Surface Production of Hydride Ions by Backscattering Hyperthermal Hydrogen Atoms
The thesis experimentally demonstrates the surface production of H^- ions by backscattering hyperthermal hydrogen atoms of energy 1-10 eV from clean molybdenum and cesiated molybdenum surfaces. Hyperthermal hydrogen atoms are produced by electron impact dissociation through Frank-Condon excitation process in a hydrogen plasma. Theoretical calculations of the hyperthermal hydrogen atom flux density in various plasmas are made by using particle balance equations. A Lisitano-Coil interdigital slow wave structure is used for plasma production. The efficient production of hyperthermal hydrogen atoms by electron impact dissociation makes the discharge source an excellent source of hyperthermal hydrogen atom beams. The total H^- ion yield, which is the ratio of the H^- ion flux density to the atom flux density, is estimated taking a Maxwellian beam distribution and a cosine angular scattering distribution of H atoms on the basis of theoretical calculation of surface charge transfer probability. More than 20% of total H^- ion yield is expected for atomic hydrogen beams of a few eV temperature from cesiated metal surfaces with an extraction electric field of thousands Volts/cm. The abundant presence of hyperthermal hydrogen atoms and the high H^- ion yield suggest that major parts of H^ - ions in most H^- ion sources are produced by the surface process by backscattering hyperthermal hydrogen atoms from cesiated walls. H^ - ions produced on external converter surfaces, located outside the plasma, are analyzed by magnetic spectroscopy developed for this purpose. The measured parallel energy distribution of H^- ions follows a Maxwellian with temperature of a few eV. Several experimental results of parallel energy distributions are obtained for different operating conditions. Experimental data on production of H^ - ions from clean molybdenum and cesiated molybdenum converter surfaces as functions of discharge power and H_2 gas flow rate for several operating modes are presented. It shows that the H^ - ion current increases faster than linearly with power, and no saturation is observed with power up to 500 watts. Maximum H^- ion current is obtained at an H_2 gas flow rate of 8-9 sccm, which corresponds to 80-90 mTorr discharge pressure; this indicates that the surface collisions dominate the loss mechanism of the hyperthermal hydrogen atoms. (Abstract shortened by UMI.).
- Pub Date:
- HYDRIDE IONS;
- MOLYBDENUM SURFACES;
- Physics: Atomic; Physics: Electricity and Magnetism; Physics: Fluid and Plasma