Research on periodic discharge characteristics of atmospheric-pressure dual-pulse-modulated microwave Ar/N2 plasma jet

Atmospheric-pressure microwave plasma jet has significant advantages, including high reactivity, high density, and direct treatment of complex-shaped surfaces. Pulse-modulated microwave technology, which periodically turns the microwave energy input on or off, provides a vital means to precisely control the spatiotemporal characteristics of the plasma jet and thereby optimize its processing efficacy. To further investigate the periodic characteristics of an atmospheric-pressure pulse-modulated microwave plasma jet, a coaxial resonator with a dual-gas channel structure was first employed by applying a dual-pulse-modulated microwave. This generated a stratified Ar/N2 plasma jet in which periodically evolving appearances were observed. Using a high-speed camera to capture dynamic images, it is found that the size of plasma jet is larger during the first three pulse cycles of each pulse train; starting from the fourth pulse cycle, both the length and radius of its luminous contour gradually decrease and eventually stabilize. Subsequently, time-resolved electron density was measured using a microwave Rayleigh scattering system. This revealed that variations in electron density are the root cause changing the optical radiation characteristics and the appearance of plasma jet, thereby leading to the differences in discharge characteristics between pulse cycles. Finally, transient spectra were measured using a monochromator, which show that the plasma jet appearance is related to changes in the relative spectral intensity of N2. Comprehensive analysis indicates that the skin effect and the development of vibrational excited states of N2 are the main physical mechanisms responsible for the transition of periodic characteristics in the atmospheric-pressure Ar/N2 plasma jet under dual-pulse-modulated microwave.