Resonance characteristics of a cavity-operated electrodeless high-pressure microwave discharge system

To characterize the resonance behavior of cavity-operated electrodeless high-pressure microwave discharge systems, a microwave circuit is developed that allows high-power network analysis during the operation of the discharge. The frequency-dependent complex impedance of the cavity including the plasma is obtained from a reflection measurement in the range 1.5 GHz-4.5 GHz. This method allows an observation of the actual resonance frequency and the actual vectorial mismatch so that a coupling efficiency >99% can always be achieved with proper adjustment of the operation frequency and coupling probe. Experimental results with a cylindrical TM₀₁₀ cavity and a coaxially situated discharge show a decrease of the resonance frequency with increasing power (increasing plasma temperature, electron density, and electrical conductivity) depending on the discharge geometry and the plasma composition. In accordance with a simple one-dimensional model, the Q factor passes through a minimum at medium power levels, where the cavity discharge system absorbs the microwave energy most effectively