Theoretical Study on the Characteristics of Atmospheric Radio Frequency Discharges by Altering Electrode Gap

In this paper, we present a theoretical study on the discharge characteristics of radio-frequency discharges at atmospheric pressure driven by a higher frequency of 40.68 MHz while the electrode gap is altered. Based on the analytical equations and simulation data from a one-dimensional fluid model, an optimal gap between electrodes, at which the largest electron density is obtained, can be observed under a constant power condition; however, as the electrode gap increases the time-averaged electron temperature decreases, and the underpinning physics is also discussed based on the simulation results. This study indicates that at a constant power by choosing an appropriate electrode spacing, the rf discharge can be effectively optimized at atmospheric pressure.     

Comparison of Sterilizing Effect of Nonequilibrium Atmospheric-Pressure He/O2 and Ar/O2 Plasma Jets

The sterilizing effect of the non-equilibrium atmospheric pressure plasma jet by applying it to the Bacillus subtilis spores is invesigated. A stable glow discharge in argon or helium gas fed with active gas （oxygen）, was generated in the coaxial cylindrical reactor powered by the radio-frequency power supply at atmospheric pressure. The experimental results indicated that the efficiency of killing spores by making use of an Ar/O2 plasma jet was much better than with a He/O2 plasma jet. The decimal reduction value of Ar/O2 and He/O2 plasma jets under the same experimental conditions was 4.5 seconds and 125 seconds, respectively. It was found that there exists an optimum oxygen concentration for a certain input power, at which the sterilization efficiency reaches a maximum value. It is believed that the oxygen radicals are generated most efficiently under this optimum condition.     

Numerical and Experimental Investigation of Electron Beam Air Plasma Properties at Moderate Pressure简

Large size of air plasma at near atmospheric pressure has specific effects in aerospace applications. In this paper, a two dimensional multi-fluid model coupled with Monte Carlo （MC） model is established, and some experiments were carried out to investigate the characteristics of electron beam air plasma at pressure of 100-170 Torr. Based on the model, the properties of electron beam air plasma are acquired. The electron density is of the order of 1016 m-3 and the longitudinal size can exceed 1.2 m. The profiles of charged particles demonstrate that the oxygen molecule is very important for air plasma and its elementary processes play a key role in plasma equilibrium processes. The potential is almost negative and a very low potential belt is observed at the edge of plasma acting as a protection shell. A series of experiments were carried out in a low pressure vacuum facility and the beam plasma densities were diagnosed. The experimental results demonstrate that electron density increased with the electron beam energy, and the relatively low pressure was favorable for gaining high density plasma. Hence in order to achieve high density and large size plasma, it requires the researchers to choose proper discharge parameters.     

Evolution of the Pulse Width in Dielectric Barrier Atmospheric Pressure Discharge

A study of the evolution of the pulse width in homogeneous dielectric barrier dis-charge at atmospheric pressure with helium as the working gas is reported by using a one-dimensional fluid model.In this paper,a new computational method is presented to estimate the pulse width through calculating the time interval between the breakdown voltage and the extinguishing voltage.The effects on the discharge characteristics of the applied voltage and exci-tation frequency are studied based on the computational data.The results of the simulation show that the pulse width is observed to be narrower and the time intervals between two consecutive current pulses decrease with increasing amplitude and excitation frequency,which indicates that the homogeneous discharge is susceptible to the filamentary mode.The simulation results support the conclusion that in order to restrain the transition from the glow mode to filamentary mode,the applied voltage and excitation frequency should be kept within an appropriate range.     

Simulation of the spatio-temporal evolution of the electron energy distribution function in a pulsed hollow-cathode discharge

This article presents the 2D simulation results of a nanosecond pulsed hollow cathode discharge obtained through a combination of fluid and kinetic models. The spatio-temporal evolution of the electron energy distribution function(EEDF) of the plasma column and electrical characteristics of the nanosecond pulsed hollow cathode discharge at a gas pressure of 5 Torr are studied. The results show that the discharge development starts with the formation of an ionization front at the anode surface. T...     

