Study on the propagation of ionization waves in a helium plasma jet at atmospheric pressure

Abstract

An atmospheric pressure plasma jet (APPJ) is the propagation of an ionization wave that contains a photoionization process. Helium APPJ is known for its versatility, but not many of its characteristics are known. This is because APPJ is influenced by various factors and it is hard to control these factors. Among these factors, the gas flow rate changes the amount of ambient air penetration and can change plasma properties but is often overlooked. Moreover, the He 23S state interacts with ambient air and changes plasma properties but the relation between the ionization wave and the distribution of He 23S is unknown. In this thesis, the spatiotemporal distribution of the He 23S state was measured by using tunable diode laser absorption spectroscopy (TDLAS), and compared with the propagation of ionization wave measured by using an intensified camera. To ease the measurement of the helium metastable state, the spatiotemporal evolution of the Voigt profile was measured, and concluded that the Voigt profile is nearly constant with regard to space and time under certain conditions. Then by using the assumption above, TDLAS image analysis was conducted. As a result, helium metastable propagation speed was slightly slower than the propagation speed of the ionization wave. Additionally, unknown phenomena were observed. First, when the gas flow rate was lower than 2.0 slm, the plasma bullet which shows the location of the ionization wave was separated. Another phenomenon observed was that when the gas flow rate was lower than 2.5 slm, helium metastable density had a double peak near the nozzle. To have further insight into this phenomenon, a measurement of the propagation of the ionization wave in the ignition phase was conducted. As a result, it was concluded that return stroke length increase by ambient air penetration into the helium column caused this phenomenon.