Electron Temperature and Electron Density Measurement of Positive Pulsed Streamer Discharge Plasma in High Pressure Gas by Laser Thomson Scattering Diagnostics

Abstract

Pulsed streamer discharge plasma, which is a type of non- thermal equilibrium plasma, has the characteristics that electron temperature is higher than gas temperature, discharge area is large, and electric field is high. Furthermore, it is known to generate various kinds of chemically active species, therefore its application field is wide as water treatment, ozone generation, exhaust gas decomposition etc. However, different applications require different kinds of chemically active species, then, the chemically active species are generated by electrons having different electron energies. Thus, it is possible to improve plasma processing efficiency by measuring the accurate electron temperature of the generated pulsed streamer discharge. Laser Thomson scattering diagnostics (LTS) is considered to be a reliable method to measure electron temperature and density in plasma. So far, it has been used mainly for measuring arc discharge and glow discharge. In the LTS, it is necessary to apply a laser to the discharge. In case of arc and glow discharge, the discharge is stable between anode and cathode electrodes, therefore, laser irradiation is relatively easy. On the other hand, in case of streamer discharge, since the discharge branches and the position of streamer head changes irregularly, it is necessary to control both the discharge itself and laser irradiation timing on a fine scale both temporally and spatially. Therefore, measuring pulsed streamer discharge using LTS diagnostics is a challenging topic. In this study, propagation progress of the pulsed streamer discharge at a needle-to-cone electrode geometry was observed with an ICCD camera, and the characteristics of the streamer discharge were investigated. In addition, LTS diagnostics was performed to clarify characteristics of the streamer discharge. In the LTS measurement, a positive high voltage was applied to the needle electrode to generate a streamer discharge between the electrodes, a laser was irradiated just below the anode needle, and the scattered light that has passed through the spectroscope was photographed with an ICCD camera. We succeeded in obtaining Thomson scattering under high pressure He gas and other gas species. The electron temperature and electron density of the pulsed streamer discharge were measured.