Negative ion enhancement in caesium-seeded hydrogen discharges—a volume or surface effect?

Measurements of plasma parameters, including H− ion densities, made in conjunction with wall temperature, visible and vacuum ultraviolet emission spectroscopy verify that there is little caesium in the plasma volume of the H− ion source. Surface work function measurements indicate that there is significant caesium coverage of the inner walls of the ion source. It is found that, as the work function of a test surface decreases due to caesium seeding, the H− ion fraction in the discharge volume increases. These observations combine to indicate that, in the present source, the H− ion enhancement mechanism is a surface dominated effect.