Investigation on Continuous and Modulated Microwave Plasma Filaments at Atmospheric Pressure

Microwave atmospheric pressure plasma jets continue to evolve with the growth of atmospheric plasma’s applications in sterilization, material modification, and cancer treatment. In this article, continuous and modulated microwave plasma filaments were produced by a hand-held microwave plasma jet, and then they were compared in plasma length, electron excitation temperature, and emission spectrum. The electron excitation temperature was measured by the emission spectroscopy diagnostic method, and the regularities of the compared parameters’ change were studied by varying duty cycles and periods of the modulating signal. The results prove that the modulated microwave can generate stable plasma filaments directly in the air without protective tubes, and it is different from the continuous microwave plasma in all compared parameters. Besides, stable dendritic tearing of plasma filaments was observed. These results imply that modulated microwave and continuous microwave plasma filaments cannot be regarded as the same, the changing manner and the reaction on the surface of the medium may be completely different even if they are the same in the ambient air in most cases. These results are helpful for the controlling and understanding of microwave plasma, and the stable tearing could be used to expand the treated area of a single plasma jet.