Low-temperature Discharge Plasma Research Articles

Generating low-temperature glow discharge plasma in the atmospheric pressure helium after spark breakdown: Modelling plasma with the prescribed properties for biomedical applications

This paper concerns computational modelling of the low-temperature glow discharge plasma in the atmospheric pressure helium after spark breakdown and research on the dependence of a spatial distribution of plasma on the initial conditions of discharge and parameters of the external electric circuit. This study analysed the influence of the initial distribution of a space charge on the generation of the glow discharge plasma after the spark breakdown between flat electrodes by means of a 2D-axial symmetric model of the atmospheric pressure helium plasma in the drift-diffusion approximation. With the discharge current of 1–12 mA, the solution for a quasi-steady state of plasma is obtained. The dependence of a type of this discharge mode on the parameters of the external electric circuit and coefficient of the secondary cathode emission is studied.

Excitation of Ar, O2, and SF6/O2 plasma discharges using tailored voltage waveforms: control of surface ion bombardment energy and determination of the dominant electron excitation mode

Using tailored voltage waveforms (TVWs) to excite a low pressure, low-temperature plasma discharge, we compare the behavior of three gas mixtures, namely Ar, O2 and SF6/O2 mixtures, the last of which is currently used for the plasma-texturing of silicon wafers for photovoltaics. The primary goal of using TVWs is to control the ion bombardment energy at the surface of the wafer, and this control is demonstrated through retarding field energy analyzer (RFEA) measurements. However, the complicated electrical response of the plasma to such waveforms makes the ab initio prediction of the ion energy difficult, although by using said RFEA measurements, we show that it can be done approximately by using measured electrical data. In addition, we utilize the response of the plasma to mirror-image ‘sawtooth’ waveforms as a predictor of the dominant electron heating mode (α or drift-ambipolar, DA). At equivalent pressures and coupled powers, the Ar and O2 mixtures always display behavior associated with electropositive plasmas (a solely α heating mode). However, with the addition of SF6 to an O2 gas flow, a transition can be observed towards a behavior associated with a more electronegative plasma (i.e. a dominant DA heating mode). This crossover in the dominant heating mode is observed through the relative self-bias voltage for each type of sawtooth waveform, and is therefore a useful predictor of the dominant electron heating mode in low pressure, cold plasma discharges.

Analysis of Oilfield Wastewater Treatment Effect Based on Low Temperature Plasma Treatment Process

In order to study the effect of different low temperature plasma treatment technology on the oil field's abolishment, the influence of low temperature plasma discharge frequency, scale inhibitor and the combined application of low temperature plasma and scale inhibitor on the quality of heavy oil wastewater and distilled water was analysed. When the discharge frequency of low temperature plasma was 900s-1, the decrease of oil content was 50.56%. The scale inhibitor also reduced the oil content of the heavy oil wastewater and the distilled water. However, the combined use of low temperature plasma and scale inhibitor would increase the content of oil and the content of metal ions in distilled water. The increase of oil content was 56%. The results showed that the low temperature plasma had obvious effect on reducing the oil content and SiO2 content in distilled water. Therefore, this study confirms the influence and mechanism of low-temperature plasma treatment on the quality of oil field wastewater.

A novel method for synthesis of arsenic sulfide films employing conversion of arsenic monosulfide in a plasma discharge

A novel plasma-based method for synthesis of arsenic sulfide films with different structural units and stoichiometries is demonstrated. For the first time As-S films have been prepared via direct conversion of arsenic monosulfide (As4S4) as a single precursor in a low-temperature non-equilibrium RF plasma discharge at low pressure. The interplay between composition, structure and properties of the chalcogenide materials prepared has been studied. Quantum-chemical calculations have been performed to gain insight into the films structure and the mechanism of its formation in the plasma discharge. Raman spectroscopy proves that with the As-content increase in the As-S films the intensity of the line, corresponding to vibrations of homopolar As-As bonds in the realgar As4S4 units and in the S2As-AsS2 bridges increases. The last ones are responsible for the appearance of a photoluminescence phenomenon in chalcogenide glasses.

On the Fractality of Microparticles from the Plasma Flow of a Vacuum Arc Discharge

Structures, namely, powder and films, deposited onto vacuum-chamber walls from low-temperature vacuum arc discharge and tokamak plasma are investigated. The fractal parameters of the given structures are compared. The possible mechanisms of their formation are discussed.

