Editor’s Choice Articles

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to readers, or important in the respective research area. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

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41 pages, 4900 KiB  
Review
A Comprehensive Review on Amplification of Laser Pulses via Stimulated Raman Scattering and Stimulated Brillouin Scattering in Plasmas
by Renju Miriam Cheriyan, Nikhil Varghese, R. S. Sooraj, Kavya H. Rao and N. Smijesh
Plasma 2022, 5(4), 499-539; https://doi.org/10.3390/plasma5040037 - 24 Nov 2022
Viewed by 1983
Abstract
The demand for high-intensity lasers has grown ever since the invention of lasers in 1960, owing to their applications in the fields of inertial confinement fusion, plasma-based relativistic particle accelerators, complex X-ray and gamma-ray sources, and laboratory astrophysics. To create such high-intensity lasers, [...] Read more.
The demand for high-intensity lasers has grown ever since the invention of lasers in 1960, owing to their applications in the fields of inertial confinement fusion, plasma-based relativistic particle accelerators, complex X-ray and gamma-ray sources, and laboratory astrophysics. To create such high-intensity lasers, free-running lasers were either Q-switched or mode-locked to increase the peak power to the gigawatt range. Later, chirped pulse amplification was developed, allowing the generation of peak power up to 1012 W. However, the next generation of high-intensity lasers might not be able to be driven by the solid-state technology alone as they are already operating close to their damage thresholds. In this scenario, concepts of amplification based on plasmas has the potential to revolutionize the laser industry, as plasma is already a broken-down medium, and hence does not pose any problems related to the damage thresholds. On the other hand, there are many other aspects that need to be addressed before developing technologies based on plasma-based amplification, and they are being investigated via theoretical and numerical methods and supported by several experiments. In this report, we review the prospects of employing plasma as the medium of amplification by utilising stimulated scattering techniques, such as the stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) techniques, to modulate high-power laser pulses, which would possibly be the key to the next generation of high-power lasers. The 1980s saw the commencement of research in this field, and possibilities of obtaining high peak powers were verified theoretically with the help of numerical calculations and simulations. The extent of amplification by these stimulated scattering schemes are limited by a number of instabilities such as forward Raman scattering (FRS), filamentation, etc., and here, magnetised plasma played an important role in counteracting these parasitic effects. The current research combines all these factors to experimentally realise a large-scale plasma-based amplifier, which can impact the high-energy laser industry in the near future. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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18 pages, 5327 KiB  
Article
Simple Parametric Model for Calculation of Lateral Electromagnetic Loads in Tokamaks at Asymmetric Vertical Displacement Events (AVDE)
by Sergey Sadakov, Fabio Villone, Guglielmo Rubinacci and Salvatore Ventre
Plasma 2022, 5(3), 306-323; https://doi.org/10.3390/plasma5030024 - 25 Jul 2022
Cited by 1 | Viewed by 1521
Abstract
This paper describes a family of relatively simple numerical models for calculation of asymmetric electromagnetic (EM) loads at all tokamak structures and coils at asymmetric vertical plasma displacement events (AVDE). Unlike currently known AVDE studies concentrated on plasma physics, these models have a [...] Read more.
This paper describes a family of relatively simple numerical models for calculation of asymmetric electromagnetic (EM) loads at all tokamak structures and coils at asymmetric vertical plasma displacement events (AVDE). Unlike currently known AVDE studies concentrated on plasma physics, these models have a practical purpose to calculate detailed time-dependent patterns of AVDE-induced EM loads everywhere in the tokamak. They are built to intrinsically assure good-enough EM load balance (opposite net forces and torques for the Vacuum Vessel and the Magnets with zero total for the entire tokamak), as needed for consequent simulation of the tokamak’s dynamic response to AVDE, as well as for the development of tokamak monitoring algorithms and tokamak simulators. To achieve these practical goals, the models work in a manner of parametric study. They do not intervene in details of plasma physics, but run at widely varied input assumptions on AVDE evolution and severity. Their outputs will fill a library of ready-for-use lateral EM loads for multiple variants of AVDE evolution and severity. The tokamak physics community can select any variant from the library, and engineers can pick ready-for-use AVDE loads. Investigated here, EM models represent one already known approach and one newly suggested. The latter attempts to reflect the helical pattern of halo currents in plasma and delivers richer outcomes and, thus, can be preferred as the single practical model for parametric calculations. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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11 pages, 3472 KiB  
Article
Chamber with Inverted Electrode Geometry for Measuring and Control of Ion Flux-Energy Distribution Functions
by Christian Schulze, He Li, Leonie Mohn, Martin Müller and Jan Benedikt
Plasma 2022, 5(3), 295-305; https://doi.org/10.3390/plasma5030023 - 23 Jun 2022
Cited by 2 | Viewed by 3514
Abstract
Measurements of ion flux-energy distribution functions at the high sheath potential of the driven electrode in a classical low-pressure asymmetric capacitively coupled plasma are technically difficult as the diagnostic device needs to float with the applied radio frequency voltage. Otherwise, the ion sampling [...] Read more.
