Influence of surface chemistry on the transport of H atoms in a supersonic hydrogen plasma jet

The transport of ground-state hydrogen atoms in the expansion of a thermal hydrogen plasma created by a cascaded arc is studied by means of two-photon absorption laser induced fluorescence. The low-dissociation degree measured at the source exit implies that H atoms flow in a H2 environment. It is shown that the H atom expansion pattern is in disagreement with the neutral gas supersonic expansion theory. Indeed the transport of H atoms in the plasma jet is strongly influenced by surface-recombination processes. Because of the large density gradients between the core of the jet and its surroundings induced by the recombination of H atoms at the reactor walls, hydrogen atoms diffuse out of the plasma jet in the course of the expansion. When the surface loss probability is high, i.e., the combination of a large wall-recombination probability with a long residence time, the losses of radicals by diffusion cannot be avoided even when the mass of the carrier gas is close to the mass of the radical.