The recovery of glow-plasma structure in atmospheric radio frequency microplasmas at very small gaps

Abstract

In atmospheric radio frequency discharges at 13.56 MHz, with the electrode gap reduced, the sheath region eventually occupies a main portion of the electrode spacing and the bulk plasma region is significantly compressed. The computational results in this letter based on a one-dimensional fluid model show that by increasing the excitation frequency over 13.56 MHz, the traditional glow-plasma structure could gradually recover even at very small sizes with a well defined quasineutral plasma region, and the electron density is improved but the electric fields in sheath region are reduced. This study indicates that the excitation frequency can be used to modulate the discharge structure and then tailor the plasma-surface interaction in atmospheric microplasmas.Atmospheric microplasmas have commanded much attention in recent years due to the considerable scientific depth and potential applications. 1-4 For atmospheric capacitively radio-frequency (rf) discharges, when the electrode gaps are confined to submillimeter dimensions, the generated microplasmas show many unique properties compared to the large-scale atmospheric plasmas, 3 such as the high energetic electrons, 5 different discharge structures, 6,7 and nonequilibrium characters. 8 The experimental 6 and computational studies 5,7 have demonstrated that in atmospheric rf discharges at 13.56 MHz, with the electrode gap reduced, the traditional glow-plasma (GP) structure will eventually transit to a new sheath-dominated-plasma (SDP) structure, where the sheath region occupies a large portion of the electrode gap and almost no distinct bulk plasma region develops. In a SDP structure, three electron groups with different electron temperatures have been revealed by the Particle-In-Cell (PIC) simulation and the high energetic electrons present a new way to interact with the electrodes or the given surfaces, moreover these electrons maybe contribute to the reactivity of atmospheric microplasmas. 5 The discharge modes in rf microplasmas have been discussed but still need to be further clarified by experiments and simulations. 5-7 Increasing frequency is widely accepted as a way to enhance the discharge stability and reduce the electron temperature at a constant power density. 10-12 Nevertheless, the frequency effects on the structure transition in atmospheric rf discharges are largely unexplored. Although the microplasmas in SDP regime show many advantages and the GP structure with quasineutral plasmas may still be very useful in many applications at very small sizes. The experimental diagnosis of atmospheric microplasmas, however, is facing many new challenges due to the small dimension and high gas pressure, 3,4 and numerical simulations provide a valuable alternative for investigating the properties of microplasmas. 2In this letter a one-dimensional fluid model is explored to study the transition of discharge structures in capacitively rf discharges at atmospheric pressure. 11,12 Briefly, the continuity equations with the drift-diffusion approximation are used to des...

Abnormal electron-heating mode and formation of secondary-energetic electrons in pulsed microwave-frequency atmospheric microplasmas

“…Moreover, an electron-heating mode and its transition, which are induced by changes in the input power and frequency, have been reported as in the cases of low-pressure discharges. 9,[13][14][15] However, the previous studies of the electron-heating mode were based on fluid models; 11,12 the effects of heating on the electron kinetics has not yet been clarified. In particular, we are interested in the electron energy distribution function (EEDF) on the electrodes in atmospheric discharges for biomedical applications.…”

“…1-8 For atmospheric radio-frequency (rf) capacitive discharges, a variety of microplasmas operating over a wide range of frequencies and sub-millimeter dimensions have been reported. 1,4,[9][10][11][12] We previously studied single-frequency (SF) 9 and dual-frequency (DF) 10 atmospheric-pressure helium microplasmas, which can be sustained over a wide range of microwave frequencies from 400 MHz to 2.45 GHz. These microwave-induced microplasmas are important because they may be used to develop safe, portable, and long-lifetime devices capable of operating at atmospheric pressure.…”

Electrode-Gap Effects on the Discharge Structure in Atmospheric Radio-Frequency Microplasmas With He/O₂ Mixtures

“…The current density with a normal sinusoidal form is taken as the input parameter [4], [6], [24], and the driving frequency is 13.56 MHz; the coupled power, actually the power per area as a result of 1-D limit, is usually fixed, to reasonably compare the evolution of discharge. The present model with pure helium as working gas has been explored to investigate the discharge characteristics at very small gaps [22], and the simulation results show a good agreement of experimental observation [21]. As the oxygen admixture was introduced, the discharge characteristics and generation of ROS at a relatively large electrode gap also have been investigated and compared with the experimental measurements [13].…”

“…The fundamental physics of RF discharges shows some unique characteristics features in microplasma regime, which have been discussed in detail in [17]- [19]. A sheath-dominated structure with a large portion of sheath region has usually been observed, and a traditional glowdischarge structure with a distinct bulk plasma and sheath could not be formed in RF microplasmas [20], [21]; in addition, the recent computational study indicated that the glow-discharge structure could also be recovered by increasing the excitation frequency even at very small gaps [22]. However, when the oxygen admixtures are added into helium microplasmas, plenty of new neutral and charged particles will be generated, and this could influence the evolution of discharge structure.…”

Enhanced transportation of energetic electrons in dual-frequency atmospheric microplasmas

“…[1][2][3][4][5][6][7][8] In particular, for atmospheric radio-frequency (rf) capacitive discharges, a large variety of single-frequency (SF) microplasmas that operate over a wide range of frequencies and sub-millimeter dimensions have been reported. 1,4,[9][10][11] Especially, in our previous study, 9 SF atmospheric-pressure helium microdischarges sustained over a wide range of microwave frequencies were the focus of our investigation. The microplasmas generated in these studies showed unique properties, such as highly energetic electrons, different discharge structures, and non-equilibrium characteristics.…”