Correlation between negative hydrogen ion production and work function of plasma grid surface in a cesium-introduced volume production-type negative hydrogen ion source

The correlation between the work function of a plasma grid surface in a Cs-introduced volume-production-type negative hydrogen ion (H-) source and the extracted H- current has been investigated experimentally. The quantum efficiency of a Cs/Mo surface has been measured as a function of the photon energy in a high-vacuum test chamber, and a relationship between the photoelectric work function and the quantum efficiencies at the wavelengths of an Ar+ laser has been obtained. Using this relationship, the work function of the plasma grid surface has been determined by measuring quantum efficiency using an Ar+ laser, When the increment of the H- current is plotted against the measured work function, an exponential dependence on the decrease of the work function is observed. The experimental result cannot be explained by a theoretical H- formation model when one assumes that H- are produced from H atoms leaving from the surface with the energy corresponding to the temperature of H atoms in the plasma. When H- are produced with the kinetic energy substantially larger than thermal energy, the theoretical result shows good agreement with the experimental result.