Numerical studies on the optimisation of volume-produced H-ions in the single-chamber system

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Abstract
Optimisation 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. The dependence of production of both H- ions and vibrationally excited hydrogen molecules H2* (vibrational level V") on plasma parameters (i.e. electron temperature Te, electron density ne, hydrogen gas pressure p, density ratio nfe/ne of fast primary electrons ef to slow plasma electrons e, energy of fast electron Efe, etc.) is explored because it is expected that H- ions are produced by the following two-step process, i.e. H2+ef to H2*(V")+Ef; H2*(V")+e to H-+H. Particular attention is also paid to wall effects, i.e. neutral particles-wall interaction, on H- production. So, a wall recombination coefficient gamma 1 for H and a wall de-excitation collision parameter gamma 2 for H2*(V") are treated as numerical parameters. It is confirmed that most H- ions are produced by the above-mentioned two-step process, and that the presence of ef with energies in excess of 40 eV is reasonable for H2*(V") production. With increasing Te (above 1 eV), H- yield decreases monotonically. Besides, ne, nfe/ne and p have some optimum values for H- production. However, the optimum condition for H- formation is not compatible with that for H2*(V") production. Another significant point is that the ion species ratios depend strongly on the wall parameters, i.e. gamma 1 and gamma 2. For H- production, the optimum condition is that gamma 1 approximately=1 and gamma 2<<1.