Plasma

20 pages, 5520 KiB

Open AccessReview

Runaway Electrons in Gas Discharges: Insights from the Numerical Modeling

by Dmitry Levko

Plasma 2025, 8(1), 12; https://doi.org/10.3390/plasma8010012 - 20 Mar 2025

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This paper reviews the state of the art of our understanding of the mechanisms of runaway electron generation in pressurized gases from the numerical modeling perspective. Since the energy relaxation length of these electrons is comparable to the interelectrode spacing, these electrons can [...] Read more.

This paper reviews the state of the art of our understanding of the mechanisms of runaway electron generation in pressurized gases from the numerical modeling perspective. Since the energy relaxation length of these electrons is comparable to the interelectrode spacing, these electrons can be captured only using the kinetic approach. Therefore, only the results from kinetic models are discussed here. Special attention is given to pulsed discharges, which play an important role in modern industry. It is concluded that the mechanisms of runaway electron generation are defined by the gap overvoltage and the discharge gap geometry. For small and moderate overvoltages, runaway electrons are primarily generated at the heads of fast ionization waves or streamers. Due to their long energy relaxation length, these electrons can pre-ionize the discharge gap far from their origin, accelerating ionization and starting new avalanches. At high overvoltages, cathode surface irregularities enhance the local electric field, leading to electron emission into the interelectrode space. These electrons, injected into the strong electric field, gain high energy and reach discharge walls with extremely high energies measuring tens and hundreds of electron volts. These electrons not only pre-ionize the gas but also stimulate the emission of high-energy photons, which can further contribute to the pre-ionization of the discharge gap. Full article

(This article belongs to the Special Issue New Insights into Plasma Theory, Modeling and Predictive Simulations)

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19 pages, 6382 KiB

Open AccessArticle

Quantifying Plasma Dose for Barley Seed Treatment by Volume Dielectric Barrier Discharges in Atmospheric-Pressure Synthetic Air

by Jiří Fujera, Petr Hoffer, Václav Prukner and Milan Šimek

Plasma 2025, 8(1), 11; https://doi.org/10.3390/plasma8010011 - 17 Mar 2025

Viewed by 164

Abstract

Plasma-assisted treatment is a potentially interesting technology for advanced seed processing. In this work, we address the issue of defining and quantifying the plasma dose during the exposure of seeds to microdischarges formed in a barrier discharge configuration fed with synthetic air at [...] Read more.

Plasma-assisted treatment is a potentially interesting technology for advanced seed processing. In this work, we address the issue of defining and quantifying the plasma dose during the exposure of seeds to microdischarges formed in a barrier discharge configuration fed with synthetic air at atmospheric pressure. Using advanced imaging and other optoelectrical diagnostics, we identify suitable conditions for the formation of microdischarges developing exclusively between the powered electrode and the seed coat, which allows for the relatively accurate quantification of the plasma dose for an individual barley seed. In addition to determining the microdischarge energy/power consumed to treat a single seed during controlled exposure, we also provide an estimate of the electric field and gas temperature, which are key parameters that can affect seed viability. In this way, each individually exposed seed can be linked to the exact exposure time, total number, energy, and temperature of the microdischarges that came into contact with it. This is fundamentally different from conventional “averaging” approaches based on the simultaneous exposure of many seeds, which makes it virtually impossible to correlate the responses of individual seeds with the corresponding individual plasma dose. Finally, we propose a minimal treatment protocol that could allow for the more direct interpretation of the results of subsequent biological tests to reveal seed responses to specific plasma–chemical stimuli during germination and seedling growth. Full article

(This article belongs to the Special Issue Processes in Atmospheric Pressure Plasmas)

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11 pages, 1336 KiB

Open AccessArticle

Network Analysis as a Method for Identifying Operational Modes of Cold Atmospheric Plasma Jets

by Blake Haist and Richard E. Wirz

Plasma 2025, 8(1), 10; https://doi.org/10.3390/plasma8010010 - 10 Mar 2025

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Abstract

Network analysis is a convenient method for analyzing cold atmospheric plasma (CAP) devices across a wide range of operating conditions. By using frequency and voltage as nodes in the network, edges are formed between nodes when the combination of voltage and frequency results [...] Read more.

