Imaging Diagnostics and Gas Temperature Measurements of Atmospheric-Microwave- Induced Air Plasma Torch

Abstract

In this article, the core of an atmospheric-microwave-induced air plasma torch is resolved and measured by adjusting the exposure time of a high-speed camera. Gas temperature measurements of the microwave plasma core are achieved by molecular emission spectrometry of the ${A}^{2}{\Sigma }^{+}\to {X}^{2}{\Pi }_{r}$ electronic system of OH radicals. How the geometric features and gas temperature of the plasma core are related to the magnetron power and gas flow rate is explored, and the radial temperature distribution of the microwave plasma is measured. In these experiments, the magnetron power is 1033–2023 W and the gas flow rate is 10–20 L/min. The results show that microwave plasma has two parts, namely, its core and outer shell. The core radius increases approximately linearly with the magnetron power, whereas it does not change substantially with the gas flow rate. Gas temperature can be used to distinguish the core from the outer shell because the former can reach 8000 K, while the latter cools down from 5500 K. Moreover, the core temperature changes with neither the magnetron power nor the gas flow rate. Finally, a method to control the radial distribution of gas temperature of microwave plasma torch is presented.