SUPPRESSION OF ACCELERATED ELECTRONS IN A HIGH-CURRENT LARGE NEGATIVE-ION SOURCE

Accelerated electrons, which would lead to high thermal load of grids, have been suppressed in a high-current large hydrogen negative ion so urce. An extraction grid, with apertures shaped as the secondary elect rons generated on the grid aperture surface would be shielded against the acceleration electric field, works well to prevent the secondary e lectrons from leaking to the acceleration gap, compared with a straigh t aperture extraction grid. Although the strong magnetic field at the extraction grid also lowers the electron leakage downstream, the apert ure shaping of the extraction grid is more effective for the suppressi on of the accelerated electrons. The acceleration efficiency, defined by the ratio of the negative-ion current to the acceleration drain cur rent, is improved to around 85%. There remains the accelerated electro ns generated in the negative ion neutralization by collision with the residual neutral molecules during the acceleration. The direct interce ption of the accelerated negative ions with the downstream grid is sma ll. The reduction of the operational gas pressure is quite important t o achieve further improvement of the acceleration efficiency. (C) 1997 American Institute of Physics.