Network analysis is a convenient method for analyzing cold atmospheric plasma (CAP) devices across a wide range of operating conditions. By using frequency and voltage as nodes in the network, edges are formed between nodes when the combination of voltage and frequency results in an ignited plasma jet. Singular value decomposition is used to identify modalities in the network that are representative of operational modes in the plasma jet. An analysis of the spectra produced by the jet provides validation of the operational modes and shows that voltage and frequency predominately affect the operation of the jet with remarkable independence. Full article

(This article belongs to the Special Issue Processes in Atmospheric Pressure Plasmas)

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17 pages, 13969 KiB

Open AccessArticle

Combined Plasma and Laser Heating of Graphite

by Aleksey Chaplygin, Mikhail Yakimov, Sergey Vasil’evskii, Mikhail Kotov, Ilya Lukomskii, Semen Galkin, Andrey Shemyakin, Nikolay Solovyov and Anatoly Kolesnikov

Plasma 2025, 8(1), 9; https://doi.org/10.3390/plasma8010009 - 4 Mar 2025

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Abstract

This paper investigates a novel combined laser and plasma heating test technique. Integrating the 1.5 kW Raycus RFL-C1500 laser source into the VGU-4 Inductively Coupled Plasma Facility (IPMech RAS) allowed the study of fine-grain MPG-7 graphite ablation in the high-temperature range from 2920 [...] Read more.

This paper investigates a novel combined laser and plasma heating test technique. Integrating the 1.5 kW Raycus RFL-C1500 laser source into the VGU-4 Inductively Coupled Plasma Facility (IPMech RAS) allowed the study of fine-grain MPG-7 graphite ablation in the high-temperature range from 2920 to 3865 K under exposure to subsonic nitrogen plasma flow and combined exposure to nitrogen plasma flow and laser irradiation. Graphite nitridation and sublimation were observed. The subsonic nitrogen plasma flow was characterized by numerical modeling, probes, and spectral measurements. The proposed experimental approach is promising for simulating the entry conditions of planetary mission vehicles into different atmospheres. Full article

(This article belongs to the Special Issue Feature Papers in Plasma Sciences 2025)

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14 pages, 4074 KiB

Open AccessArticle

Characterization of Tungsten Sputtering Processes in a Capacitively Coupled Argon Plasma

by Espedito Vassallo, Miriam Saleh, Matteo Pedroni, Anna Cremona and Dario Ripamonti

Plasma 2025, 8(1), 8; https://doi.org/10.3390/plasma8010008 - 28 Feb 2025

Viewed by 284

Abstract

A capacitively coupled radio-frequency argon plasma, used for tungsten sputtering deposition, is characterized using Langmuir probe measurements. Druyvesteyn’s method is used to evaluate plasma parameters through the integral of the Electron Energy Distribution Function (EEDF). In the pressure range analyzed (0.6–10 Pa), the [...] Read more.

A capacitively coupled radio-frequency argon plasma, used for tungsten sputtering deposition, is characterized using Langmuir probe measurements. Druyvesteyn’s method is used to evaluate plasma parameters through the integral of the Electron Energy Distribution Function (EEDF). In the pressure range analyzed (0.6–10 Pa), the obtained distributions are not Maxwellian, which suggests some depletion of electrons with higher energies. The obtained plasma parameters are compared with those derived from the graphical method. The electron temperature obtained via the graphical method is always lower than the effective temperatures derived from EEDFs, and vice versa, the electron density is overestimated by the graphical method. Optical Emission Spectroscopy is used to monitor the atoms sputtered in the plasma process. The behavior of excited species correlates with the plasma parameters. Full article

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8 pages, 1370 KiB

Open AccessArticle

An Implicit Flux-Corrected Transport Algorithm Used for Gas Discharge Calculations

by Richard Morrow

Plasma 2025, 8(1), 7; https://doi.org/10.3390/plasma8010007 - 28 Feb 2025

Viewed by 273

Abstract

An implicit flux-corrected transport (FCT) and diffusion algorithm was developed and used in many gas discharge calculations. Such calculations require the use of a fine mesh where the electric field changes rapidly; that is, near electrodes or in a streamer front. If diffusion [...] Read more.

An implicit flux-corrected transport (FCT) and diffusion algorithm was developed and used in many gas discharge calculations. Such calculations require the use of a fine mesh where the electric field changes rapidly; that is, near electrodes or in a streamer front. If diffusion is included using an explicit method, then the von Neumann stability condition severely limits the time-step that can be used; however, this limitation does not apply to implicit methods. Further, for gas discharge calculations including space-charge effects, it is necessary to solve the continuity equations with no negative number densities nor point-by-point oscillation in the number density. This is because the electron number densities are finely balanced with the ion number densities to determine the space-charge distribution and hence the electric field which drives the motion of the particles. An efficient way to solve the particle transport equation, with the required properties, is to use FCT. The most accurate form of FCT developed by the author is implicit fourth-order FCT; hence, the method presented incorporates implicit diffusion into the implicit fourth-order FCT scheme to produce a robust algorithm that has been successfully used in many calculations. Full article

(This article belongs to the Special Issue Recent Advances of Dielectric Barrier Discharges)

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18 pages, 1488 KiB

Open AccessArticle

Plasma-Induced Abatement of Tar from Syngas Produced in Municipal Waste Gasification: Thermodynamic Modeling with Experimental Validation

by Mobish A. Shaji, Francis Eboh, Alexander Rabinovich, Liran Dor and Alexander Fridman

Plasma 2025, 8(1), 6; https://doi.org/10.3390/plasma8010006 - 16 Feb 2025

Viewed by 423

Abstract

Municipal waste gasification presents a promising avenue to extract useful energy from waste through syngas. This technology’s application is limited by tar formation (long-chain hydrocarbons), which can decrease energy conversion efficiency and applications of raw syngas. Non-thermal plasma-based tar degradation is a simple [...] Read more.

