An improved conversion of the microwave energy into plasma in an optimized microwave plasma sheet source at 2.45 GHz designed for surface treatment

Abstract

Abstract Efficient conversion of the microwave energy into an atmospheric-pressure argon plasma in a microwave plasma sheet source (MPSS) operating at 2.45 GHz in ambient air is the subject of this work. The MPSSs, capable of providing an atmospheric pressure plasma in the shape of a narrow sheet are an innovative technology for variety of surface treatments with no impact on the environment. The existing designs of the MPSS exhibit unsatisfactory efficiency of the conversion of microwave energy produced by the microwave source into the microwave plasma (about 80%), which hinders implementation of the MPSSs in the industry. In this paper, after an analytical approach based on the equivalent electrical circuit method we proposed an optimization of the design of MPSS in terms of the microwave energy conversion efficiency. The MPSS design improvement consisted in reducing the electrical impedance in the input plane of the MPSS by introducing a capacitive diaphragm into the MPSS transmission line, which compensated the inductive character of the plasma sheet. The experimental measurement of the electrodynamic characteristics of the MPSS with diaphragm showed that using the diaphragm significantly decreased the microwave power losses due to the microwave reflection from about (20–30)% to a level of several percent. This substantially increased the MPSS energy conversion efficiency up to about 97%, 93% and 87% for incident powers P I of 500 W, 750 W and 1000 W, respectively. The electrodynamic characteristics of the MPSS appeared a very convenient tool for determining the conditions of maximum efficiency of conversion of the microwave energy into the microwave plasma. The electrodynamic characteristics were measured at argon flow rate of 20 Nl min−1. The analysis of the calculated input impedances of the MPSSs with and without diaphragm, based on the equivalent electrical circuits of both MPSSs confirmed that the change of the character of the input reactance of the MPSS from inductive to capacitive after introducing the diaphragm is the cause of the increase in the MPSS energy conversion efficiency. The MPSSs having the energy conversion efficiency such high as presented in this paper appears to be attractive innovative technology for economic use in industrial applications.