping loss of negative ions and magnetic fields in the accelerator were included in the calculation to examine the beam trajectories precisely. It was clarified that the beamlet deflection angle was larger than 10 mrad due to space charge repulsion among the beamlets and magnetic fields, which resulted in excess heat loads of more than 20 kW/cm² at the grounded grid. To compensate for beamlet deflections, an aperture offset and field shaping plate were designed.