Dissociation of hydrogen molecules on metal filaments in H⁻ion sources

Atomic hydrogen causes significant deterioration of H- ion source characteristics through vibration-translation relaxation of vibrationally excited H2 molecules. We propose a method for atomic hydrogen density determination based on its selective absorption by a metal (Ta in our case) covered with a monoatomic non-metal film. The temperature-dependence of the dissociation rate by the Ta filament and the contribution of dissociation on Ta filaments to the atomic density in the H- ion source are measured and reported. This contribution is noticeable but not dominant when the energy injected into the discharge is large enough. It is shown that the filament surface of W, Ta, Nb or W-Th behaves as a clean one with respect to H2 dissociation under the actual conditions of H- ion sources. According to a phenomenological theory, filaments of W, Ta and Nb dissociate H2 independently of their temperature and of the gas pressure with probability about 0.2-0.3 per single impact, while a W-Th filament dissociates H2 with much lower probability, which depends on pressure and temperature.