Measurements of H-density and work function of a plasma electrode in a negative ion source

The correlation between the work function of the plasma electrode of a H-ion source and H-density was investigated in H2 plasma, which contains the vapor of an alkali metal (cesium or rubidium). For the work function measurement, the photoelectric current was measured with laser irradiation to the plasma electrode, while the negative ion signal was measured by the laser photodetachment technique simultaneously with an electrostatic probe. As the work function of the plasma electrode was reduced due to alkali metal adsorption, the ratio of H- density to electron density, n-/ne, increased gradually. The n-/ne ratio became the maximum value when the work function reached the minimum value. The measured work function was well correlated with the n-/ne ratio. The dependence of plasma parameters on the discharge current is compared between pure H2 and H2 + (Rb/Cs) mixture discharge