Mode transition in radio-frequency atmospheric argon discharges with and without dielectric barriers

In this letter, basic characteristics of glow modes and their mode transition are studied for radio-frequency rf atmospheric argon discharges with bare and dielectrically insulated electrodes. Through input power control, large-volume rf atmospheric argon discharges with bare electrodes are achieved in the mode via an abrupt transition from a constricted mode, whereas dielectrically insulated electrodes result in large argon discharges in both the and modes with gradual mode transition. Current dependence of the 750 nm line intensity and of the gas temperature are shown to capture clearly the signature of mode transition

Mode characteristics of radio-frequency atmospheric glow discharges

Building on recent experimental and numerical evidence of different glow modes in atmospheric plasmas, this paper reports a systematic study of these modes in radio-frequency (RF) glow discharges in atmospheric helium. Using a one-dimensional (1-D) hybrid computer model, we present detailed characterization of three glow modes, namely the α mode, the α - γ transitional mode, and the γ-mode in a 13.56-MHz atmospheric glow discharge over a wide range of root mean square (RMS) current density from 5 m A / cm2 to 110 m A / cm2. Our focus is on sheath dynamics through spatial and temporal profiles of charged densities, electric field, electron mean energy, sheath thickness, and sheath voltage, and when appropriate our results are compared against experimental data of atmospheric glow discharges and that of glow discharges at reduced gas pressure below 1 torr. Fundamental characteristics of the three glow modes are shown to be distinctively different, and these can be used as a hitherto unavailable route to tailor the operation of radio-frequency atmospheric glow discharges to their intended applications

Nonthermal atmospheric plasmas sustained without dielectric barrier in the kilohertz range

We report observation of nonthermal atmospheric discharges produced between two bare metallic electrodes over a wide frequency range from 20 to 260 kHz, in which generation of stable atmospheric glow discharges has so far necessitated dielectric barrier to be added to at least one electrode. Measured current and voltage characteristics suggest a distinctively different plasma-sustaining mechanism from that of atmospheric dielectric-barrier discharges. This is confirmed by hydrodynamic simulation

Radio-frequency dielectric-barrier glow discharges in atmospheric argon

In this letter, an experimental investigation is presented to characterize the properties and benefits of radio-frequency (rf) dielectric-barrier discharges (DBDs) in atmospheric argon. Compared to rf atmospheric glow discharges generated with bare electrodes, atmospheric argon rf DBDs are shown to remain stable and uniform over a large current range from the α and the γ modes. Optical emission spectroscopy is used to show an active underpinning plasma chemistry and a gas temperature range of 461–562 K. These highlight the advantages of argon rf DBD as a surface processing technique over more expensive helium-based rf atmospheric glow discharges

Cathode fall characteristics in a dc atmospheric pressure glow discharge

Atmospheric pressure glow discharges are attractive for a wide range of material-processing applications largely due to their operation flexibility afforded by removal of the vacuum system. These relatively new atmospheric plasmas are nonequilibrium plasmas with gas temperature around 100 °C and electron temperature in the 1–10 eV range. Their appearance is characteristically diffuse and uniform, and their temporal features are repetitive and stable. Of the reported numerical studies of atmospheric glow discharges, most are based on the hydrodynamic approximation in which electrons are assumed to be in equilibrium with the local electric field. Spectroscopic and electrical measurements suggest however that the cathode fall region is fundamentally nonequilibrium. To this end we consider a hybrid model that treats the cathode fall region kinetically but retains a hydrodynamic description for the region between the thin cathode fall layer and the anode. Using this hybrid model, a helium discharge system excited at dc is studied numerically for a very wide current density range that spans from Townsend dark discharge, through normal glow discharge, to abnormal glow discharge. Numerical results confirm many distinct characteristics of glow discharges and compare well with that of low-pressure glow discharges. Generic relationships, such as that between the electric field and the current density, are also established and are in good agreement with experimental data. This hybrid model is simple and insightful as a theoretical tool for atmospheric pressure glow discharges

Large-volume and low-frequency atmospheric glow discharges without dielectric barrier

It is widely believed that, at low frequencies of 1–100 kHz, the generation of atmospheric pressure glow discharges (APGD) requires a dielectric barrier added to at least one electrode. This letter reports the experimental observation of a uniform and stable APGD generated between two bare electrodes without a dielectric barrier over a wide frequency range from 20 kHz to 260 kHz. Below 70 kHz, it is shown that preionization in the rising phase of the applied voltage is important and plasma generation occurs in the voltage-falling phase. Mechanism of barrier-free APGD is found to be different from both atmospheric dielectric-barrier discharges and radio-frequency APGD

Expansion of the plasma stability range in radio-frequency atmospheric-pressure glow discharges

Reliable applications of atmospheric-pressure glow discharges (APGDs) depend critically on their plasma stability. A common technique of ensuring APGD stability is to keep their operation well within their stability range by decreasing their discharge current. However, this reduces the achievable densities of the reactive plasma species and, thereby, compromises the application efficiency. In this letter, the use of high excitation frequencies in radio-frequency APGD is shown to substantially expand their stability range. It is also demonstrated that high-frequency operation introduces an added benefit of higher electron energy and greater electron density, thus enabling more abundant reactive plasma species and improved application efficiency

Sheath dynamics in radio-frequency atmospheric glow discharges

This paper presents images of dynamic sheath evolution in a radio-frequency atmospheric pressure glow discharge at 13.56 MHz. Electron mean energy, electron production rate, electron density, and electric field are used to highlight volumetric gas ionization at low-current densities and localized gas ionization at large current densities

Electron trapping in radio-frequency atmospheric-pressure glow discharges

In this letter, the authors present experimental evidence of electron trapping in radio-frequency rf atmospheric-pressure glow discharges. By linking electron density to nanosecond plasma images and optical emission spectroscopy, they show that electron trapping occurs under most discharge conditions. The level of electron trapping increases with increasing discharge current or/and increasing excitation frequency, and manifests itself in the change of the differential conductivity at the point of the gas breakdown. Finally, they demonstrate that electron trapping is largely related to whether the half rf period is shorter than the electron transition time across the electrode gap

Mitigating plasma constriction using dielectric barriers in radio-frequency atmospheric pressure glow discharges

It is known that radio-frequency (rf) atmospheric glow discharges with bare electrodes are susceptible to plasma constriction at large discharge currents. This is undesirable for large-scale applications, even though large currents usually lead to abundant plasma reactive species and high application efficiency. In this letter, an experimental investigation is presented to demonstrate that plasma constriction can be mitigated by introducing dielectric barriers to the electrodes. The resulting atmospheric rf dielectric-barrier discharge is shown to operate in the γ mode of large discharge current while maintaining its discharge volume. This improves significantly plasma stability and the application potential