Measurements of ion flux-energy distribution functions at the high sheath potential of the driven electrode in a classical low-pressure asymmetric capacitively coupled plasma are technically difficult as the diagnostic device needs to float with the applied radio frequency voltage. Otherwise, the ion sampling is disturbed by the varying electric field between the grounded device and the driven electrode. To circumvent such distortions, a low-pressure plasma chamber with inverted electrode geometry, where the larger electrode is driven and the smaller electrode is grounded, has been constructed and characterized. Measurements of the ion flux-energy distribution functions with an energy-selective mass spectrometer at the high sheath potential of the grounded electrode are presented for a variety of conditions and ions. The potential for suppressing low-energy ions from resonant charge transfer collisions in the sheath by the dilution of the working gas is demonstrated. Additionally, the setup is supplemented by an inductively coupled plasma that controls the plasma density and consequently the ion flux to the substrate while the radio frequency bias controls the ion energy. At high ion energies, metal ions are detected as a consequence of the ionization of sputtered electrode material. The proposed setup opens a way to study precisely the effects of ion treatment for a variety of substrates such as catalysts, polymers, or thin films. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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15 pages, 1241 KiB  
Article
Impact of Internal Faraday Shields on RF Driven Hydrogen Discharges
by David Rauner, Dominikus Zielke, Stefan Briefi and Ursel Fantz
Plasma 2022, 5(3), 280-294; https://doi.org/10.3390/plasma5030022 - 21 Jun 2022
Cited by 1 | Viewed by 3005
Abstract
At RF plasma reactors operated at high power, internal Faraday shields are required to shield dielectric vessel or windows from erosion due to isotropic heat and particle fluxes. By utilizing a flexible and diagnostically well-equipped laboratory setup, crucial effects that accompany the application [...] Read more.
At RF plasma reactors operated at high power, internal Faraday shields are required to shield dielectric vessel or windows from erosion due to isotropic heat and particle fluxes. By utilizing a flexible and diagnostically well-equipped laboratory setup, crucial effects that accompany the application of internal Faraday shields at low-pressure hydrogen (and deuterium) RF discharges are identified and quantified in this contribution. Both an inductively coupled plasma (ICP) utilizing a helical coil and a low-field helicon discharge applying a Nagoya-type III antenna at magnetic fields of up to 12 mT are investigated. Discharges are driven at 4 MHz and in the pressure range between 0.3 and 10 Pa while the impact of the Faraday shields on both the RF power transfer efficiency and spectroscopically determined bulk plasma parameters (electron density and temperature, atomic density) is investigated. Three main effects are identified and discussed: (i) due to the Faraday shield, the measured RF power transfer efficiency is globally reduced. This is mainly caused by increased power losses due to induced eddy currents within the electrostatic shield, as accompanying numerical simulations by a self-consistent fluid model demonstrate. (ii) The Faraday shield reduces the atomic hydrogen density in the plasma by one order of magnitude, as the recombination rate of atoms on the metallic (copper) surfaces of the shield is considerably higher compared to the dielectric quartz walls. (iii) The Faraday shield suppresses the transition of the low-field helicon setup to a wave heated regime at the present conditions. This is attributed to a change of boundary conditions for wave propagation, as the plasma is in direct contact with the conductive surfaces of the Faraday shield rather than being operated in a laterally fully dielectric vessel. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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11 pages, 1601 KiB  
Article
Pulsed Spherical Tokamak—A New Approach to Fusion Reactors
by Mikhail Gryaznevich, Valery A. Chuyanov and Yuichi Takase
Plasma 2022, 5(2), 247-257; https://doi.org/10.3390/plasma5020019 - 18 May 2022
Cited by 4 | Viewed by 2929
Abstract
Traditionally, spherical tokamak (ST) reactors are considered to operate in a steady state. This paper analyses the advantages of a pulsed ST reactor. The methodology developed for conventional tokamak (CT) reactors is used and it is shown that advantages of a pulsed operation [...] Read more.