Municipal waste gasification presents a promising avenue to extract useful energy from waste through syngas. This technology’s application is limited by tar formation (long-chain hydrocarbons), which can decrease energy conversion efficiency and applications of raw syngas. Non-thermal plasma-based tar degradation is a simple and cost-effective alternative to existing thermal and catalytic tar mitigation methods. While plasma stimulates tar reformation reactions like steam reformation, there are thermodynamic energy requirements associated with these endothermic processes. Determining thermodynamic energy requirements and the equilibrium composition of products during tar reformation can aid with the proper optimization of the treatment process. In the present study, thermodynamic modeling and experimental validation are conducted to study energy requirements and product formation during the plasma-assisted steam reformation of tar present in raw syngas with an inlet temperature of 300 °C and 30% moisture content. The thermodynamic study evaluated the effect of adding air into the system (to increase the temperature by oxidizing a portion of raw syngas). Results show that up to 75% of energy requirement can be brought down by adding up to 30% air; experimental validation using gliding arc discharge with 30% air addition agrees with the thermodynamic model finding. The thermodynamic model predicted an increase in H₂ and CO concentration with the degradation of tar, but experimental validation reported a reduction in H₂ and CO concentration with the degradation of tar, as syngas was consumed to increase the temperature to support oxidation, owing to the low temperature (300 °C) and significant moisture presence (~30%) of raw syngas analyzed in this study. Full article

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25 pages, 7953 KiB

Open AccessArticle

A Study of Particle Heating and Oxidation Protection in a Modified Internally Injected Ar–H₂ Atmospheric Plasma Spraying Torch

by Mahrukh Mahrukh, Sen-Hui Liu, Li Zhang, Sohail Husnain, Cheng-Chung Yang, Xiao-Tao Luo and Chang-Jiu Li

Plasma 2025, 8(1), 5; https://doi.org/10.3390/plasma8010005 - 13 Feb 2025

Viewed by 594

Abstract

This study employs computational fluid dynamics (CFD) to analyze the in-flight dynamics of particles in an Ar–H₂ atmospheric plasma spray (APS) torch with a modified diverging-type nozzle. The focus is on optimizing injection parameters—plasma gas flow rates, input power, and carrier gas [...] Read more.

This study employs computational fluid dynamics (CFD) to analyze the in-flight dynamics of particles in an Ar–H₂ atmospheric plasma spray (APS) torch with a modified diverging-type nozzle. The focus is on optimizing injection parameters—plasma gas flow rates, input power, and carrier gas flow rates—to enhance coating microstructure and deposition efficiency by achieving superheated molten metal droplets. Using a discrete phase model, the heat and momentum transfer of Ni/Al/C (2 wt.% diamond) composite powders (30–110 µm) within the plasma jet were simulated. Results show that particle characteristics, such as temperature and oxidation, can be controlled by adjusting plasma jet temperature (T∞) and velocity (U∞). Smaller particles heat faster, reaching higher temperatures with increased evaporation and oxidation rates. The modified nozzle enables Ni-based alloy particles to reach ~2500 °C, reducing oxygen inclusion in the plasma jet core. This setup allows for the control of the onset of carbon and oxygen reactions, wherein carbon serves as a sacrificial element, protecting the base alloy elements (such as aluminum) from excessive oxidation. Full article

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13 pages, 1929 KiB

Open AccessArticle

An Approach to Nuclear Fusion Utilizing the Dynamics of High-Density Electrons and Neutrals, Part I

by Alfred YiuFai Wong and Chun-Ching Shih

Plasma 2025, 8(1), 4; https://doi.org/10.3390/plasma8010004 - 31 Jan 2025

Viewed by 617

Abstract

An approach to achieve nuclear fusion utilizing the formation of high densities of electrons and neutrals is described. The abundance of low energy free electrons produces intense electric fields that reduce the Coulomb barrier in nuclear fusion. Meanwhile, high-density rotating neutrals provide high [...] Read more.

