Simulation and experimental research on the parameter distribution of low-pressure Ar/O2 inductivly coupled plasma

doi:10.1016/j.vacuum.2017.08.029

Vacuum

Volume 145, November 2017, Pages 77-85

https://doi.org/10.1016/j.vacuum.2017.08.029 Get rights and content

Highlights

•
A fluid model of the Ar/O₂ ICP is used to investigate the parameter distribution, while the electron energy distribution function (EEDF) and the transport coefficients are obtained by the Boltzmann equation solver module to improve the accuracy.
•
An experiment is carried out to diagnose the discharge parameters, and the results indicate that the fluid model with the Boltzmann equation module can well describe the parameters distribution of the Ar/O₂ ICP.

Abstract

Adding electronegative gas to the inert gas is an important means of adjusting the plasma parameter distribution. In this paper, a fluid model of the Ar/O₂ inductivly coupled plasma (ICP) is used to investigate the parameter distribution, while the electron energy distribution function (EEDF) and the transport coefficients are obtained by the Boltzmann equation solver module to improve the accuracy. The spatial-temporal evolution of the electron density and the electron temperature of the ICP are obtained and the influence of the discharge power and the oxygen mole ratio on the distribution of the ICP parameters is compared. Additionally, the physical mechanism is researched using the diffusion-transport theory of the plasma. In order to verify the reliability of the model results, an experiment is carried out to diagnose the discharge parameters. The results of the simulation and the experiment are performed for different power and oxygen mole ratios and a good qualitative and numerical agreement is obtained, indicating that the inclusion of the simulation results in a high accuracy.

Introduction

Inductivly coupled plasma (ICP) is a stable and uniform high-density plasma source with low radio-frequency (RF) power and discharge pressure [1], a simple device structure, and parameters that are easy to adjust [2]. Thus, ICP has a great potential application in reducing the electromagnetic scattering characteristics of the inlet and radar cabin of stealth aircraft [3], [4], [5]. Adding electronegative gas to the inert gas is an important means of adjusting the plasma parameter distribution; therefore, the study of the discharge characteristics of the ICP at different argon-oxygen mixing ratios is of important significance [6].

For an analysis of the electromagnetic scattering properties of a target by ICP, the dielectric parameter is required and it is mainly dependent on the electron density (n_e) and the electron temperature (T_e). Therefore, it is necessary to accurately diagnose the spatial distribution of the plasma's characteristic parameters. It is difficult to demonstrate the evolution of a parameter's variation on a small temporal scale using an experimental diagnostic method. Instead, a numerical simulation method can obtain the distribution and transformation laws of the plasma's parameters on a multi-temporal scale.

The fluid model is one of the commonly used methods for the numerical simulation of low-temperature plasma. The method not only uses an iterative feedback process in the ICP but also simplifies the collision using a Monte Carlo approach [7], [8]. The advantage of the fluid model is that a simple hypothesis function is used to describe the energy distribution function of the particle, such as the Maxwellian distribution [9], to reduce the computational cost; thus, the fluid model can better represent the macroscopic characteristics of the plasma under medium and high air pressure. However, under low pressure, the high-energy electrons will affect the accuracy of the model [10].

In this paper, the ICP module and the Boltzmann equation module are solved coherently using the COMSOL multi-physics software. The Boltzmann equation module is used to solve the EEDF and determine the electron transport/diffusion coefficient and the electron impact reaction rate coefficient. The effects of the discharge power and the oxygen molar ratio on the distribution of the ICP parameters are researched and the results are compared with the experimental results to validate the accuracy of the model.

Section snippets

Simulation model

As shown in Fig. 1 (a), the ICP cavity of the metal wall is designed and fabricated. The geometry of the cavity is cylindrical and the quartz window is embedded in the metal wall. The thickness of the quartz window on both sides and the cavity wall is 1 cm. The advantage of the metal cavity wall is that the processing technology is mature, the cost is low, and a Langmuir probe can be used to achieve a high-resolution diagnosis of the electron density and the electron temperature distribution.

Parameters distribution of ICP

Fig. 3(a) shows the EEDF versus discharge power when the Oxygen mole ratio is 0.2, the EEDF as the function of the mean energy presents double-temperature distribution character because the low-energy electrons are constrained by the bipolar electron potential trap while it is mainly the medium-energy electrons that involved in the heating process. In this case we can see that the low-energy region of EEDF is less affected by the discharge power, for the increasing power will mainly increase

Experimental research of the ICP

In order to further validate the results of the model, this section describes the relevant experiments to conduct a comparative study. Fig. 10 (a) and (b) show the discharge patterns when the power is 500 W and the η_O2 is 0 and 20% respectively. There is a large area of relatively uniform plasma in the central portion of the cavity and the brightness decreases gradually towards the edge. After adding oxygen, the brightness of the ICP clearly decreases, which is consistent with the spatial

Conclusion

Low-pressure ICP has a great potential application in radar stealth of military objects. Adding electronegative gas to the inert gas is an important means of adjusting the plasma parameter distribution, therefore, this paper focus on the discharge characteristics of the ICP at different argon-oxygen mixing ratios by simulation and experiment.

The results of the model and the experiment show that:I. The fluid model with the Boltzmann equation module can well describe the parameters distribution

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Simulation and experimental research on the parameter distribution of low-pressure Ar/O2 inductivly coupled plasma

Highlights

Abstract

Introduction

Section snippets

Simulation model

Parameters distribution of ICP

Experimental research of the ICP

Conclusion

Vacuum

J. Comput. Phys.

J. Appl. Phys.

Acta Phys. Sin.

IEEE Trans. plasma Sci.

Vacuum

Phys. Plasmas

Acta Phys. Sin.

Appl. Phys.

Simulation and experimental research on the parameter distribution of low-pressure Ar/O₂ inductivly coupled plasma