Infrared and Raman spectroscopy study of As[sbnd]S chalcogenide films prepared by plasma-enhanced chemical vapor deposition

AsS chalcogenide films, where As content is 60–40at.%, have been prepared via a RF non-equilibrium low-temperature argon plasma discharge, using volatile As and S as the precursors. Optical properties of the films were studied in UV–visible-NIR region in the range from 0.2 to 2.5μm. Infrared and Raman spectroscopy have been employed for the elucidation of the molecular structure of the newly developed material. It was established that PECVD films possess a higher degree of transparency (up to 80%) and a wider transparency window (>20μm) in comparison with the “usual” AsS thin films, prepared by different thermal methods, which is highly advantageous for certain applications.

Perspective: The physics, diagnostics, and applications of atmospheric pressure low temperature plasma sources used in plasma medicine

Low temperature plasmas have been used in various plasma processing applications for several decades. But it is only in the last thirty years or so that sources generating such plasmas at atmospheric pressure in reliable and stable ways have become more prevalent. First, in the late 1980s, the dielectric barrier discharge was used to generate relatively large volume diffuse plasmas at atmospheric pressure. Then, in the early 2000s, plasma jets that can launch cold plasma plumes in ambient air were developed. Extensive experimental and modeling work was carried out on both methods and much of the physics governing such sources was elucidated. Starting in the mid-1990s, low temperature plasma discharges have been used as sources of chemically reactive species that can be transported to interact with biological media, cells, and tissues and induce impactful biological effects. However, many of the biochemical pathways whereby plasma affects cells remain not well understood. This situation is changing rather quickly because the field, known today as “plasma medicine,” has experienced exponential growth in the last few years thanks to a global research community that engaged in fundamental and applied research involving the use of cold plasma for the inactivation of bacteria, dental applications, wound healing, and the destruction of cancer cells/tumors. In this perspective, the authors first review the physics as well as the diagnostics of the principal plasma sources used in plasma medicine. Then, brief descriptions of their biomedical applications are presented. To conclude, the authors' personal assessment of the present status and future outlook of the field is given.

A parallelization method for time periodic steady state in simulation of radio frequency sheath dynamics

In order to reduce the computing time in simulation of radio frequency (rf) plasma sources, various numerical schemes were developed. It is well known that the upwind, exponential, and power-law schemes can efficiently overcome the limitation on the grid size for fluid transport simulations of high density plasma discharges. Also, the semi-implicit method is a well-known numerical scheme to overcome on the simulation time step. However, despite remarkable advances in numerical techniques and computing power over the last few decades, efficient multi-dimensional modeling of low temperature plasma discharges has remained a considerable challenge. In particular, there was a difficulty on parallelization in time for the time periodic steady state problems such as capacitively coupled plasma discharges and rf sheath dynamics because values of plasma parameters in previous time step are used to calculate new values each time step. Therefore, we present a parallelization method for the time periodic steady state problems by using period-slices. In order to evaluate the efficiency of the developed method, one-dimensional fluid simulations are conducted for describing rf sheath dynamics. The result shows that speedup can be achieved by using a multithreading method.

The Feasibility of Applying AC Driven Low-Temperature Plasma for Multi-Cycle Detonation Initiation**supported by National Natural Science Foundation of China (No. 51176001)

Ignition is a key system in pulse detonation engines (PDE). As advanced ignition methods, nanosecond pulse discharge low-temperature plasma ignition is used in some combustion systems, and continuous alternating current (AC) driven low-temperature plasma using dielectric barrier discharge (DBD) is used for the combustion assistant. However, continuous AC driven plasmas cannot be used for ignition in pulse detonation engines. In this paper, experimental and numerical studies of pneumatic valve PDE using an AC driven low-temperature plasma igniter were described. The pneumatic valve was jointly designed with the low-temperature plasma igniter, and the numerical simulation of the cold-state flow field in the pneumatic valve showed that a complex flow in the discharge area, along with low speed, was beneficial for successful ignition. In the experiments ethylene was used as the fuel and air as oxidizing agent, ignition by an AC driven low-temperature plasma achieved multi-cycle intermittent detonation combustion on a PDE, the working frequency of the PDE reached 15 Hz and the peak pressure of the detonation wave was approximately 2.0 MPa. The experimental verifications of the feasibility in PDE ignition expanded the application field of AC driven low-temperature plasma.

Influence of the preparation technique on the optical properties and content of heterophase inclusions of AS_2S_3 chalcogenide glasses

Bulk samples of As-S chalcogenide glasses were prepared by an interaction of vapors of volatile precursors in low-temperature non-equilibrium plasma discharge (plasma enhanced chemical vapor deposition (PECVD process)). Elemental arsenic and sulfur (As + S), and arsenic monosulfide and sulfur (As4S4 + S) were used as initial substances. In parallel, the As-S bulk samples were synthesized by “traditional” melting of the initial substances in the evacuated quartz ampoule from the same precursors. The optical properties of the bulk samples were compared. The exhausted gas mixtures were analyzed to clarify the difference in carbon impurities content. 3D laser ultra microscopy was used to determine the content of heterophase inclusions in the samples.

