Evaluation of Reactive Species in a Frequency-Tunable Resonant Microwave Plasma Jet

Plasma jets generate a stream of chemical species, including ions, electrons, and radicals, which are crucial in various biomedical and industrial applications. Hence, alongside a power-efficient performance, control over reactive species production is essential. Among the existing technologies, the evanescent-mode cavity resonator structure has emerged as a promising candidate for achieving power-efficient plasma sources [1]. This technology enhances and confines electromagnetic fields, enabling the ignition and sustenance of plasma jets even at power levels in the milliwatt range. The type and concentration of chemical species generated by such sources can be controlled through several operating parameters such as input power, background gas type and pressure, and gas flow rate. Introducing frequency tunability offers an additional means to manipulate these reactive species. Motivated by this potential, we developed a frequency-tunable microwave plasma jet based on the evanescent-mode cavity resonator technology structure. By utilizing surface mountable varactors [2], the resonant frequency of a prototype plasma jet was tuned within the range of 2.6 to 2.9 GHz. It is demonstrated that by adjusting the frequency within this frequency range, the concentration of hydroxyl (OH) radicals can be varied from 10¹⁴ cm^-3 to 10¹⁵ cm^-3 when helium is used as the jet flow gas to atmospheric air. The inclusion of frequency as a tunable parameter, in conjunction with other operating parameters, provides a broader range of control over the types and concentrations of reactive species being generated.

References:

1. A. Semnani and K. S. Kabir, "A Highly Efficient Microwave Plasma Jet Based on Evanescent-Mode Cavity Resonator Technology," IEEE Transactions on Plasma Science, vol. 50, no. 10, pp. 3516-3524, Oct. 2022.

2. A. Anand, J. Small, D. Peroulis and X. Liu, "Theory and Design of Octave Tunable Filters with Lumped Tuning Elements," IEEE Transactions on Microwave Theory and Techniques, vol. 61, no. 12, pp. 4353-4364, Dec. 2013.