Characteristic Study of an Atmospheric-Pressure Radio-Frequency Capacitive Argon/Nitrogen Plasma Discharge

A developed atmospheric-pressure capacitive discharge source driven by radio-frequency power supply at 13.56 MHz is capable of producing a homogeneous and cold glow discharge in N₂/Ar. The measurements of its electrical parameters are made systematically with respect to the nitrogen component when diluted into argon plasma discharge at various concentration ratio, and the variation tendencies of the atomic nitrogen and the N₂ \((C^{{3}}\Pi _{{\rm u}})\) states with the concentration ratio of nitrogen to argon are investigated by means of optical emission spectroscopy. It is shown that although the admixture of nitrogen into argon plasma changes the electric characteristics greatly, the vibrational distribution function alters slightly at the same mixture ratio, and the production of N-atom is proportional to the nitrogen component.

I. Introduction

Atmospheric-pressure glow discharge (APGD) is one of the highlighted research areas in low-temperature plasma physics because it offers a low-cost and chamber-free rout for numerous applications, including material surface cleaning and modification, sterilization, film deposition, and cancer cell treatment [1]. Therefore, various kinds of APGD sources have been developed upon request of different application backgrounds, such as capacitively coupled atmospheric-pressure plasma jet (APPJ) [2]–[7], atmospheric-pressure plasma needle [8], the cold plasma torch [9], dielectricbarrier discharge (DBD) [10], [11], and atmospheric-pressure corona discharge [12]. They produce nonequilibrium plasmas with gas temperatures between 50°C and 400°C. However, most APGDs make use of the helium-dominant gas as the working gas due to ready control of its stability. The economical argon gas is usually employed as carrier gas in industrial applications, and the replacement of helium with argon in APGDs is very desirable to lower cost, especially in application of the large-area homogeneous APPJ. It is known that the large-area argon/nitrogen APGDs are much more difficult to achieve operating in homogenous mode with the conventional configuration of bare electrodes [4]. Therefore, the dielectric-insulated electrodes are usually employed to compose a DBD configuration driven by radio frequency (RF) [5]–[7]. Although the introduction of dielectric barriers improves the stability over a large range of the discharge current [5], the Joule heat released in plasma discharge becomes severe due to the poor conductivity of dielectric barrier, large Joule heat generated in argon discharge, and the low value of thermal conductivity of argon. One of the large-area homogeneous APPJs proposed in [13] and [14] makes it possible to sustain a stable -mode plasma discharge in Ar/O₂ between two bare metal electrodes at atmospheric pressure with an enhanced mixture ratio of oxygen to argon up to 1.0 vol.%, while maintaining operation at low plasma temperature. One of its applications is in surface-cleaning work; however, the abundant ozone produced in the Ar/O₂ APPJs requires additional special process to eliminate its negative effect on the environment. Fortunately, it is found that when the oxygen component in Ar/O₂ APPJs is replaced with nitrogen gas in the same plasma source, the generated homogeneous Ar/N₂ APPJs are working as well as Ar/O₂ APPJs in surface cleaning without production of the undesirable ozone. Furthermore, the nitrogen and nitrogen-containing plasmas have a wide variety of other industrial applications, including metal surface treatment, material deposition, and semiconductor processing [15].