Emission of Negative Ions From Surfaces With High Emissivity

Spectra of the negative ion fraction in the low field emission from MPG-8 graphite with rough surface and tungsten surface with nano fuzz as a function of the residual gas composition and pressure are measured and analyzed. The intensity of the emitted negative ions increases linearly with the pressure of the working gas in the range of $10^{-5}$ to $10^{-7}~{\rm Torr}$ and depends on the atomic mass of the residual gas. A simple model based on the secondary negative ion emission of adsorbed particles under positive ion bombardment of the cathode is proposed.