Low-pressure Discharge Plasma Research Articles

Modeling and experimental comparison of pulsed-DC driven low-pressure plasma discharge in a metal tube

High-voltage pulsed-DC can be used to deposit films on the inner surfaces of tubes and pipes. However, the characteristics of the plasma discharge in the tubes and mechanisms of film deposition are not well understood. In this study, a discharge is formed in a metal tube inserted with a coaxial anode by microsecond (μs) pulsed power to explore the influence of the pulse parameters on the plasma characteristics, including the temporal and spatial evolution near the tube walls experimentally and by simulation. The discharge waveform is monitored by a voltage-current probe to analyze the impact of the electrical characteristics on the plasma. The plasma properties are diagnosed by a Langmuir probe and numerically calculated by the fluid model. The source voltage and current jointly influence the plasma density and electron temperature, and secondary electron emission by energetic ion bombardment affects not only the distribution of electron density measured by the probe, but also the temporal distribution during the pulse-on and pulse-off time. The electron temperature varies conversely with the density from the plasma zone to the cathode suggesting the typical glow discharge in the pulsed-DC tube. Secondary electron emission is created at a higher discharge frequency, and a sustained plasma between pulse cycles appears to be produced at a higher frequency. The results impart information about the pulsed-DC driven plasma discharge at low pressure as well as the role energetic ions play on the cathode and elucidate the mechanisms of plasma deposition on the interior tube wall.

On the chemistry mechanism for low-pressure chlorine process plasmas

A chemical reaction mechanism of chlorine plasma under low-pressure conditions that is widely used in the literature is validated against the experimental data of Y. Wang and J. K. Olthoff [J. Appl. Phys. 85, 6358 (1999)] for an inductively coupled plasma reactor. The model used in the present study is a self-consistent two-dimensional fluid plasma model coupled with Maxwell’s equations. The quantities of interest in the plasma are the fluxes and energy distribution functions of Cl2+ and Cl+ ions. We find that the charge exchange reaction between Cl+ and Cl2 that is typically included in chlorine plasma reaction mechanisms results in poor predictability of the model compared to experiments. Neglecting this reaction allows for a correct prediction of the dominant ion species in the low-pressure chlorine plasma and dependence of their fluxes on the gas pressure. Additionally, neglecting the charge exchange reaction allows for a rather accurate prediction of ion energy distribution functions at the grounded electrode. Overall, we conclude that the rate coefficient of the charge exchange reaction between Cl+ and Cl2 reported in the literature significantly exceeds what may in fact explain the role of this process in a low-pressure plasma discharge.

Activation of water in the downstream of low-pressure ammonia plasma discharge

In the present work, we study the physicochemical changes that arise in water named plasma processed water (PPW) when it is exposed to the downstream low-pressure discharge of ammonia (NH3) gas. Optical emission spectroscopy and voltage-current characteristics of NH3 plasma are studied to identify species formed in NH3 plasma along with plasma characterization. A three-way full factorial design of experiment is performed to study the effect of process parameters named applied voltage, post-discharge gas-water interaction time, and NH3 gas pressure on physicochemical properties of PPW. The obtained results are analyzed using analysis of variance, standardized effect estimation, regression analysis, and response surfaces. The optimum values of these properties and PPW process parameters are estimated using MATLAB fmincon solver with experimental constraints. The emission spectrum of NH3 plasma showed strong intensity N2 + lines along with weak intensity N2, NH, and N+ lines. The obtained results showed the post-discharge gas-water interaction time and applied voltage had a significant impact on physicochemical properties and ammonium ions concentration in PPW. The obtained optimum value of voltage and time is 550 V and 15 min with given experimental constraints.

Data driven discovery of a model equation for anode-glow oscillations in a low pressure plasma discharge

A plasma glow discharge tube, a versatile device widely employed in several scientific and industrial applications, is also a useful tool for many basic plasma studies in the laboratory. Anode glow oscillations are well-known phenomena in such devices that arise from an instability of the plasma glow around a small positively charged electrode. Depending upon the gas pressure, the applied DC voltage, and the distance between the electrodes, these oscillations can display a rich dynamical behavior. Over a certain parametric regime, these nonlinear oscillations exhibit a stable limit cycle behavior that has been modeled in the past by a Van der Pol like equation. While such a model equation provides a qualitative description of the observations, it lacks quantitative agreement and does not have any predictive capability. We employ the sparse identification of nonlinear dynamics (SINDy) method to obtain a model equation directly from a time series of the experimental data. Our model captures well the main features of the experimental data in a quantitative manner. It also shows a significant deviation from the Van der Pol model due to additional contributions that are akin to nonlinear damping in a Rayleigh oscillator. Such a hybrid Van der Pol–Rayleigh oscillator model could provide a useful paradigm for future explorations of the nonlinear dynamics of this system.