Traditionally, spherical tokamak (ST) reactors are considered to operate in a steady state. This paper analyses the advantages of a pulsed ST reactor. The methodology developed for conventional tokamak (CT) reactors is used and it is shown that advantages of a pulsed operation are even more pronounced in an ST reactor because of its ability to operate at a higher beta, therefore achieving a higher bootstrap current fraction, which, together with a lower inductance, reduces requirements for magnetic flux from the central solenoid for the plasma current ramp-up and sustainment. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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10 pages, 2621 KiB  
Article
Poly (O-Aminophenol) Produced by Plasma Polymerization Has IR Spectrum Consistent with a Mixture of Quinoid & Keto Structures
by Natalie M. Stuart and Karl Sohlberg
Plasma 2022, 5(2), 196-205; https://doi.org/10.3390/plasma5020015 - 14 Apr 2022
Viewed by 1932
Abstract
A vibrational analysis of various poly(o-aminophenol) structures has been undertaken using first principles methods. It is shown that a mixture of quinoid and keto forms of poly(o-aminophenol) gives rise to a simulated spectrum that replicates the experimental infrared spectra of plasma-produced poly(o-aminophenol) better [...] Read more.
A vibrational analysis of various poly(o-aminophenol) structures has been undertaken using first principles methods. It is shown that a mixture of quinoid and keto forms of poly(o-aminophenol) gives rise to a simulated spectrum that replicates the experimental infrared spectra of plasma-produced poly(o-aminophenol) better than either the quinoid or keto poly(o-aminophenol) spectra alone. An unassigned peak in the spectrum is attributed to hydrogen bonding to the silica substrate. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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22 pages, 544 KiB  
Article
Adaptive Algorithm for the Generation of Superconfigurations in Hot-Plasma Opacity Calculations
by Jean-Christophe Pain
Plasma 2022, 5(1), 154-175; https://doi.org/10.3390/plasma5010012 - 04 Mar 2022
Cited by 4 | Viewed by 2392
Abstract
In hot plasmas, such as the ones encountered in astrophysics or laser-fusion studies, the number of ionic excited states may become huge, and the relevant electron configurations cannot always be handled individually. The Super Transition Array approach enables one to calculate the massic [...] Read more.
In hot plasmas, such as the ones encountered in astrophysics or laser-fusion studies, the number of ionic excited states may become huge, and the relevant electron configurations cannot always be handled individually. The Super Transition Array approach enables one to calculate the massic photo-absorption cross-section (or radiative opacity) in a statistical manner consisting of grouping configurations close in energy into superconfigurations. One of the main issues of the method, beyond its spectral resolution, is the determination of the most relevant configurations that contribute to opacity. In this work, we discuss different aspects of the generation of superconfigurations in a hot plasma and propose a new adaptive algorithm. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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19 pages, 8889 KiB  
Article
Application of Nitrogen Piezoelectric Direct Discharge for Increase in Surface Free Energy of Polymers
by Dariusz Korzec, Florian Hoppenthaler, Thomas Andres, Sophia Guentner and Simona Lerach
Plasma 2022, 5(1), 111-129; https://doi.org/10.3390/plasma5010009 - 09 Feb 2022
Cited by 4 | Viewed by 2722
Abstract
The subject of this study is the application of the piezoelectric direct discharge (PDD) operated with nitrogen to control the surface free energy (SFE) of polymers. The activation area, defined as the area of the zone reaching the SFE of 58 mN/m for [...] Read more.
