One-dimensional Discharge Research Articles

Simulation of the Impact of Humidity on the Species Generated by a One-Dimensional Discharge of Helium Gas

Simulation of the Impact of Humidity on the Species Generated by a One-Dimensional Discharge of Helium Gas

Theoretical-Computational Study of Atmospheric DBD Plasma and Its Utility for Nanoscale Biocompatible Plasmonic Coating.

In this study, time-dependent, one-dimensional modeling of a surface dielectric barrier discharge (SDBD) device, driven by a sinusoidal voltage of amplitude 1–3 kV at 20 kHz, in argon is described. An SDBD device with two Cu-stripe electrodes, covered by the quartz dielectric and with the discharge gap of 20 × 10−3 m, was assumed, and the time-dependent, one-dimensional discharge parameters were simulated versus time across the plasma gap. The plasma device simulated in the given arrangement was constructed and used for biocompatible antibacterial/antimicrobial coating of plasmonic particle aerosol and compared with the coating strategy of the DBD plasma jet. Simulation results showed discharge consists of an electrical breakdown, occurring in each half-cycle of the AC voltage with an electron density of 1.4 × 1010 cm−3 and electric field strength of 4.5 × 105 Vm−1. With SDBD, the surface coating comprises spatially distributed particles of mean size 29 (11) nm, while with argon plasma jet, the nanoparticles are aggregated in clusters that are three times larger in size. Both coatings are crystalline and exhibit plasmonic features in the visible spectral region. It is expected that the particle aerosols are collected under the ionic wind, induced by the plasma electric fields, and it is assumed that this follows the dominant charging mechanisms of ions diffusion. The cold plasma strategy is appealing in a sense; it opens new venues at the nanoscale to deal with biomedical and surgical devices in a flexible processing environment.

High-fidelity modeling of breakdown in helium: initiation processes and secondary electron emission

Understanding the role of physical processes contributing to breakdown is critical for many applications in which breakdown is undesirable, such as capacitors, and applications in which controlled breakdown is intended, such as plasma medicine, lightning protection, and materials processing. The electron emission from the cathode is a critical source of electrons which then undergo impact ionization to produce electrical breakdown. In this study, the role of secondary electron yields due to photons (γ ph) and ions (γ i) in direct current breakdown is investigated using a particle-in-cell direct simulation Monte Carlo model. The plasma studied is a one-dimensional discharge in 50 Torr of pure helium with a platinum cathode, gap size of 1.15 cm, and voltages of 1.2–1.8 kV. The current traces are compared with experimental measurements. Larger values of γ ph generally result in a faster breakdown, while larger values of γ i result in a larger maximum current. The 58.4 nm photons emitted from He(21P) are the primary source of electrons at the cathode before the cathode fall is developed. Of the values of γ ph and γ i investigated, those which provide the best agreement with the experimental current measurements are γ ph = 0.005 and γ i = 0.01. These values are significantly lower than those in the literature for pristine platinum or for a graphitic carbon film which we speculate may cover the platinum. This difference is in part due to the limitations of a one-dimensional model but may also indicate surface conditions and exposure to a plasma can have a significant effect on the secondary electron yields. The effects of applied voltage and the current produced by a UV diode which was used to initiate the discharge, are also discussed.

Incorporating Improved Chemical and Electrochemical Reaction Schemes in Electrochemical Engineering Models for Lithium Sulfur Batteries