Reaction dynamic process of low pressure discharge plasma in microwave resonant cavity

Low-pressure discharge is an important problem that restricts the development of microwave components of spacecraft toward high-power and miniaturization. To clarify the mechanism of low-pressure discharge of microwave component in spacecraft, we build an emission spectroscopy diagnostic platform for studying the low-pressure radio frequency (RF) discharge plasma, and investigate the plasma reaction dynamics of low-pressure RF discharge of microwave cavity resonator and the damage effect of discharge on microwave component. The emission spectra of the plasma inside the resonator under different gas pressure conditions are obtained, and it is found that the density of hydroxyl OH (A-X), excited nitrogen molecules N<sub>2</sub> (C-B) and oxygen atoms O (3p<sup>5</sup>P→3s<sup>5</sup>S<sup>0</sup>) in the plasma each show a first-increasing and then decreasing trend with the increase of gas pressure. The kinetic mechanism of the plasma reaction behind this phenomenon is analyzed, and it is found that the gas pressure can influence the concentration magnitude of each species in the plasma by changing the species production and consumption paths as well as the average electron temperature of the plasma. The variation law of plasma emission spectrum with the input power is studied, and the trends of linear increase of particle concentration with the increase of input power at different air pressures are found. This study provides a reference for investigating the mechanism of low-pressure RF discharge and the reliable design of spacecraft microwave components.

Response of the low-pressure hot-filament discharge plasma to a positively biased auxiliary disk electrode

In a steady-state plasma, the loss rate of plasma particles to the chamber wall and surfaces in contact with plasma is balanced by the ionization rate of background neutrals in the hot-filament discharges. The balance between the loss rate and ionization rate of plasma particles (electrons and ions) maintains quasi-neutrality of the bulk plasma. In the presence of an external perturbation, it tries to retain its quasi-neutrality condition. In this work, we studied how the properties of bulk plasma are affected by an external DC potential perturbation. An auxiliary biased metal disk electrode was used to introduce a potential perturbation to the plasma medium. A single Langmuir probe and an emissive probe, placed in the line of the discharge axis, were used for the characterization of the bulk plasma. It is observed that only positive bias to the auxiliary metal disk increases the plasma potential, electron temperature, and plasma density but these plasma parameters remain unaltered when the disk is biased with a negative potential with respect to plasma potential. The observed plasma parameters for two different-sized, positively as well as negatively biased, metal disks are compared and found inconsistent with the existing theoretical model at large positive bias voltages. The role of the primary energetic electrons population in determining the plasma parameters is discussed. The experimentally observed results are qualitatively explained on the basis of electrostatic confinement arising due to the loss of electrons to a biased metal disk electrode.

The Effect of Low-Pressure Dielectric Barrier Discharge (LPDBD) Plasma in Boosting Germination, Growth, and Nutritional Properties in Wheat

Plasma agriculture is an emerging technology, although the application of non-thermal plasma in wheat productivity is still in its early stage. This study deciphers the effect and mechanistic basis of non-thermal air-generated LPDBD (low-pressure dielectric barrier discharge) plasma in boosting germination, growth and nutritional properties in wheat. Seeds treated with LPDBD plasma exhibited cracked periphery and discernible expansion during seed germination. LPDBD plasma applied for 6 min showed a 22.11% increase in the germination percentage and a substantial increase in iron content in grains compared to non-treated controls. At the cellular level, the concentration of H2O2 in leaves significantly increased (3.56 µM g−1 FW) due to LPDBD treatment compared to controls. This increased level of H2O2 may act as a stimulating agent to trigger the physiological functions in wheat plants. In addition, plants sprouted from air-treated seeds exhibited a marked elevation in CAT and SOD activity accompanied by the increased expression of TaCAT and TaSOD genes in roots of wheat. Interestingly, the grain yield of wheat increased by 27.06% in response to plasma treatment compared to control. Further, grains harvested from plasma-treated plants showed a substantial elevation in iron and fat content as well as decreased moisture content that may contribute to the increased shelf life. The study will open up a new avenue for practical application of plasma in agriculture.