The subject of this study is the application of the piezoelectric direct discharge (PDD) operated with nitrogen to control the surface free energy (SFE) of polymers. The activation area, defined as the area of the zone reaching the SFE of 58 mN/m for high-density polyethylene (HDPE) and poly (methyl methacrylate) (PMMA), is characterized. For HDPE, the activation area was characterized as a function of the distance from 1 to 16 mm, the nitrogen flow from 5 to 20 SLM, and the treatment time from 1 to 32 s. For larger distances, where SFE does not exceed 58 mN/m, the water contact angle is evaluated. The activation area for nitrogen PDD is typically a factor of 3 higher than for air with all other conditions the same. A maximum static activation area of 15 cm2 is reached. The plasma treatment of lens panels made of PMMA is presented as application example. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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16 pages, 2998 KiB  
Article
Plasma Co-Polymerization of HMDSO and Limonene with an Atmospheric Pressure Plasma Jet
by Gerrit Wulf, Bernd Mayer and Uwe Lommatzsch
Plasma 2022, 5(1), 44-59; https://doi.org/10.3390/plasma5010004 - 04 Jan 2022
Cited by 6 | Viewed by 3130
Abstract
Plasma co-polymers (co-p) were deposited with an atmospheric pressure plasma jet (APPJ) using a precursor mixture containing hexamethyldisiloxane (HMDSO) and limonene. A coating with fragments from both precursors and with siloxane, carbonyl and nitrogen functional groups was deposited. The flow rate of limonene [...] Read more.
Plasma co-polymers (co-p) were deposited with an atmospheric pressure plasma jet (APPJ) using a precursor mixture containing hexamethyldisiloxane (HMDSO) and limonene. A coating with fragments from both precursors and with siloxane, carbonyl and nitrogen functional groups was deposited. The flow rate of limonene was found to be an important parameter for plasma co-polymerization to tune the formation and structure of the functional groups. The FTIR and XPS analysis indicates that with increasing flow rate of limonene a higher proportion of carbon is bound to silicon. This is related to a stronger incorporation of fragments from limonene into the siloxane network and a weaker fragmentation of HMDSO. The formation mechanism of the nitroxide and carboxyl groups can be mainly differentiated into in-plasma and post-plasma reactions, respectively. Full article
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14 pages, 2015 KiB  
Article
The Effect of Excited Species on the Collisional Energy of Argon Inductively Coupled Plasmas: A Global Model Study
by Júlia Karnopp, Bernardo Magaldi, Julio Sagás and Rodrigo Pessoa
Plasma 2022, 5(1), 30-43; https://doi.org/10.3390/plasma5010003 - 04 Jan 2022
Cited by 2 | Viewed by 3869
Abstract
Global modeling of inductively coupled plasma (ICP) reactors is a powerful tool to investigate plasma parameters. In this article, the argon ICP global model is revisited to explore the effect of excited species on collisional energy through the study of different approaches to [...] Read more.
Global modeling of inductively coupled plasma (ICP) reactors is a powerful tool to investigate plasma parameters. In this article, the argon ICP global model is revisited to explore the effect of excited species on collisional energy through the study of different approaches to particle and energy balance equations. The collisional energy loss is much more sensitive to modifications in the balance equations than the electron temperature. According to the simulations, the multistep ionization reduces the collisional energy loss in all investigated reaction sets and the inclusion of heavy species reactions has negligible influence. The plasma parameters obtained, such as total energy loss and electron temperature, were compared with experimental results from the literature. The simulated cases that have more excited species and reactions in the energy balance are in better agreement with the experimental measurements. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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18 pages, 3883 KiB  
Article
The Nanosecond Impulsive Breakdown Characteristics of Air, N2 and CO2 in a Sub-mm Gap
by Ting Liu, Igor Timoshkin, Mark P. Wilson, Martin J. Given and Scott J. MacGregor
Plasma 2022, 5(1), 12-29; https://doi.org/10.3390/plasma5010002 - 30 Dec 2021
Cited by 5 | Viewed by 2587
Abstract
The present paper investigates the breakdown characteristics—breakdown voltage, with breakdown occurring on the rising edge of the applied HV impulses, and time to breakdown—for gases of significance that are present in the atmosphere: air, N2 and CO2. These breakdown characteristics [...] Read more.
