The trajectories of the H? and the secondary particle in a multi-aperture accelerator for negative ion sources were calculated with a 3D numerical code, taking into account non-uniformity in magnetic field and negative ion density, and the heat loading on acceleration grids was estimated. The heat loading on a grounded grid mainly originates from the secondary electron impact and the non-uniform transverse magnetic field enhances the possibility of the secondary electron impacts on the grounded grid. Perveance condition of beamlets strongly depends on the plasma density profile and the direct interception of the H? by an extraction grid is enhanced in the non-uniform plasma case.
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