Understanding the energy balance of a surface barrier discharge for various molecular gases by a multi-diagnostic approach

Abstract

Electrical and calorimetrical measurements were performed to obtain insight into the energy transformation from input power to the power available for the plasma treatment of substrates in a diffuse coplanar surface barrier discharge used as plasma source and operated in air, N 2, O 2, and CO 2 at input power of up to 380 W. Overall input power and applied electrical power were measured, and a conversion of 90% was determined with a constant loss of 30 W for the operation of the control unit. Measurements of the temperature gradient were performed for the electrode oil cooling to identify the power loss due to cooling. With roughly 50% of the applied electrical power, it turned out to be the largest loss term. A ceramic passive thermal probe was used to determine the energy flux from the plasma to a substrate. Highest energy flux values were found to be about 500 mW / cm 2 using air as working gas. Conversion efficiencies from the available electrical power via discharge to the power used for substrate treatment of 50%-35% depending on the working gas (highest for air, lowest for CO 2) were determined. Investigation on the spatial expansion of the surface discharge showed a dependence of the energy flux from the distance above the electrode. The energy flux maximum shifts to larger distances if no oxygen is present in the working gas.