THE INFLUENCE OF THE GAS VELOCITY ON DISSOCIATION DEGREE AND GAS TEMPERATURE IN A FLOWING MICROWAVE HYDROGEN DISCHARGE

We report on the influence of the gas velocity on the discharge equili brium in a low-pressure (1 Torr) microwave driven hydrogen plasma. We show that the gas velocity has a great influence on the dissociation d egree, the key parameter being the residence time of the molecules in the plasma with respect to their effective dissociation time. The powe r balance (plasma length, electric field...) is substantially modified by the variations of the dissociation degree induced by the change in gas velocity. On the contrary, the gas temperature, determined by Dop pler broadening of Il atom Balmer lines, is not directly dependent on the gas velocity. Simple calculations confirm that the gas temperature is determined by local parameters and that heat is not axially transp orted. Surprisingly, dissociation of the H-2 molecules does not seem t o play any significant role for the heating of the gas. (C) 1998 Ameri can Institute of Physics.