The present paper investigates the breakdown characteristics—breakdown voltage, with breakdown occurring on the rising edge of the applied HV impulses, and time to breakdown—for gases of significance that are present in the atmosphere: air, N2 and CO2. These breakdown characteristics have been obtained in a 100 µm gap between an HV needle and plane ground electrode, when stressed with sub-µs impulses of both polarities, with a rise time up to ~50 ns. The scaling relationships between the reduced breakdown field Etip/N and the product of the gas number density and inter-electrode gap, Nd, were obtained for all tested gases over a wide range of Nd values, from ~1020 m−2 to ~1025 m−2. The breakdown field-time to breakdown characteristics obtained at different gas pressures are presented as scaling relationships of Etip/N, Nd, and Ntbr for each gas, and compared with data from the literature. Full article
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51 pages, 3297 KiB  
Review
Electrostatic Solitary Structures in Space Plasmas: Soliton Perspective
by Gurbax Singh Lakhina, Satyavir Singh, Rajith Rubia and Selvaraj Devanandhan
Plasma 2021, 4(4), 681-731; https://doi.org/10.3390/plasma4040035 - 21 Oct 2021
Cited by 20 | Viewed by 3411
Abstract
Occurrence of electrostatic solitary waves (ESWs) is ubiquitous in space plasmas, e.g., solar wind, Lunar wake and the planetary magnetospheres. Several theoretical models have been proposed to interpret the observed characteristics of the ESWs. These models can broadly be put into two main [...] Read more.
Occurrence of electrostatic solitary waves (ESWs) is ubiquitous in space plasmas, e.g., solar wind, Lunar wake and the planetary magnetospheres. Several theoretical models have been proposed to interpret the observed characteristics of the ESWs. These models can broadly be put into two main categories, namely, Bernstein–Green–Kruskal (BGK) modes/phase space holes models, and ion- and electron- acoustic solitons models. There has been a tendency in the space community to favor the models based on BGK modes/phase space holes. Only recently, the potential of soliton models to explain the characteristics of ESWs is being realized. The idea of this review is to present current understanding of the ion- and electron-acoustic solitons and double layers models in multi-component space plasmas. In these models, all the plasma species are considered fluids except the energetic electron component, which is governed by either a kappa distribution or a Maxwellian distribution. Further, these models consider the nonlinear electrostatic waves propagating parallel to the ambient magnetic field. The relationship between the space observations of ESWs and theoretical models is highlighted. Some specific applications of ion- and electron-acoustic solitons/double layers will be discussed by comparing the theoretical predictions with the observations of ESWs in space plasmas. It is shown that the ion- and electron-acoustic solitons/double layers models provide a plausible interpretation for the ESWs observed in space plasmas. Full article
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221 pages, 61334 KiB  
Review
Update on the Scientific Status of the Plasma Focus
by Sunil Auluck, Pavel Kubes, Marian Paduch, Marek J. Sadowski, Vyacheslav I. Krauz, Sing Lee, Leopoldo Soto, Marek Scholz, Ryszard Miklaszewski, Hellmut Schmidt, Alexander Blagoev, Maurizio Samuelli, Yeow Sing Seng, Stuart Victor Springham, Alireza Talebitaher, Cristian Pavez, Mohammad Akel, Seong Ling Yap, Rishi Verma, Karel Kolacek, Paul Lee Choon Keat, Rajdeep S. Rawat, Ali Abdou, Guixin Zhang and Tõnu Laasadd Show full author list remove Hide full author list
Plasma 2021, 4(3), 450-669; https://doi.org/10.3390/plasma4030033 - 07 Sep 2021
Cited by 29 | Viewed by 6178
Abstract
This paper is a sequel to the 1998 review paper “Scientific status of the Dense Plasma Focus” with 16 authors belonging to 16 nations, whose initiative led to the establishment of the International Center for Dense Magnetized Plasmas (ICDMP) in the year 2000. [...] Read more.
