Wall Effects on the Production Processes of Positive and Negative Ions in Hydrogen Plasmas

Positive and negative ion species ratios in hydrogen plasmas are calculated numerically as a function of plasma parameters by using the particle balance model. The model considers important processes for the production of both H^- and positive ions. Particular attention has been paid to wall effects on volume production of H^- ions. Therefore, the recombination coefficient γ_1 for H and the de-excitation parameter γ_2 for vibrationally excited molecules H_2^* at the wall surface are treated as numerical parameters. The calculated results have confirmed that H^- ions are generated by dissociative attachment to highly excited H^2^* (vibrational level ν”=5-10) and these molecules are produced by fast electrons with enegies in excess of 30 V. The most significant point is that ion species ratios depend strongly not only on plasma parameters but also on wall parameters, γ_1 and γ_2. With decreasing γ_1,i.e. γ_1<1,H and H^+ increase while H^- ratio decreases because of the attenuation of H_2^* and the enhanced loss process, H^- +H^+ →2H.