An investigation into plasma surface interactions, focussing on surface produced, and surface assisted, negative ion generation

The study of negative ions in low temperature, low pressure, radio-frequency plasmas has become increasingly important for a number of applications, including neutral beam injection (NBI). Additionally, as nuclear fusion reactors increase in size, more stringent requirements are placed upon NBIs. Unfortunately, the use of positive ions is infeasible at these higher energies, as their neutralisation efficiency rapidly decreases at higher energies/nucleon; this is not the case for negative ions. However, generating negative ions in sufficient quantities is challenging. Currently, the only feasible method is through using caesiated metallic surfaces. Whilst caesiated metallic surfaces are an acceptable replacement in the short term, an alternative is necessary, due to the inherent problems with using caesium.
This work has measured H- ion densities through using laser photodetachment above multiple metallic surfaces; these have been shown to have an influence on the volume produced H- ion densities. It was discovered that tungsten and stainless steel surfaces result in a higher H- ion density than copper, aluminium, and molybdenum surfaces. This was explained by considering that the recombination of atomic hydrogen at the surface could produce vibrationally excited molecular hydrogen, that in turn undergoes dissociative attachment to form H- ions. Two-photon absorption laser induced fluorescence and phase-resolved optical emission spectroscopy were used to investigate this hypothesis; the results of which are presented within this work.
Finally, for the first time, nitrogen doped diamond samples and a nanoporous 12CaO 7Al2O3 (C12A7) electride were investigated for their ability to produce H- ions. Consequently, nitrogen doping was observed to have an influence on the H- ion yield; this was not observed with previously investigated doped diamond samples. Comparisons to other non-caesiated surfaces therefore suggest that both materials are worthy of further study for their ability to produce a large quantity of negative ions.