This paper is a sequel to the 1998 review paper “Scientific status of the Dense Plasma Focus” with 16 authors belonging to 16 nations, whose initiative led to the establishment of the International Center for Dense Magnetized Plasmas (ICDMP) in the year 2000. Its focus is on understanding the principal defining characteristic features of the plasma focus in the light of the developments that have taken place in the last 20 years, in terms of new facilities, diagnostics, models, and insights. Although it is too soon to proclaim with certainty what the plasma focus phenomenon is, the results available to date conclusively indicate what it is demonstrably not. The review looks at the experimental data, cross-correlated across multiple diagnostics and multiple devices, to delineate the contours of an emerging narrative that is fascinatingly different from the standard narrative, which has guided the consensus in the plasma focus community for several decades, without invalidating it. It raises a question mark over the Fundamental Premise of Controlled Fusion Research, namely, that any fusion reaction having the character of a beam-target process must necessarily be more inefficient than a thermonuclear process with a confined thermal plasma at a suitably high temperature. Open questions that need attention of researchers are highlighted. A future course of action is suggested that individual plasma focus laboratories could adopt in order to positively influence the future growth of research in this field, to the general benefit of not only the controlled fusion research community but also the world at large. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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12 pages, 774 KiB  
Article
Aerosol Charging with a Piezoelectric Plasma Generator
by Helmut Krasa, Mario A. Schriefl, Martin Kupper, Alexander Melischnig and Alexander Bergmann
Plasma 2021, 4(3), 377-388; https://doi.org/10.3390/plasma4030027 - 16 Jul 2021
Cited by 2 | Viewed by 3176
Abstract
A novel piezoelectric plasma generator developed by TDK Electronics GmbH & Co. OG, the CeraPlas®, was investigated for its feasibility as a charger for aerosol particles. The CeraPlas® charger was directly compared to a commercially available bipolar X-ray charger regarding [...] Read more.
A novel piezoelectric plasma generator developed by TDK Electronics GmbH & Co. OG, the CeraPlas®, was investigated for its feasibility as a charger for aerosol particles. The CeraPlas® charger was directly compared to a commercially available bipolar X-ray charger regarding its efficiency of charging atomized NaCl particles in a size range from 30 nm to 100 nm. First results show the ability of the CeraPlas® to perform bipolar aerosol charging with high reproducibility, and measurements of the charge distribution in the Nit product yielded about 1012 m−3 s for our experimental charging configuration. Unwanted generation of ozone was suppressed by a dedicated charging chamber and operation in N2 atmosphere. Full article
(This article belongs to the Special Issue Piezoelectric Direct Discharge)
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13 pages, 3810 KiB  
Article
RF and Microwave Ion Sources Study at Institute of Modern Physics
by Qian Y. Jin, Yu G. Liu, Yang Zhou, Qi Wu, Yao J. Zhai and Liang T. Sun
Plasma 2021, 4(2), 332-344; https://doi.org/10.3390/plasma4020022 - 06 Jun 2021
Cited by 9 | Viewed by 7511
Abstract
Intense ion beam production is of high importance for various versatile applications from accelerator injectors to secondary ion mass spectrometry (SIMS). For these purposes, different types of ion beams are needed and, accordingly, the optimum plasma to produce the desired ion beams. RF-type [...] Read more.
Intense ion beam production is of high importance for various versatile applications from accelerator injectors to secondary ion mass spectrometry (SIMS). For these purposes, different types of ion beams are needed and, accordingly, the optimum plasma to produce the desired ion beams. RF-type plasma features a simple structure, high plasma density and low plasma temperature, which is essential for negative ion beam production. A very compact RF-type ion source using a planar coil antenna has been developed at IMP for negative molecular oxygen ion beam production. In terms of high-intensity positive ion beam production, 2.45 GHz microwave power-excited plasma has been widely used. At IMP, we developed a 2.45 GHz plasma source with both ridged waveguide and coaxial antenna coupling schemes, tested successfully with intense beam production. Thanks to the plasma built with an external planar coil antenna, high O2 production efficiency has been achieved, i.e., up to 43%. With 2.45 GHz microwave plasma, the ridged waveguide can support a higher power coupling of high efficiency that leads to the production of intense hydrogen beams up to 90 emA, whereas the coaxial antenna is less efficient in power coupling to plasma but can lead to attractive ion source compactness, with a reasonable beam extraction of several emA. Full article
(This article belongs to the Special Issue Low Temperature Plasmas for Ion Beam Generation)
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15 pages, 814 KiB  
Article
An Investigation into the Approximations Used in Wave Packet Molecular Dynamics for the Study of Warm Dense Matter
by William A. Angermeier and Thomas G. White
Plasma 2021, 4(2), 294-308; https://doi.org/10.3390/plasma4020020 - 27 May 2021
Cited by 4 | Viewed by 3026
Abstract
Wave packet molecular dynamics (WPMD) has recently received a lot of attention as a computationally fast tool with which to study dynamical processes in warm dense matter beyond the Born–Oppenheimer approximation. These techniques, typically, employ many approximations to achieve computational efficiency while implementing [...] Read more.
