Experimental investigation and parameter diagnosis of air high-pressure ring-shaped inductively coupled plasma
Authors: Wei Xiao-Long  , Xu Hao-Jun  , Li Jian-Hai  , Lin Min  , Song Hui-Min  ,   Science and Technology on Plasma Dynamics Laboratory, Air Force Engineering University, Xi'an 710038, China;,   College of Aeronautics and Astronautics Engineering, Air Force Engineering University, Xi'an 710038, China
Published: December 2020

The variable parameters like electron destiny (ne), electron collision frequency, covered-area and thickness have been regarded as the key factors for the electromagnetic scattering characteristics of the covering target. Therefore, an air inductively coupled plasma (ICP) generator of all-quartz chamber of 20 cm × 20 cm × 7 cm without magnetic confinement and grounded metal surface of substantial area is designed and conducted to study the discharge process and diagnose the parameters in this paper. The shape, E-H mode transition, and structure of inductively coupled plasma are observed, and the width and thickness change due to change of power and pressure are measured in experiments. Results show that the plasma is nearly uniformly full of the chamber in E-mode, while the shape of plasma rapidly changes to a ring in H-mode and the structure of inductively coupled plasma stratified into an electronegative core and an electropositive halo. It is observed clearly that the luminance of plasma increases slowly with the RF power in E-mode, but increases significantly in H-mode, which are proved through the relative spectral intensity variation of nitrogen 337.1 nm spectral lines due to the change of power and pressure. The width and thickness of the core region increase significantly with power, while decrease apparently with increasing pressure, which could be logically explained by the variation of RF magnetic induction amplitude distribution with power and by the theoretical diffusion analyses of electronegative gas. Since a mass of oxygen electronegative ion exists in the air inductively coupled plasma, the electron density (ne) diffusion models are different for the electronegative core and the electropositive halo. It is proved also by the theoretical drift-diffusion analyses that the electron density is distributed nearly uniformly in the electronegative core and decreased sharply in the electropositive halo. The model of electromagnetic wave propagation in the ICP generator is given and the microwave interferometry is discussed in detail. The electron density in the core region under different discharge conditions is diagnosed by microwave interferometer and the electron density of edge halo is calculated by using the high-pressure diffusion model. And the electron density increases with increasing power and pressure, which range from 0.65×1011 to 3.71×1011 cm-3. But decay rate of electron density in the halo is less affected by the power at 100 Pa, while the rate is accelerated with increasing pressure. Finally, the electronic excitation temperature of the electronegative core and the electropositive halo are diagnosed by Boltzmann graphic method using emissive spectrum of auxiliary Ar. Results show that the electronic excitation temperature of the core, which ranges from 4201 to 4390 K, increases with increasing power, but decreases with increasing pressure.