Lithium sulfur batteries have gained popularity due to the promise of high energy density, predicted to be 2-3 times than of lithium ion batteries.1 The challenges that limit commercial success for lithium sulfur batteries stem from the poor cyclability of lithium metal, insulating sulfur products, and the polysulfide shuttle mechanism where soluble sulfur species react with the lithium metal anode, resulting in permanent active material loss.2 The complexity of the challenges facing the adoption of lithium sulfur batteries has led to a focus on electrolyte engineering, additives, and anode coatings. The complex reaction pathway is sensitive to changes in electrolyte and additives, which can alter the shape of the voltage curve and even capacity.3,4 Understanding the effects of different electrolyte systems is important to characterizing the performance. Various techniques like UV-vis and Raman spectroscopy are useful to identifying the speciation. The complex speciation pathway has shown to include dissociation and disproportionation reactions and stabilization of radical species through UV-vis and Raman spectroscopy.5–10 Physics-based modeling can help elucidate underlying phenomena within batteries. Kumaresan and White11 developed a one-dimensional discharge model for lithium sulfur batteries. The model utilized included 5-step reduction scheme with Sn 2- species (n=8,6,4,2,1). The subsequent models that were developed used a similar reaction scheme or a reduced reaction scheme with only 2 or 3 reactions. The chemistry has not been updated to include the new insights available from spectroscopy work. In this work, the following homogeneous dissociation reaction that has been identified in many sulfur electrochemical systems12–17 is added to the reaction schemeS6 2- ↔ S3 ·- (1)The S6 2- species balance is modified to include a sink for the equilibrium reaction according to reaction (1). The material balance for S3 ·- radical species is added with the only source as the equilibrium reaction; the S3 ·- species is assumed to be electrochemically inactive. When there is significant S3 ·- species, the S6 2- species essentially becomes trapped as the S3 ·- radical species, removing it from the reduction scheme. When the reaction is pushed farther to S3 ·- radical species, the electrolyte system favors the stabilization of this radical species over the S6 2- species. The forward rate of reaction (1) sets the reaction timescale, and the equilibrium constant determines the ratio of S3 ·- and S6 2-. The resulting interplay of these dynamics on the electrochemical behavior of the battery cell will be explored, and the implications on electrolyte engineering will be discussed.Acknowledgments: The authors are thankful to the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy through the Advanced Battery Materials Research (BMR) Program (Battery500 Consortium) for funding of this work.

Multi-fluid modelling of pulsed discharges for flow control applications

Experimental evidence suggests that short-pulse dielectric barrier discharge actuators are effective for speeds corresponding to take-off and approach of large aircraft, and thus are a fruitful direction for flow control technology development. Large-eddy simulations have reproduced some of the main fluid dynamic effects. The plasma models used in such simulations are semi-empirical, however, and need to be tuned for each flowfield under consideration. In this paper, the discharge physics is examined in more detail with multi-fluid modelling, comparing a five-moment model (continuity, momentum, and energy equations) to a two-moment model (continuity and energy equations). A steady-state, one-dimensional discharge was considered first, and the five-moment model was found to predict significantly lower ionisation rates and number densities than the two-moment model. A two-dimensional, transient discharge problem with an elliptical cathode was studied next. Relative to the two-moment model, the five-moment model predicted a slower response to the activation of the cathode, and lower electron velocities and temperatures as the simulation approached steady-state. The primary reason for the differences in the predictions of the two models can be attributed to the effects of particle inertia, particularly electron inertia in the cathode layer. The computational cost of the five-moment model is only about twice that of the simpler variant, suggesting that it may be feasible to use the more sophisticated model in practical calculations for flow control actuator design.

Simulation Study on Multi-Pulse Phenomena of Atmospheric Pressure Argon Dielectric Barrier Discharge

In this paper, a one-dimensional discharge model is employed to study multi-pulse phenomena in Ar dielectric barrier discharge (DBD) under atmospheric pressure. The finite-element method is employed to solve the model. The influences of applied voltage amplitude and frequency as well as gas gap distance on the variation of multi discharge pulses are investigated and discussed. The simulation results show that, both the intensity of discharge current and the number of discharge pulses increase with the amplitude of applied voltage, and narrower gas gap is more favorable for the formation of multi pulses. It is revealed that Ar DBDs behave in glow discharge mode when the applied voltage and gas gap distance vary from 2 kV to 6 kV and from 1 mm to 3 mm, respectively. With the frequency decreasing from 250 Hz to 125 Hz, the intensity of discharge current weakens and the number of discharge pulses increases, and the discharges behave in the typical Townsend discharge mode.

