Voltage holding capability of large-size acceleration grid with multiple-apertures and multiple-stage for negative ion source

Voltage holding capability of a large negative ion source for fusion application is experimentally examined, which is characterized by multiple-stage acceleration with multiple-apertures over 1000 on large-area grids of 2 m2 for the multiple-beamlet accelerations. From the observation of the vacuum discharge between the grids, it was found that the aperture generated 10 times larger dark current than the flat region and initiated the vacuum discharge associated with the breakdown. As a result, it was found that the sustainable voltages were dominated by not only the surface area but also the number of the apertures. Because these effects were originated in the area effects by weak and strong electric field profiles, these results implied the surface integration of the electric field were the key parameter for the vacuum insulation. (author)