An approach to achieve nuclear fusion utilizing the formation of high densities of electrons and neutrals is described. The abundance of low energy free electrons produces intense electric fields that reduce the Coulomb barrier in nuclear fusion. Meanwhile, high-density rotating neutrals provide high centrifugal forces to achieve the extreme pressure gradients of electrons and consequent negative electric fields to reduce the ion repulsive Coulombic fields. These high-density neutrals also provide better stability and higher reaction rates. Ion–neutral coupling is responsible for the control of neutral dynamics. Since high-frequency excitations favor the generation of free electrons, pulsed operations are recommended to achieve fusion with higher gains. Full article

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30 pages, 11200 KiB

Open AccessArticle

Shock Waves in Ion-Beam-Depleted Spin-Polarized Quantum Plasma with Ionic Pressure Anisotropy

by Manoj K. Deka, Balaram Pradhan, Apul N. Dev, Deepsikha Mahanta, Jalil Manafian and Khaled H. Mahmoud

Plasma 2025, 8(1), 3; https://doi.org/10.3390/plasma8010003 - 8 Jan 2025

Viewed by 698

Abstract

In this study, the effects of pressure anisotropy and viscosity on the propagation of shock waves in spin-polarized degenerate quantum magnetoplasma are studied under the influence of the streaming energy of ion beams. The effects of different suitable plasma parameters on the shock [...] Read more.

In this study, the effects of pressure anisotropy and viscosity on the propagation of shock waves in spin-polarized degenerate quantum magnetoplasma are studied under the influence of the streaming energy of ion beams. The effects of different suitable plasma parameters on the shock wave’s potential profile are studied using the steady state solution of the Zakharov–Kuznetsov–Burgers (Z–K–B) equation, as well as the numerical simulation of the governing non-linear Z–K–B equation. First-order analysis of the non-linear wave propagation depicted a new beam-induced stable mode whose Mach number may be subsonic or supersonic depending on the anisotropic pressure combination in the presence of different spin density polarization ratios. This is the first observation of this new beam-induced stable mode in ion beam plasma, apart from the other existing modes of ion beam plasma systems, namely, the fast beam mode, the slow beam mode, the inherent ion acoustic mode, and the coupled mode, which also has unique propagation characteristics compared to the other modes. The spin density polarization ratio of spin-up and spin-down electrons have an unprecedented effect on the polarity and the direction of propagation of different shock wave modes in such plasma systems. Apart from the spin effect, anisotropic pressure combinations, as well as the viscosity of ions and ion beams, also play an outstanding role in controlling the nature of propagation of shock waves, especially in the newly detected beam-induced stable mode, and depending on the viscosity parameters of ions and ion beams, both oscillatory and monotonic shock waves can propagate in such plasma. Full article

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7 pages, 1387 KiB

Open AccessArticle

Effects of Spiralling Trajectories on White Dwarf Spectra: Remarks on Different Calculations

by Spiros Alexiou

Plasma 2025, 8(1), 2; https://doi.org/10.3390/plasma8010002 - 3 Jan 2025

Viewed by 501

Abstract

The purpose of this paper is to address conflicting results regarding a simple criterion that has been proposed as decisive in determining whether accounting for spiralling electron trajectories increases or decreases the widths of hydrogen lines in a parameter range relevant to the [...] Read more.

The purpose of this paper is to address conflicting results regarding a simple criterion that has been proposed as decisive in determining whether accounting for spiralling electron trajectories increases or decreases the widths of hydrogen lines in a parameter range relevant to the spectral lines of white dwarfs. We analyse the claims in detail and also provide explicit calculations. It is shown that the recent attempts to justify a simple theory are erroneous and miss important physics. Full article

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8 pages, 1714 KiB

Open AccessArticle

Longitudinally Resolved Terahertz Radiation Characteristics Along Two-Color Filament in Air

by Juan Long, Tiejun Wang, Fukang Yin, Yaoxiang Liu, Yingxia Wei, Chengpu Liu and Yuxin Leng

Plasma 2025, 8(1), 1; https://doi.org/10.3390/plasma8010001 - 29 Dec 2024

Viewed by 723

Abstract

The evolution of the THz waveform generated from the two-color air filament was experimentally investigated by moving an iris along the plasma channel. By taking the differentiation of the measured THz waveforms, the local longitudinally resolved THz waves along a 54 mm-long filament [...] Read more.

The evolution of the THz waveform generated from the two-color air filament was experimentally investigated by moving an iris along the plasma channel. By taking the differentiation of the measured THz waveforms, the local longitudinally resolved THz waves along a 54 mm-long filament were obtained. The local THz pulse underwent periodic phase shifts. A theoretical deduction indicates that the phase shifts are mainly caused by the dispersion in the plasma channel which plays a dominant role in the evolution of the local THz waveforms. Full article

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Plasma, Volume 8, Issue 1 (March 2025) – 12 articles

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