Deposition of Al2O3 coatings in Ar-O2 low-pressure discharge plasma under a high dissociation degree of O2

The deposition of Al2O3 coating with a corundum structure was done by anodic evaporation in a low-pressure arc with a self-heated hollow cathode. The conditions were created for increasing the energy of plasma electrons and a corresponding increase in the frequency of O2 dissociation by contraction of the discharge in the anode region. The discharge was maintained in a combined mode with a constant current (70 – 100 A), on which current pulses (100 μs, 1 kHz) with adjustable amplitude (up to 220 A) were superimposed. This mode ensured a change in the degree of O2 dissociation in the range of 0.3 – 0.5 at constant average discharge current and Al evaporation rate. It is shown that an increase in the degree of O2 dissociation leads to an increase in the rate of coating deposition by a factor of 1.3 and promotion of the preferred (300) orientation of crystallites. The effect is due to the features of the adsorption of molecular and atomic oxygen on the Al2O3.

Experimental Investigation into the Influence of Plasma Technology on Seed Surface Wettability

The influence of atmospheric plasma discharge (APD) of the Gliding Arc type and low-pressure microwave plasma discharge (LPMD) on the class of various seeds was investigated. Pea, wheat, and sunflower seeds, representing legumes, cereals, and oilseeds, respectively, were selected for the treatment. Our study aimed to verify the effect of plasma treatment on the water contact angle and the wettability of the seeds. Treatment time, working gas flow and microwave power were varied to determine their impact. All treated and untreated variants were used to conduct the water contact angle measurements to compare their wettability. APD treatment seemed to be utterly ineffective for improving the wettability for most process parameters. On the contrary, LPMD manifested a much more efficient impact. The maximum effect was found for the 800 W microwave power applied to the pea seed for 30 s. The contact angle achieved decreased by approximately 50% compared to the untreated sample. These results indicate that LPMD may be an effective alternative to traditional pre-sowing seed treatments used in agriculture for water intake enhancement. Still, it is strongly dependent on the seed’s type and the used process parameters.

Development of a lumping methodology for the analysis of the excited states in plasma discharges operated with argon, neon, krypton, and xenon

In this paper, a methodology is presented to compute the plasma properties (e.g.,, density and temperature) accounting for the dynamics of the excited states. The proposed strategy applies to both zero-dimensional (0D) models and multidimensional fluid and hybrid codes handling low-pressure (<50 mTorr) plasma discharges filled with argon, neon, krypton, and xenon gases. The paper focuses on two main aspects: (i) a lumping methodology is proposed to reduce the number of reactions and species considered in order to keep at bay the computational cost without a major loss of accuracy; (ii) the influence that different datasets of cross sections have on the results has been assessed. First, the lumping methodology has been implemented in a 0D model accounting for singly charged ions, neutrals, along with 1s and 2p excited states (Paschen notation). Metastable and resonant are treated as two separate species within the 1s energy level (1sM and 1sR, respectively). The results have been benchmarked against those obtained treating each energy level of the excited states as an individual species. Differences lower than 1% have been obtained. Second, the results of the 0D model have been compared against measurements of electron density and temperature performed on an inductively coupled plasma. Numerical predictions and experiments present a disagreement up to 20%–30%, which is comparable to the uncertainty band of the measurements. Finally, the lumping strategy has been implemented in a 2D fluid code to assess its computational affordability, and the results have been compared against the experiments as well. A variance up to 30% in electron density and temperature is registered adopting different datasets of cross sections.

Determination and control of ion parameters in a complex plasma of a DC discharge

Complex plasmas, comprising a low-pressure DC discharge plasma and charged microparticles, form traps that can accumulate and localize ions. A loss of ions and electrons always occurs on the surfaces of the microparticles, which can lead to a decrease in the ion and electron densities in the plasma bulk. However, in most cases, the collective effect of the interaction between the discharge plasma and the dust cloud leads to an increase in the ion density—both around and inside the dust cloud—in comparison with that in pure plasma at the same discharge currents. Complex plasmas are open dissipative synergetic systems. Feedback between plasma processes allows one to control their properties using the external influences of various physical fields. Such influences thus represent a tool for controlling the dust cloud parameters and the ion density. In this letter, we report a method for determining the ion density inside a dust cloud using an electric field-current diagram (ECD). This method is based on the statement that only a single electric field can relate the particular values of the ion density, the discharge, and the dust cloud parameters. Our method includes a procedure for superimposing the ECD on the ion density-current diagram. To calculate the plasma parameters, a fluid model of a DC discharge with microparticles is employed. The method proposed herein allows us to find the conditions under which the ion density in a plasma with microparticles is less than that in pure plasma without microparticles—or vice versa, increases up to five times.

