Photoelectric work function measurement of a cesiated metal surface and its correlation with the surface-produced H− ion flux

For application in plasma heating, fueling, and current drive of magnetic fusion devices, high current negative deuterium ion sources for intense neutral beam injectors are being developed using efficient production of negative hydrogen isotope ions on low work function metal substrates imbedded in hydrogen plasmas. In order to investigate the correlation between work function and negative hydrogen ion production, photoelectron emission from a cesiated metal surface, which is immersed in a hydrogen plasma with an electron density less than 5×1010/cm3, was measured in the photon energy range of 1.3 to 4.1 eV. The work function determination was based on Fowler’s analysis, and at the optimum coverage a work function of less than 1.5 eV was observed for a Cs–Cu surface. Measured values of work functions for different Cs coverages were compared to the negative hydrogen currents produced at the metal surface in the discharge; the surface production of negative hydrogen ion current is monotonically increasing with decreasing work function.