Mechanism of negative ion production in a cesium seeded ion source

The production mechanism of negative ions has been investigated experimentally in a cesium seeded volume type negative ion source. The cesium light emission intensity, Cs ion volume density, plasma parameters, H^- current, and work function of the plasma grid were measured. The change in plasma parameters was small even in a high Cs ion density region(up to 10¹² cm^-3) which was also confirmed by zero-dimensional simulation. Upon heating the plasma grid, the work function decreased and the negative ion current increased to become three times larger than that of the low plasma grid temperature condition. This increase was in good agreement with the decrease of the work function of the plasma grid surface. Therefore, it is considered that the Cs contribution for the negative ion production is the surface effect in the negative ion source.