Microhollow cathode discharge stability with flow and reaction

Under certain conditions, microhollow cathode (MHC) discharges display self-pulsing, with relaxation oscillations in voltage (V_d) and current (I_d). An equivalent circuit model of the discharge and circuit demonstrates that relaxation oscillations occur only if the load line crosses the discharge characteristic in the region of negative differential resistivity R_d ≡ ∂V_d/∂I_d. The pulsing and steady-state current regimes could have implications on the use of the discharges as reactors. We present measurements and model results in a study of high pressure MHC discharges as flow reactors in the steady-state current regime. Flow of molecular gases through the intense discharge induces chemical modifications such as molecular decomposition. The MHC behaves approximately as a plug flow reactor with reactant conversion depending primarily on residence time in the plasma. Measured peak gas temperatures in the plasma of the order of 1000-2000 K suggest that endothermic reaction conversion should be thermodynamically favoured. Comparisons to literature values of thermal decomposition kinetics indicate that the MHC plasma has the decomposition activity of gas at 2000-3000 K. High gas temperatures and molecular dissociation induce a significant pressure drop through the plasma. A model calculation for flow through a cylindrical tube containing an intense plasma demonstrates that the increase of pressure drop across the plasma zone is due to the increase in gas mass-averaged velocity as a result of lower mass density associated with the temperature increase and creation of molecular fragments.