Theoretical simulation of high-voltage discharge with runaway electrons in sulfur hexafluoride at atmospheric pressure

The results of theoretical simulation of runaway electron generation in high-pressure pulsed gas discharge with inhomogeneous electric field are presented. Hydrodynamic and kinetic approaches are used simultaneously to describe the dynamics of different components of low-temperature discharge plasma. Breakdown of coaxial diode occurs in the form of a dense plasma region expanding from the cathode. On this background there is a formation of runaway electrons that are initiated by the ensemble of plasma electrons generated in the place locally enhanced electric field in front of dense plasma. It is shown that the power spectrum of fast electrons in the discharge contains electron group with the so-called “anomalous” energy.

PIC/MCC simulation of capacitively coupled discharges: Effect of particle management and integration

A PIC/MCC simulation model for the analysis of low-temperature discharge plasmas is represented which takes the common leapfrog and the velocity Verlet algorithm for the particle integration, adaptive particle management as well as parallel computing using MPI into account. Main features of the model including the impact of super particle numbers, adaptive particle management and the time step size for the different integration methods are represented. The investigations are performed for low-pressure capacitively coupled radio frequency discharges in helium and argon. Besides a code verification by comparison with benchmark simulation results in helium it is shown that an adaptive particle management is particularly suitable for the simulation of discharges at elevated pressures where boundary effects and processes in the sheath regions are important. Furthermore, it is pointed out that the velocity Verlet integration scheme allows to speed up the PIC/MCC simulations compared to the leapfrog method because it makes the use of larger time steps at the same accuracy possible.

Theoretical simulation of the picosecond runaway-electron beam in coaxial diode filled with SF6 at atmospheric pressure

This paper presents detailed results of gas discharge theoretical simulation and the explanation of probabilistic mechanism of fast-electrons generation. Within the framework of a hybrid mathematical model, the hydrodynamic and the kinetic approaches are used simultaneously in order to describe the dynamics of different components of a low-temperature discharge plasma. The breakdown of a coaxial diode occurs in the form of a dense plasma region expanding from the cathode. On this background there is a formation of runaway electrons that are initiated by the ensemble of plasma electrons generated in the region of locally enhanced electric field within the front of the dense plasma. It is shown that the power spectrum of fast electrons in the discharge contains the group of electrons with the so-called “anomalous” energies. Comparison of the calculation results with the existent experimental data gives a good agreement for all major process parameters.

Graphene Flakes in Arc Plasma: Conditions for the Fast Single-Layer Growth

The results of systematic numerical studies of graphene flakes growth in low-temperature arc discharge plasmas are presented. Diffusion-based growth model was developed, verified using the previously published experiments, and used to investigate the principal effects of the process parameters such as plasma density, electron temperature, surface temperature and time of growth on the size and structure of the plasma-grown graphene flakes. It was demonstrated that the higher growth temperatures result in larger graphene flakes reaching 5 μm, and simultaneously, lead to much lower density of the carbon atoms adsorbed on the flake surface. The low density of the carbon adatoms reduces the probability of the additional graphene layer nucleation on surface of growing flake, thus eventually resulting in the synthesis of the most valuable single-layered graphenes.

Coulomb collisions in the Boltzmann equation for electrons in low-temperature gas discharge plasmas

This paper investigates the effects of electron-electron and electron-ion Coulomb collisions on the electron distribution function and transport coefficients obtained from the Boltzmann equation for simple dc gas discharge conditions. Expressions are provided for the full Coulomb collision terms acting on both the isotropic and anisotropic parts of the electron distribution function, which are then incorporated in the freeware Boltzmann equation solver BOLSIG+. Different Coulomb collision effects are demonstrated and discussed on the basis of BOLSIG+ results for argon gas. It is shown that the anisotropic part of the electron-electron collision term, neglected in previous work, can in certain cases have a large effect on the electron mobility and is essential when describing the transition towards the Coulomb-collision dominated regime characterized by Spitzer transport coefficients. Finally, a brief overview is presented of the discharge conditions for which different Coulomb collision effects occur in different gases.