Radial control of plasma uniformity and electron temperature by external plate biasing in a back diffused partially magnetized plasma

The possibility of achieving uniform and controllable plasma distribution remain a fundamental challenge in low pressure plasma discharges, which has numerous applications in plasma technologies. An external object when introduced in a magnetized plasma can significantly alter the spatial plasma distribution. In this paper, the effect of external plate biasing on both axial as well as radial characteristics of an expanding, partially magnetized plasma column, created by hot cathode filament inside a linear plasma device is presented. It is found that by applying a positive potential to a conducting external electrode, placed at a remote location away from the primary discharge region; the back diffused plasma tends to become uniform. The plate biasing also results in an overall increase in electron temperature of the expanding plasma; however, it does not appear to alter the radial plasma characteristics inside the source and the biasing region. A possible mechanism behind observing this effect has been provided.

Highly Hydrophobic Organosilane-Functionalized Cellulose: A Promising Filler for Thermoplastic Composites.

The aim of this work is to design and optimize the process of functionalization of cellulose fibers by organosilane functional groups using low-pressure microwave plasma discharge with hexamethyldisiloxane (HMDSO) precursor in order to prepare a compatible hydrophobic filler for composites with nonpolar thermoplastic matrices. Particular attention was paid to the study of agglomeration of cellulose fibers in the mixture with polypropylene. In our contribution, the dependence of the surface wettability on used process gas and treatment time was investigated. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analyses were applied to characterize the surface morphology and chemical composition of the cellulose fibers. It was observed that the plasma treatment in oxygen process gas led to the functionalization of cellulose fibers by organosilane functional groups without degradation. In addition, the treated cellulose was highly hydrophobic with water contact angle up to 143°. The use of treated cellulose allowed to obtain a homogeneous mixture with polypropylene powder due to the significantly lower tendency of the functionalized cellulose fibers to agglomerate.

Impact of Plasma Pre-Treatment on the Tribological Properties of DLC Coatings on PDMS Substrates.

The processes of the deposition of carbon coatings on PDMS (polydimethylsiloxane) substrates using plasma techniques are widely used in a large number of studies, in applications ranging from electronic to biological. That is why the potential improvement of their functional properties, including tribological properties, seems very interesting. This paper presents an analysis of the impact of plasma pre-treatment on the properties of the produced diamond-like carbon (DLC) coatings, including changes in the coefficients of friction and wear rates. The initial modification processes were performed using two different techniques based on low-pressure plasma (RF PACVD, radio-frequency plasma-assisted chemical vapour deposition) and dielectric barrier discharge (DBD) plasma. The effects of the above-mentioned treatments on the geometric structure of the PDMS surface and its water contact angles and stability over time were determined. The basic properties of the DLC coatings produced on unmodified substrates were compared to those of the coatings subjected to plasma pre-treatment. The most interesting effects in terms of tribological properties were achieved after the DBD process and production of DLC coatings, achieving a decrease in wear rates to 2.45 × 10−8 mm3/Nm. The tests demonstrate that the cross-linking of the polymer substrate occurs during plasma pre-treatment.

Compact scanning retarding potential analyzer

A miniature scanning electrostatic energy analyzer has been developed for measurements of the ion energy distribution in low-pressure plasma discharges. The retarding potential analyzer utilizes four grids to reject the incident electron flux and discriminate the ion energy. It features a compact size of 5-mm diameter and 2.5-mm thickness and uses a simple configuration to mount the grids and provide voltage isolation using high temperature ceramic cement that avoids complicated grid alignment or MEMS fabrication techniques. Mounting on a fast-scanning two-axis probe drive eliminates overheating of the ceramics and subsequent current leakage, which permits insertion into challenging locations and provides profiles of the ion energy in the plasma discharge. Guidelines for the analyzer design for high density operation are discussed, and the performance of the analyzer is shown for measurements in a hollow cathode plume in xenon plasmas.

