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

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 A²Σ⁺ → X²Π_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.

I. Introduction

As a type of plasma with a moderate and controllable gas temperature and as an efficient source of highly active species, microwave plasmas have achieved good results in the fields of surface modification, nanomaterial preparation, and exhaust gas treatment in recent years [1]–[5]. Compared with arc plasmas and dielectric barrier discharge plasmas, microwave plasmas have many advantages, such as 1) requiring no electrodes and 2) being easy to generate a large volume of plasma at atmospheric pressure. Therefore, microwave plasmas have wide application prospects for future use.