Microhollow cathode discharge stability with flow and reaction

Abstract

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.