Plasma power balance: methodology and investigations of microwave capillary discharges

Abstract

Abstract Nowadays, where strong energy constraints are set by the environmental and economical context, the real power coupled in plasmas becomes a critical aspect particularly for applications where plasma technologies are facing competition. This issue is enhanced for atmospheric pressures micro-plasmas, where the local power density can be very high (e.g. 105 W cm−3) which implies fast heat exchanges. The precise knowledge of the power coupled to plasma is also a key for fundamental understanding of discharge properties. This is often used as an input parameter for plasma modeling and its inaccuracy can mitigate the predictive quality of plasma simulation tools. In the present paper the macroscopic power balance was established for continuous microwave (MW) plasmas generated in capillaries in argon gas flows. The macroscopic power budgets were performed based on measurements of microwave leak fluxes, heat fluxes (thermal radiation, convection, conduction), optical radiation and chemical powers. It was proven that the traditional power measurements at the MW generator would largely overestimate the real power coupled to plasmas, namely by at least a factor two. An important power fraction is found in the heat transfer through the MW launcher, the impedance mismatching being here better assessed. The proposed methodology, which is based on the physics of transport phenomena, can be employed for characterization of other (micro-) plasma sources.