Numerical Simulation on Tandem Negative Ion Source

Abstract

It is commonly accepted that in a hydrogen volume source the negative hydrogen ions H^- are formed by dissociative attachment of slow plasma electrons e to vibrationally excited molecules H₂ ^*(v″), where v″ is the vibrational quantum number.^1,2 The vibrational excitation is attributed to electron impact excitation of hydrogen molecules H₂ by the fast primary electrons e_f in the discharge.³ To clarify the mechanism of H^- production and to show the dependence of H^- ions on plasma parameters concretely, we have proposed a set of particle balapce equations for a steady-state hydrogen plasma in a single chamber system.^4,5 According to our recent simulation results,^6–8 most H^- ions are produced by the two-step process mentioned above, i.e. H₂+e_f→H₂ ^*(v″)+e_f; H₂ ^*(v″)+e→H^-+H. Besides, the presence of e_f with energies E_fe in excess of 40 eV is reasonable for H₂ ^*(v″) production while plasma electrons e with electron temperature ${{T}_{e}} \cong 1eV$ are optimum for H^- formation.