Analysis of the characteristics of the plasma of an RF driven ion source for a neutral beam injector

A radio frequency(RF)driven ion source is a very important component of a neutral beam injector for large magnetic confinement fusion devices.In order to study the key technology and physics of an RF driven ion source for a neutral beam injector in China,an RF ion source test facility was developed at the Institute of Plasma Physics,Chinese Academy of Sciences.In this paper,a two-dimensional fluid model is used to simulate the fundamental physical characteristics of RF plasma discharge.Simulation results show the relationship of the characteristics of plasma(such as electron density and electron temperature)and RF power and gas pressure.In order to verify the effectiveness of the model,the characteristics of the plasma are investigated using a Langmuir probe.In this paper,experimental and simulation results are presented,and the possible reasons for the discrepancies between them are given.This paper can help us understand the characteristics of RF plasma discharge,and give a basis for further R&D for an RF ion source.     

Study on an Atmospheric Pressure Plasma Jet and its Application in Etching Photo-Resistant Materials

An atmospheric pressure radio-frequency plasma jet that can eject cold plasma has been developed. In this paper, the configuration of this type of plasma jet is illustrated and its discharge characteristics curves are studied with a current and a voltage probe. A thermal couple is used to measure the temperature distribution along the axis of the jet stream. The temperature distribution curve is generated for the He/O2 jet stream at the discharge power of 150 W. This jet can etch the photo-resistant material at an average rate of 100 nm/min on the surface of silicon wafers at a right angle.     

Uniformity of Plasma Density and Film Thickness of Coatings Deposited Inside a Cylindrical Tribe by Radio Frequency Sputtering

Plasma surface modification of the inner wall of a slender tube is quite difficult to achieve using conventional means. In the work described here, an inner coaxial radio frequency （RF） copper electrode is utilized to produce the plasma and also acts as the sputtered target to deposit copper films in a tube. The influence of RF power, gas pressure, and bias voltage on the distribution of plasma density and the uniformity of film thickness is investigated. The experimental results show that the plasma density is higher at the two ends and lower in the middle of the tube. A higher RF power and pressure as well as larger tube bias lead to a higher plasma density. Changes in the discharge parameter only affect the plasma density uniformity slightly. The variation in the film thickness is consistent with that of the plasma density along the tube axis for different RF power and pressure. Although the plasma density increases with higher tube biases, there is an optimal bias to obtain the highest deposition rate. It can be attributed to the reduction in self-sputtering of the copper electrode and re-sputtering effects of the deposited film at higher tube biases.     

Simulation of Vacuum Arc Characteristics at Different Moments Under Power-Frequency Current

In this paper,based on the quasi-stationary magneto-hydrodynamic(MHD) model,vacuum arc characteristics are simulated and analyzed at different moments under power-frequency current.For a vacuum arc with sinusoidal current under a uniform axial magnetic field(AMF),simulation results show that at the moment of peak value current,maximal values appear in the ion number density,axial current density,heat flux density,electron temperature,plasma pressure and azimuthal magnetic field.At the same time,the distributions of these parameters along the radial position are mostly nonuniform as compared with those at other moments.In the first 1/4 cycle,the ion number density,axial current density and plasma pressure increase with time,but the growth rate decreases with time.Simulation results are partially compared with experimental results published in other papers.Simulations and experimental results both show that the arc light intensity near the cathode side is stronger than that near the anode side for diffusing vacuum arcs.     

Spectral Measurement of Atmospheric Pressure Plasma by Means of Digital camera

A digital camera measuring system has been used sucessfully to measure the space fluctuation behaviors of Induced Dielectric Barrier Discharge (IDBD) plasma at atmospheric pressure.The experimental results showed that:(1) The uniformity of elctron Temperature of space depends on discharge condition and structure of web electrode,For a certain web electrode the higher the discharge volgage is ,the more uniform distribution of electron temperature in space will be.For a certain discharge the finer and denser the holes on web electrode are,the more uniform distribution of electron temperature in space will be(2).Digital camera is an available equipment to measure some behaviors of the plasma working at atmospheric pressure.     

