Dependence of vacuum electrical breakdown field and field enhancement factor on the number of apertures drilled in small electrodes

Pre-breakdown current based on field-emission current is regarded as one of factors causing the vacuum electrical breakdown (BD) between the multi-aperture acceleration grids in the JT-60 negative ion source. In this paper, we focused on field enhancement factor β which is a key parameter of field-emission on the cathode. Vacuum BD tests for small-size electrodes drilled different number of apertures were carried out to investigate the β and BD field characteristics in the conditioning process of each electrode. Experimental results revealed that the increase of the number of apertures drilled in the electrode causes the increase of β value, and high β value reduces the dielectric strength of these electrodes. This result demonstrates that flowing field-emission current between the multi-aperture electrodes causes vacuum BD event.