Thermal and Optical Study on the Frequency Dependence of an Atmospheric Microwave Argon Plasma Jet

This paper presents the frequency- and power-dependent thermal and spectral properties of a microwave argon plasma jet in the gigahertz range. These properties are determined at 1.3 GHz, 2.4 GHz, and 3.5 GHz for input powers from 2 W to 15 W and gas-flows from 1 to 4 Lm(-1), including the influence of an argon/nitrogen mixture. These investigations are performed to determine the specific influence of the excitation frequency on the spectral properties of the plasma. The gas temperature is determined via the hydroxyl radical (OH) rotational band around 310 nm by fitting the simulated spectrum to the measurements. It is shown that high temperatures above 1350 K are generated inside the plasma for an input power of 10 W. At the same time, selective discharge 2-D mapping is realized as a result of charge-coupled device (CCD) imaging through optical bandpass filters, corresponding to the OH, N-2, Ar, and H spectral emission lines. It is found that all the investigated species change their size proportional to the size of the plasma. Influence of nitrogen is investigated by generating an argon/nitrogen gas mixture. The nitrogen admixtures up to 2% are examined. These investigations are based on measurements of the N-2-band around 370 nm. A strong dependence of the discharge appearance on the nitrogen content is found.