Modeling Approach and Analysis of the Structural Parameters of an Inductively Coupled Plasma Etcher Based on a Regression Orthogonal Design

The geometry of an inductively coupled plasma (ICP) etcher is usually considered to be an important factor for determining both plasma and process uniformity over a large wafer. During the past few decades, these parameters were determined by the "trial and error" method, resulting in wastes of time and funds. In this paper, a new approach of regression orthogonal design with plasma simulation experiments is proposed to investigate the sensitivity of the structural parameters on the uniformity of plasma characteristics. The tool for simulating plasma is CFD-ACE+, which is commercial multi-physical modeling software that has been proven to be accurate for plasma simulation. The simulated experimental results are analyzed to get a regression equation on three structural parameters. Through this equation, engineers can compute the uni- formity of the electron number density rapidly without modeling by CFD-ACE+. An optimization performed at the end produces good results.     

Improvement of Azimuthator Based on Particle Simulation

The azimuthator is an important part of plasma optical mass separation.The existing design for an azimuthator is based on the single particle orbit theory and focused on the movement of ions.In this paper,the particle simulation method is adopted to study the behavior of plasma crossing an azimuthator.The results show that electrons are bounded at the entrance of the azimuthator and an axial electric field is produced due to the charge separation.In order to better achieve the function of the azimuthator,a cathode is designed to transmit the electrons and to obtain a quasi-neutral plasma beam.     

In-situ reduction of silver by surface DBD plasma: a novel method for preparing highly effective electromagnetic interference shielding Ag/PET

Electromagnetic interference(EMI) shielding composites with good flexibility and weatherability properties have attracted increased attention. In this study, we combined the surface modification method of sub-atmospheric pressure glow discharge plasma with in situ atmospheric pressure surface dielectric barrier discharge plasma(APSDBD) reduction to prepare polyethylene terephthalate supported silver(Ag/PET). Due to the prominent surface modification of PET film, mild plasma reduction, and effective control of the silver morphology by polyvinylpyrrolidone(PVP), a 3.32 μm thick silver film with ultralow sliver loading(0.022 wt%) exhibited an EMI shielding efficiency(SE) of 39.45 d B at 0.01 GHz and 31.56 d B at 1.0 GHz(30 d B in the range of 0.01–1.0 GHz). The SEM results and EMI shielding analysis indicated that the high performance originated from the synergistic effect of the formation of silver nanoparticles(Ag NPs) with preferentially oriented cell-like surface morphologies and layer-by-layer-like superimposed microstructures inside, which demonstrated strong microwave reflection properties. Fourier transform infrared spectrometer and x-ray diffractometer showed that the surface structures of the heat-sensitive substrate materials were not destroyed by plasma.Additionally, APSDBD technology for preparing Ag/PET had no special requirements on the thickness, dielectric constant, and conductivity of the substrate, which provides an effective strategy for manufacturing metal or alloy films on surfaces of heat-sensitive materials at a relatively low cost.     

Decomposition of Glycerine by Water Plasmas at Atmospheric Pressure

High concentration of aqueous glycerine was decomposed using a direct current (DC) plasma torch at atmospheric pressure. The torch can generate the plasma with water as the plasma-supporting gas in the absence of any additional gas supply system and cooling devices. The results indicated that 5 mol% glycerine was completely decomposed by water plasmas at arc powers of 0.55～1.05 kW. The major products in the effluent gas were H 2 (68.9%～71.1%), CO 2 (18.9%～23.0%), and CO (0.2%～0.6%). However, trace levels of formic acid (HCOOH) and formaldehyde (HCHO) were observed in the liquid effluent. The results indicated that the water plasma waste treatment process is capable of being an alternative green technology for organic waste decomposition.     

