Study on discharge modes and laser Thomson scattering diagnostics of an atmospheric pressure argon jet

With the ability to achieve separation between the plasma generation area and the application region, the atmospheric pressure argon plasma jet (APPJ) generates a remote plasma plume rich in various active particles in ambient air. Therefore, APPJ has been extensively used in lots of application fields. Previous studies have shown that helium APPJ can operate in either a guided streamer mode or a continuous discharge mode, depending on experimental parameters. However, it is not clear whether the continuous discharge mode exists in an argon APPJ. Aiming to this research status, an argon APPJ with a needle-plate electrode geometry is employed, which is excited by a sinusoidal voltage. With varying discharge power and gap width, three distinct operational modes are observed: the guided streamer mode, the transition mode, and the continuous discharge mode. Results indicate that as discharge power increases or gap width decreases, the argon APPJ undergoes transitions from the guided streamer mode, through the transition mode, and finally to the continuous discharge mode. In addition, temporal evolutions of electron density and electron temperature are determined by laser Thomson scattering for the continuous discharge mode, revealing that both electron density and electron temperature exhibit periodic variations during each voltage half-cycle. The changes in electron density and electron temperature follow the temporal profile of the gap voltage. Moreover, the results of the laser Thomson scattering are verified by the line intensity ratio method obtained from optical emission spectroscopy. Additionally, the gas temperature is obtained by fitting the rotational system band of OH radicals, which keeps almost unchanged over time within an acceptable margin of error.