CORRELATION OF NEGATIVE HYDROGEN ION PRODUCTION AND THE WORK FUNCTION OF A SURFACE IN A HYDROGEN PLASMA

Surface-plasma negative hydrogen ion sources are being developed as possible parts for future neutral beam systems. In these ion sources, negative hydrogen ions (H('-)) are produced at low work function metal surfaces immersed in hydrogen plasmas. To investigate the correlation between the work function and the H('-) production at the surface with a condition similar to the one in the actual plasma ion source, these two parameters were simultaneously measured in the hydrogen plasma environment.;As Cs density was increased in the hydrogen discharge, the work function decreased until it reached a minimum value. This value of the lowest work function was approximately 1.4 eV for both Mo and Cu surfaces, and the detected total H('-) current was a maximum at this condition. Further introduction of Cs into the discharge from this point increased the work function and decreased the total H('-). Thus, at any coverage of Cs on the surface, the total H('-) current was monotonically increasing for decreasing work function.;When the negative bias potential to the H('-) production surface was increased, the work function changed in a way corresponding to a decrease in Cs coverage. Due to this effect a bias potential for maximum H('-) production was observed for steady state operation of the ion source with a fixed density of Cs. When H('-) currents from surfaces of similar work functions, but with different bias potentials, were compared, the total H('-) current was always larger at the higher bias potential.;The photoelectron emission currents from Mo and Cu surfaces in a cesiated hydrogen discharge were measured in the photon energy range from 1.45 to 4.14 eV, to determine the work function based on Fowler's theory. A small magnetic line cusp plasma container was specially designed to minimize the plasma noise and to realize the efficient collection of incident light onto the target. The photoelectron current was detected phase sensitively and could be measured with reasonable accuracy up to about 5 x 10('11) cm('-3) of the plasma electron density.