<RESEARCH REPORT>Optimization of Volume Produced H^- Ions in a Low-Pressure Hydrogen Plasma

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Optimization of volume produced H^- ions is studied by using a set of particle balance equations in a steady state hydrogen plasma with a single chamber system. Dependences of both H^- and vibrationally excited hydrogen molecules H^*_2 productions on plasma parameters (i.e. electron temperature T_e, hydrogen gas pressure p, electron density n_e, density ratio n_<fe>/n_e of fast electron e_f to cold plasma electron e, energy of fast electron E_<fe>) are explored because H^- ions are generated by the following two-step process, i.e. H_2+e_f⟶H^*_2+e_f; H^*_2+e⟶H^-+H. Particular attention is also paid to wall effects on H^- production. The wall recombination coefficient γ_1 for H and the wall de-excitaion collision parameter γ_2 are treated as numerical parameters. It is confirmed that the presence of e_f with energies in excess of 40 eV is essential to H^- production. With increasing T_e (above 1eV), H^- yield decreases monotonically. n_e, n_<fe>/n_e and p have some optimum values for H^- production. These plasma parameter dependences for H^- ions, namely optimum conditions, are not compatible with those for H^*_2 formation. Another significant point is that ion species ratios depend strongly on wall parameters, i.e. γ_1 and γ_2. For H^- production, the optimum condition is that γ_1≃1 and γ_2<<1.
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