The response of a capacitively coupled discharge to the formation of dust particles: Experiments and modeling

The influence of dust particles on the properties of a capacitively coupled Ar–C2H2 discharge is studied both experimentally and theoretically. The results of measurements of the intensity and spatial distribution of the emitted light, the line width of the fast component of Hα line and of the electron density during the particle growth are presented. To analyze the experimental results a one-dimensional discharge model is developed. Using the model the effects of dust grains on the power absorption (taking into account stochastic and Ohmic heating in the plasma sheaths), the optical emission intensity profile, the sheath size, the rf electric field and on the energy of positive ions bombarding the electrodes are investigated. In particular, it is shown that the decrease of the power absorption in the sheaths of complex plasmas is due to the dependence of the stochastic and Ohmic heating in the plasma sheaths on the electron temperature and the current flowing across the discharge plates. The results of the calculations are compared with the available experimental data and found to be in good agreement.

One-dimensional discharge simulation of nitrogen DBD atmospheric pressure plasma

Numerical calculations with time dependant one-dimensional fluid model are performed at various discharge conditions in nitrogen atmospheric pressure plasmas to calculate electrical characteristics and plasma parameters such as charged and metastable molecules densities. The amplitude and frequency of external driving voltages are used as control parameters to change the discharge behavior of nitrogen plasmas. The simulation results show that the discharge structure of nitrogen atmospheric pressure DBD plasma similar to the Townsend discharge. The increase of the amplitude and frequency of external voltage lead to the increase of plasma density. Oxygen gases added to nitrogen plasmas are confirmed to act as a N 2(A 3∑ u +) quencher by observing the decrease of metastable molecule and charged particle densities. Dielectric constants of the barrier materials have also shown a strong influence on the discharge structure.

Theoretical Investigations on the Relation between Neutral Pressure and Beta in HANBIT Mirror Discharges

A numerical study is made of the effects of neutral pressure on the characteristics of HANBIT mirror plasmas. One-dimensional discharge simulations are carried out under various neutral pressure conditions and the dependence of β (=plasma pressure/magnetic pressure) on the neutral pressure P0 is investigated. It is found that β and ion temperature increase rapidly with decreasing pressure below P0 ~ 2 × 10-6 Torr in HANBIT mirror discharges.

Analysis for discharge-radiation dynamics in alternating current plasma display panels

An analytical method to study the discharge-radiation dynamics (DRD) in alternating current plasma display panels was developed. The input parameters for this DRD analysis were experimentally determined panel voltage and current wave forms. Discharge voltage, current, and power wave forms in the discharge volume of a cell were first obtained from the measured panel voltage and current wave forms using known geometrical configurations and electric circuit calculations. Intrinsic discharge parameters, such as electron temperature and density, were then determined to satisfy these discharge wave forms under the assumption of a hydrodynamic approach. A one-dimensional discharge structure with two regions (cathode fall and positive column) and several other assumptions which are plausible from the discharge physics point of view were also adopted. These assumptions took account of known cross sections and energies of electron-impact excitation and ionization of discharge gas atoms, and a secondary electron emission coefficient of the dielectric surface at the cathode side induced by ion bombardment. Radiation intensities from the discharge were calculated using the determined intrinsic discharge parameters, and the results were compared with those measured for the respective panel conditions used in the calculations, yielding a fair agreement. The luminous efficiency, defined as the radiation intensity divided by the discharge power, was also determined using the intrinsic discharge parameters. Discussion on the luminous efficiency change for different panel operating conditions revealed that the efficiency improvement at a lower voltage was attributable to a lower electron temperature for this condition.

Spatiotemporal evolution of a low-pressure glow discharge

The spatiotemporal evolution of a pulsed low-pressure glow discharge is investigated theoretically and experimentally. The discharge is modelled phenomenologically by a reaction-diffusion equation in which the cathode fall and the negative glow of the discharge are described as a bistable layer. A laterally one-dimensional discharge is realized experimentally by a hollow cathode discharge in a slotted cathode. With the help of probe measurements, a homogeneous evolution with a laterally uniform current rise and an inhomogeneous evolution starting from special sites and progressing along the cathode are found. The main experimental results are reproduced by model calculations.