Dependence of volume-produced H− ions on the wall recombination probability of H atoms in a low pressure H2 positive column Original Research Article

A kinetic study of a low-pressure hydrogen positive column, at moderate currents, is carried out by solving the electron Boltzmann equation coupled to a system of rate balance equations for the populations of the vibrational levels H2(X 1Σg+,v), dissociated atoms H(1s), and negative ions H−. A systematic investigation of the effects of the wall recombination probability γH of H atoms on the enhancement of H− concentration is presented. It is shown that relative concentrations [H−]/ne larger than one can be obtained, at p=1–5 Torr and I=20–50 mA, as the probability for wall losses of H atoms increases beyond γH≃10−2. On the contrary, for lower values of γH, the concentrations of H atoms are high enough to produce a fast depopulating mechanism of the vibrational levels H2(X,v) by V–T exchanges in H2–H collisions, preventing H− formation by dissociative attachment from H2(X,v) molecules.