Search

Traditional plasma systems typically operate at low pressures and centimeter scale reactors. However, during the last decade or so, there is an active interest in the downscaling of devices and plasmas are no exception. Because of the popular pd (pressure times gap size) scaling, such plasmas have to operate at or near atmospheric pressure and are referred to as microplasmas or microdischarges. While downscaling the plasma device, field emission of electrons and their interaction with micro discharge due to high electric fields has shown to affect both pre-and post-breakdown operation of these discharge. In this context, we present computational data for the ionization coefficient (α) at high electric fields. A zero-dimensional Monte Carlo code is used to determine the variation of α as a function of electric field for various gases to use in device-level models. Results are also presented for the dependence of alpha (α) in a spatially varying electric field. While alpha (α) represents the volume process in a discharge, the most important surface process in microdischarge is field emission which is characterized by the field enhancement factor (β). Comparison of experimental data with theoretical data for argon, hydrogen, carbon dioxide and dry air are presented to predict an inverse dependence of βeff on electric field. Finally, device-level models are also formulated for the post-breakdown operation of field emission assisted microdischarges as motivated by novel cathodes with excellent field emission properties. In this context we present a non-linear sheath model for direct current field emitted assisted microdischarges. The main focus of this work is to develop a self-consistent sheath model that includes the effects of field induced electron emission without assuming a linear electric field. The results obtained from the non-linear sheath model for various parameters including current-voltage characteristics, and current density profiles of ion/electro are validated with PIC/MCC simulation of an argon microdicharge.

The aim of this paper is to investigate the process of growth of different carbon deposits in low-current electrical microdischarges in argon with an admixture of cyclohexane as the carbon feedstock. The method of synthesis of carbon structures is based on the decomposition of hydrocarbons in low-temperature plasma generated by an electrical discharge in gas at atmospheric pressure. The following various types of microdischarges generated at this pressure were tested for both polarities of supply voltage with regard to their applications to different carbon deposit synthesis: Townsend discharge, pre-breakdown streamers, breakdown streamers and glow discharge. In these investigations the discharge was generated between a stainless-steel needle and a plate made of a nickel alloy, by electrode distances varying between 1 and 15 mm. The effect of distance between the electrodes, discharge current and hydrocarbon concentration on the obtained carbon structures was investigated. Carbon nanowalls and carbon microfibers were obtained in these discharges.

A microplasma thruster has been developed of electrothermal type using azimuthally symmetric microwave-excited microplasmas. The microplasma source was ~2 mm in diameter and ~10 mm long, being operated at around atmospheric pressures; the micronozzle was a converging-diverging type, having a throat ~0.2 mm in diameter and ~1 mm long. Numerical and experimental results with Ar as a working gas demonstrated that this miniature electrothermal thruster gives a thrust of >1 mN, a specific impulse of ~100 s, and a thrust efficiency of ~10 % at a microwave power of

Diffuse Coplanar Surface Barrier Discharge (DCSBD) is a novel type of atmospheric-pressure plasma source developed for high-speed large-area surface plasma treatments. The statistical behavior of microdischarges of DCSBD generated in artificial air atmosphere was studied using time-correlated optical and electrical measurements. Changes in behavior of microdischarges are shown for various electrode gap widths and input voltage amplitudes. They are discussed in the light of correlation of the number of microdischarges and the number of unique microdischarges’ paths per discharge event. The ‘memory effect’ was observed in the behavior of microdischarges and it manifests itself in a significant number of microdischarges reusing the path of microdischarges from previous half-period. Surprisingly this phenomenon was observed even for microdischarges of the same half-period of the discharge, where mechanisms other than charge deposition have to be involved. The phenomenon of discharge paths reuse is most pronounced for wide electrode

The parameters for microdischarges of plasma medicine in air and argon vortex flows at atmospheric pressure for different shapes of electrodes (outlet nozzle and axis electrode diameters ratio set) have been investigated. The current-voltage characteristics of the designed systems have been analyzed, the parameters of the generated jets plasma have been investigated by means of the optical emission spectroscopy, and the form of plasma jets has been studied by using video camera. Keywords: microdischarge, gliding microdischarge, microjet, glow discharge, emission spectroscopy. DOI: 10.1515/chem-2015-0043received February 07, 2014; accepted May 30, 2014. 1 Introduction Microdischarge plasma generation is considered to be one of the most promising areas of non-equilibrium plasma chemistry today and can be applied in different fields: for the environmental protection, in biology and biomedicine, as an ultraviolet and vacuum ultraviolet intense source, and for the surface activation and films deposition. Currently there is no full understanding of the microdischarge physics nature, and therefore it is impossible to do the detailed analysis of their parameters. Microdischarge plasma is characterized by significant changes of the excited states distribution function and ions and electrons energy in a relatively short time. It is possible to use such discharges as the sources of reactions active components.The medical area is interested in the different microplasma systems to ensure by their ability to generate low-temperature non-equilibrium atmospheric pressure plasma (NAPP). The electron temperature in NAPP is a few eV, which is usually sufficient, to initiate a group of chemical reactions including – proteins, cell membranes and DNA modification; and producing radicals. At the same time, the heavy particle temperature is close to room temperature; therefore the plasma treatment does not destroy temperature sensitive biological tissues. The disinfection, decontamination and sterilization processes that uses active plasma particles (radicals, charged particles, excited particles, UV radiation) occurs quite rapidly under such conditions. Experiments have shown several seconds to several minutes of the plasma treatment is necessary for different types of viruses and bacteria neutralization on surfaces and in aqueous solutions. The following applications of plasma systems in medicine are known from the literature [1-3]: sterilization, cancer cells destruction, blood coagulators (to stop bleeding), and wound healing, whitening and cleaning teeth. It was shown that plasma usage in combination with well-known traditional medical methods significantly increases its efficiency. Currently, there are two main applications of NAPP for disinfection, decontamination and sterilization. For the first application, the plasma generation is remote, and the products of its afterglow are delivered to the biological tissue in a loop. The therapeutic effect is probably provided by the neutral particles and radicals with long-term existence, while the majority of charged particles disappear outside the plasma generation zone. Usually, in order to avoid plasma instability, the work should be carried out with helium or argon diluted gases, and the discharge is doped with just a few percent of molecular

International audience; The failure mechanisms of micro hollow cathode discharges (MHCD) in silicon have been investigated using their I-V characteristics, high speed photography and scanning electron microscopy. Experiments were carried out in helium. We observed I–V instabilities in the form of rapid voltage decreases associated with current spikes. The current spikes can reach values more than 100 times greater than the average MHCD current. (The peaks can be more than 1 Ampere for a few 10’s of nanoseconds.) These current spikes are correlated in time with 3–10 μm diameter optical flashes that occur inside the cavities. The SEM characterizations indicated that blister-like structures form on the Si surface during plasma operation. Thin Si layers detach from the surface in localized regions. We theorize that shallow helium implantation occurs and forms the ‘blisters’ whenever the Si is biased as the cathode. These blisters ‘explode’ when the helium pressure inside them becomes too large leading to the transient micro-arcs seen in both the optical emission and the I–V characteristics. We noted that blisters were never found on the metal counter electrode, even when it was biased as the cathode (and the Si as the anode). This observation led to a few suggestions for delaying the failure of Si MHCDs. One may coat the Si cathode (cavities) with blister resistant material; design the MHCD array to operate with the Si as the anode rather than as the cathode; or use a gas additive to prevent surface damage. Regarding the latter, tests using SF6 as the gas additive successfully prevented blister formation through rapid etching. The result was an enhanced MHCD lifetime.