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A novel method for measuring microwave plasma impedance is presented. Plasma impedance is measured in real-time using three coupling probes and a microwave detector, rather than using a vector network analyzer or directional coupler, spectrometer. The measured impedance of a plasma filament at atmospheric pressure demonstrates that the impedance changes significantly with power variation, whereas the influence of airflow is minor. This implies that the impedance matching of atmospheric pressure microwave plasmas should take into account the effects of microwave power variations, and the use of a fixed matcher is not recommended for impedance matching of plasma jets unless they are operating at a fixed power. This measurement method avoids the intervention of other microwave power or expensive instruments, which facilitates the development of microwave plasma models and the tuning of efficiency. [ABSTRACT FROM AUTHOR]

Plasma-based microwave power limitation is experimentally investigated in a microstrip transmission line integrating a direct current (dc) plasma discharge for preionization. Steady state and transient high-power microwave measurements demonstrate that the preionization by the dc plasma discharge helps in reducing limiting power thresholds and time responses to about 30 dBm and tens to few microsecond, respectively. Besides, optical diagnostics also exhibit interesting behaviors of the plasma discharge such as diffusion and parasitic breakdowns. Finally, the effect of plasma confinement is discussed, and it is shown that reducing the volume devoted to plasma expansion leads to a decrease in the dynamic range of the microwave power limitation. [ABSTRACT FROM AUTHOR]