On 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 equilibrium in a low-pressure (1 Torr) microwave driven hydrogen plasma. We show that the gas velocity has a great influence on the dissociation degree, the key parameter being the residence time of the molecules in the plasma with respect to their effective dissociation time. The power 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 Doppler broadening of H 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 transported. Surprisingly, dissociation of the H₂ molecules does not seem to play any significant role for the heating of the gas.