Analysis of the Plasma Properties Affected by Magnetic Confinement with Special Emphasis on Helicon Discharges

A one-dimensional radial non-uniform fluid model is employed to study plasma behaviors with special emphasis laid on helicon discharges. The plasma density ne, electron temperature Te, electron azimuthal and radial drift velocities are investigated in terms of the plasma radius rp, magnetic field intensity B0 and gas pressure p0, by assuming radial ambipolar diffusion and negligible ion cyclotron movement. The results show that the magnetic confinement plays an important role in the discharge equilibrium, especially at low pressure, which significantly reduces Te compared with the case of a negligible magnetic field effect, and higher B0 leads to a greater average plasma density. Te shows little variations in the plasma density range of 1011 cm-3- 1013 cm-3 for p0 〈 3.0 mTorr. Comparison of the simulation results with experiments suggests that the model can make reasonable predictions of Te in low pressure helicon discharges.     

A Brief Study on the Ignition of the Non-Thermal Atmospheric Pressure Plasma Jet from a Double Dielectric Barrier Configured Plasma Pencil

To understand the self sustained propagation of the plasma jet/bullet in air under atmospheric pressure, the ignition of the plasma jet/bullet, the plasma jet/bullet ignition point in the plasma pencil, the formation time and the formation criteria from a dielectric barrier configured plasma pencil were investigated in this study. The results were confirmed by comparing these results with the plasma jet ignition process in the plasma pencil without a dielectric barrier. Electrical, optical, and imaging techniques were used to study the formation of the plasma jet from the ignition of discharge in a double dielectric barrier configured plasma pencil. The investigation results show that the plasma jet forms at the outlet of the plasma pencil as a donut shaped discharge front because of the electric field line along the outlet’s surface. It is shown that the required time for the formation of the plasma jet changes with the input voltage of the discharge. The input power calculation for the gap discharge and for the whole system shows that 56% of the average input power is used by the first gap discharge. The estimated electron density inside the gap discharge is in the order of 1011cm-3 . If helium is used as a feeding gas, a minimum 1.48×10-8C charge is required per pulse in the gap discharge to generate a plasma jet.     

Characteristics of Nanosecond Pulsed Discharges in Atmospheric Helium Microplasmas

Microplasmas are very interesting due to their unique properties and achievable regimes maintained at atmospheric pressures. Due to the small scales, numerical modeling could contribute to the understanding of underlying phenomena as it provides access to local parameters — and complements experimental global characteristics. A self-consistent formalism,applied to nanosecond pulsed atmospheric non-equilibrium helium plasmas, reveals that several successive discharges can persist as a result of a combined volume and dielectric surface effects. The valuable insights provided by the spatiotemporal simulation results show the critical importance of coupled gas and plasma dynamics — namely gas heating and electric field reversals.     

Investigation of Atmospheric Pressure Plasma Discharge and Its Application to Surface Modification of Blood-Filtering Material

Melt-blown polybutylene terephthalate (PBT) nonwoven materials treated by using plasma is regarded as one of the excellent materials to filter white blood cells (WBC) from blood.In this paper, dielectric barrier discharge (DBD) plasma source at an improved quasi-stable atmospheric pressure is achieved when discharge voltage, discharge current, and gap between the electrodes are carefully controlled. This plasma source has been used to modify the surface of PBT melt-blown nonwoven materials. Experimental results indicate that both the wettablity and permeation of treated PBT melt-blown nonwoven materials are greatly improved.     

Carbon Dioxide Reforming of Methane to Syngas by Thermal Plasma

Experiments were conducted on syngas preparation from dry reforming of methane by carbon dioxide with a DC arc plasma at atmospheric pressure.In all experiments,nitrogen gas was used as the working gas for thermal plasma to generate a high-temperature jet into a horizontal tube reactor.A mixture of methane and carbon dioxide was fed vertically into the jet.In order to obtain a higher conversion rate of methane and carbon dioxide,chemical energy efficiency and fuel production efficiency,parametric screening studies were conducted,in which the volume ratio of carbon dioxide to methane in fed gases and the total flux of fed gases were taken into account.Results showed that carbon dioxide reforming of methane to syngas by thermal plasma exhibited a larger processing capacity,higher conversion of methane and carbon dioxide and higher chemical energy efficiency and fuel production efficiency.In addition,thermodynamic simulation for the reforming process was conducted.Experimental data agreed well with the thermodynamic results,indicating that high thermal efficiency can be achieved with the thermal plasma reforming process.