Production of H- Ions by Surface Mechanisms in Cs-free Multi-dipolar Microwave Plasma

This study demonstrates the feasibility of the recently proposed idea of enhancing recombinations of the hydrogen atoms from the plasma on a surface in order to produce highly vibrationally excited molecules that can be attached and dissociated by the cold electrons of the plasma, hence creating negative ions that could be used as a Cs-free negative ion source. The negative ion density was obtained for a) two distinct materials, i.e. tantalum and stainless steel, and for b) two different degrees of molecular hydrogen dissociation, the higher degree of dissociation resulting from the cooling of the walls of the source. The relative negative ion density n-/n_e was measured by laser photodetachment and the electron density was obtained from Langmuir probe measurements. The pre-sheath was studied by emissive and conventional Langmuir probes to evaluate the potential drop near the surface. Laser photodetachment measurements performed in the vicinity of the investigated material consisting of a disk inserted in the source, evidence the production of negative ions by surface mechanisms. With cooled walls a tantalum disk at floating potential increases the negative ion density by 60-100% (depending on the probe distance to the disk) compared to a stainless steel disk under the same conditions. The observations strongly suggest surface-assisted recombination processes involving H atoms such as the Langmuir-Hinshelwood (diffusion), Eley-Rideal (direct impact) and hot atom (indirect collisional) mechanisms, followed by dissociative attachment.