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

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 H 2 * (vibrational level V") on plasma parameters (i.e. electron temperature T e , electron density n e , hydrogen gas pressure p, density ratio n fe /n e of fast primary electrons e f to slow plasma electrons e, energy of fast electron E fe , etc.) is explored because it is expected that H - ions are produced by the following two-step process, i.e. H 2 +e f to H 2 *(V")+E f ; H 2 *(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 H 2 *(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 e f with energies in excess of 40 eV is reasonable for H 2 *(V") production. With increasing T e (above 1 eV), H - yield decreases monotonically. Besides, n e , n fe /n e and p have some optimum values for H - production. However, the optimum condition for H - formation is not compatible with that for H 2 *(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.