Characterization of Low-Pressure Nitrogen Plasma Discharge and Pre-Oxidation on the Surface Characteristics of Activated Carbons

Commercial activated carbon (CAC) was modified using low-pressure radio frequency nitrogen plasma discharge (NPD) operating at 0.3 mbar and 40 kHz. The surface chemistry of CAC was modified using HNO3 pre-oxidation to possibly influence the reactivity of NPD. The results of x-ray photoelectron spectroscopy (XPS) suggested that pre-oxidation reduces aromaticity, generates aliphatic carbons (C-C and C-H), and increases carboxylic functional groups (COOH) which probably enhances the nitrogen plasma functionalization based on the N/C ratio for CAC-O-P (4.29 %) compared to CAC-P (2.88 %). FTIR was used to confirm such effects of pre-oxidation from the functional groups present on the carbon surface. The total surface area was identified using Langmuir and Brunauer–Emmett–Teller (BET) N2 adsorption isotherms at 77 K. Both pre-oxidation and plasma treatment caused an increase in the surface area of CAC up to 150 percent. Carbon t-plot method was used to determine the micropore volume, micropore area, and external surface area. The total surface area of each activated carbon was mostly constituted of micropore area which was identified to be directly proportional to the micropore volume. Scanning electron microscope (SEM) confirms the destruction of the surface morphology for CAC-O that might have caused the increase in surface area. Development of surface threadlike structures were observed for the NPD treated CAC-O. NPD favors the development of NH2 functionalities and reduces the aromaticity of activated carbons while enhancing the surface morphology and the surface area.

An exploration of the effects of low-pressure plasma discharge on the physicochemical properties of chia (Salvia hispanica L.) flour

This work explores the preliminary feasibility of employing the low-pressure coldplasma technology for the modification of the properties of chia flour. Chia flour wasexposed to low pressure plasma in air for 5 min, 10 min, and 15 min, at two differentpower levels (40 W and 60 W). The oils extracted from untreated and treated chiaflour were exhaustively characterized for fatty acid composition, nutritional value,and rancidity indices using thermal calorimetric methods (DSC/TGA). The resultsindicated a significant change in the colour of flour with an increase in lightness.Infrared and ultraviolet spectroscopy indicated changes in the tocopherol groups ofthe oil extracted from plasma treated chia flour. However, the oil extracted fromplasma treated chia flour revealed a loss of conjugated dienes and formation oftrans-fatty acids as seen in conventional hydrogenation of edible oils. DSC and TGAresults revealed better oxidative stability of low-pressure plasma treated oils thancontrol, which was linked to a relative increase of MUFA in the former.

Effects of Microwave and Cold Plasma Assisted Hydrodistillation on Lemon Peel Oil Extraction

Abstract This study aimed to investigate the effect of low-pressure dielectric barrier discharge (DBD) plasma on microwave-assisted hydrodistillation of lemon peel oil extraction. Microwave pre-treated lemon peel powder was exposed to plasma treatment (1.0, 1.5, 2.0, and 2.5 kV) for 10 min. The treated lemon peel powders were subjected to hydrodistillation to extract the essential oil and the extraction yields were calculated. The extracted oil was analyzed for chemical composition with gas chromatography-mass spectrometry (GC-MS). Effect of plasma on the surface morphology of the lemon peel was observed in a scanning electron microscope (SEM) which revealed the formation of fissures and cracks owing to the higher extraction yield. Plasma treatment at 2.5 kV was observed higher extraction yield than conventional hydrodistillation (149.34 % rise) and the chemical composition of plasma treated sample essential oil remains significantly unchanged. Thus, DBD plasma could be a promising technique to enhance the lemon peel essential oil extraction.

Simulator design for low pressure plasma discharges in planar geometry

A design of a simulator code to determine the plasma characteristics in planar geometry discharges is presented. The experimentally obtained parameters are compared to the code predictions. The experimental plasma discharge characteristics were carried out using 99.99% nitrogen as the working gas at typical pressure values in the range 0.1–0.6 Torr. The simulator code was used to obtain the discharge current-voltage, showing compatibility with experimental values. The minimum breakdown voltage at the minimum gas pressure was also obtained. The relation between the discharge current at different anode-cathode distances was also determined.

Low-Pressure Discharge Plasma Treatment of Aqueous Solutions with Mn, Cr and Fe

Low-Pressure Discharge Plasma Treatment of Aqueous Solutions with Mn, Cr and Fe