1D simulation of runaway electrons generation in pulsed high-pressure gas discharge

The results of theoretical modelling of runaway electron generation in the high-pressure nanosecond pulsed gas discharge are presented. A novel hybrid model of gas discharge has been successfully built. Hydrodynamic and kinetic approaches are used simultaneously to describe the dynamics of different components of low-temperature discharge plasma. To consider motion of ions and low-energy (plasma) electrons the corresponding equations of continuity with drift-diffusion approximation are used. To describe high-energy (runaway) electrons the Boltzmann kinetic equation is included. As a result of the simulation we obtained spatial and temporal distributions of charged particles and electric field in a pulsed discharge. Furthermore, the energy spectra calculated runaway electrons in different cross-sections, particularly, the discharge gap in the anode plane. It is shown that the average energy of fast electrons (in eV) in the anode plane is usually slightly higher than the instantaneous value of the applied voltage to the gap (in V).

Modelling of tokamak glow discharge cleaning I: physical principles

Glow discharge cleaning (GDC) is a common technique for the conditioning of tokamak vessel walls in order to improve plasma performance and will be one of the primary conditioning techniques in ITER. The GDC discharge is a dc low-temperature plasma discharge, operated in the absence of the toroidal magnetic field, between one or more anodes inserted into the vessel, and the entire vessel wall serving as a cathode. This paper presents a self-consistent 2D model of the GDC discharge with the aim of improving fundamental understanding and predicting the wall ion current density distribution in ITER. The model combines a standard fluid model of the quasineutral plasma bulk with non-standard fluid equations for the fast electrons accelerated by the cathode sheath, based on transport coefficients and rate coefficients deduced from a Monte Carlo simulation. Examples of model results are shown in order to illustrate the general principles of the GDC discharge and the influence of the model input parameters. An important insight gained from this work is that the GDC discharge operates basically as a hollow-cathode discharge: the plasma is sustained mainly by ionization by secondary electrons emitted from the cathode, accelerated ballistically through a thin cathode sheath, penetrating the plasma as a fast electron beam, and trapped by the cathode fall surrounding the plasma on all sides. The electric field distribution inside the plasma, which determines the ion flux distribution on the vessel walls, is controlled by low-energy plasma bulk electrons. The relatively small surface area of the anode leads to the formation of an anode glow affecting the plasma uniformity. Comparisons with experimental data and predictions for ITER are presented in a companion paper.

Cross-field diffusion in low-temperature plasma discharges of finite length**This article is part of the special issue ‘Transport in B-fields in low-temperature plasmas’, published in Plasma Sources Sci. Technol. vol 23, issue 4 (http://iopscience.iop.org/0963-0252/23/4).

The long-standing problem of plasma diffusion across a magnetic field (B-field) is reviewed, with emphasis on low-temperature linear devices of finite length with the magnetic field aligned along an axis of symmetry. In these partially ionized plasmas, cross-field transport is dominated by ion–neutral collisions and can be treated simply with fluid equations. Nonetheless, electron confinement is complicated by sheath effects at the endplates, and these must be accounted for to get agreement with experiment.

Propagation of light along the hollow-core photonic crystal fibers filled with plasma

Using the finite element method, the propagation of light along the hollow-core photonic crystal fibers filled with low-temperature glow discharge plasma is investigated. The refractive index of glow discharge plasmas is a function of plasma parameters and incident wavelength. In weakly ionized helium, neon and He–Ne plasmas with low electron density and low electron temperature, the rise in the electron density will increase the plasma oscillation frequency, while the rise in the electron temperature and helium content will increase the collision frequency. In the end, the increase of the wavelength, the plasma oscillation and the collision frequency will cause an increase in the imaginary part of the refractive index. With the air holes occupied by plasmas, the optical field distribution is simulated in photonic bandgap fibers for two different wavelengths. The results show the plasma in microstructured fibers will change the transmission attenuation slightly, and the influence of plasma parameters and plasma components on attenuation has a similar tendency to that on refractive index.

Development of Simple Designs of Multitip Probe Diagnostic Systems for RF Plasma Characterization

Multitip probes are very useful diagnostics for analyzing and controlling the physical phenomena occurring in low temperature discharge plasmas. However, DC biased probes often fail to perform well in processing plasmas. The objective of the work was to deduce simple designs of DC biased multitip probes for parametric study of radio frequency plasmas. For this purpose, symmetric double probe, asymmetric double probe, and symmetric triple probe diagnostic systems and their driving circuits were designed and tested in an inductively coupled plasma (ICP) generated by a 13.56 MHz radio frequency (RF) source. Using I-V characteristics of these probes, electron temperature, electron number density, and ion saturation current was measured as a function of input power and filling gas pressure. An increasing trend was noticed in electron temperature and electron number density for increasing input RF power whilst a decreasing trend was evident in these parameters when measured against filling gas pressure. In addition, the electron energy probability function (EEPF) was also studied by using an asymmetric double probe. These studies confirmed the non-Maxwellian nature of the EEPF and the presence of two groups of the energetic electrons at low filling gas pressures.