Wave packet molecular dynamics (WPMD) has recently received a lot of attention as a computationally fast tool with which to study dynamical processes in warm dense matter beyond the Born–Oppenheimer approximation. These techniques, typically, employ many approximations to achieve computational efficiency while implementing semi-empirical scaling parameters to retain accuracy. We investigated three of the main approximations ubiquitous to WPMD: a restricted basis set, approximations to exchange, and the lack of correlation. We examined each of these approximations in regard to atomic and molecular hydrogen in addition to a dense hydrogen plasma. We found that the biggest improvement to WPMD comes from combining a two-Gaussian basis with a semi-empirical correction based on the valence-bond wave function. A single parameter scales this correction to match experimental pressures of dense hydrogen. Ultimately, we found that semi-empirical scaling parameters are necessary to correct for the main approximations in WPMD. However, reducing the scaling parameters for more ab-initio terms gives more accurate results and displays the underlying physics more readily. Full article
(This article belongs to the Special Issue Laser–Plasma Interactions and Applications)
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13 pages, 7521 KiB  
Article
Multi-Device Piezoelectric Direct Discharge for Large Area Plasma Treatment
by Dariusz Korzec, Florian Hoppenthaler, Anatoly Shestakov, Dominik Burger, Andrej Shapiro, Thomas Andres, Simona Lerach and Stefan Nettesheim
Plasma 2021, 4(2), 281-293; https://doi.org/10.3390/plasma4020019 - 25 May 2021
Cited by 7 | Viewed by 3444
Abstract
The piezoelectric cold plasma generators (PCPG) allow for production of the piezoelectric direct discharge (PDD), which is a kind of cold atmospheric pressure plasma (APP). The subjects of this study are different arrays of PCPGs for large-area treatment of planar substrates. Two limiting [...] Read more.
The piezoelectric cold plasma generators (PCPG) allow for production of the piezoelectric direct discharge (PDD), which is a kind of cold atmospheric pressure plasma (APP). The subjects of this study are different arrays of PCPGs for large-area treatment of planar substrates. Two limiting factors are crucial for design of such arrays: (i) the parasitic coupling between PCPGs resulting in minimum allowed distance between devices, and (ii) the homogeneity of large area treatment, requiring an overlap of the activation zones resulting from each PCPG. The first limitation is investigated by the use of electric measurements. The minimum distance for operation of 4 cm between two PCPGs is determined by measurement of the energy coupling from an active PCPG to a passive one. The capacitive probe is used to evaluate the interference between signals generated by two neighboring PCPGs. The second limitation is examined by activation image recording (AIR). Two application examples illustrate the compromising these two limiting factors: the treatment of large area planar substrates by PCPG array, and the pretreatment of silicon wafers with an array of PCPG driven dielectric barrier discharges (DBD). Full article
(This article belongs to the Special Issue Piezoelectric Direct Discharge)
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13 pages, 1666 KiB  
Article
Delayed Discharge Bridging Two Sputtering Modes from Modulated Pulsed Power Magnetron Sputtering (MPPMS) to Deep Oscillation Magnetron Sputtering (DOMS)
by Masaomi Sanekata, Hiroshi Nishida, Tatsuya Watabe, Yuki Nakagomi, Yoshihiro Hirai, Nobuo Nishimiya, Masahide Tona, Hiroaki Yamamoto, Naoyuki Hirata, Keizo Tsukamoto, Keijiro Ohshimo, Fuminori Misaizu and Kiyokazu Fuke
Plasma 2021, 4(2), 239-251; https://doi.org/10.3390/plasma4020016 - 21 Apr 2021
Cited by 4 | Viewed by 2750
Abstract
Delayed discharges due to electrical breakdown are observed in modulated pulsed pow er magnetron sputtering (MPPMS) plasma of titanium. The delayed discharge, which is remarkable with decreasing argon gas pressure, transforms the discharge current waveform from a standard modulated pulsed discharge current waveform [...] Read more.
Delayed discharges due to electrical breakdown are observed in modulated pulsed pow er magnetron sputtering (MPPMS) plasma of titanium. The delayed discharge, which is remarkable with decreasing argon gas pressure, transforms the discharge current waveform from a standard modulated pulsed discharge current waveform to a comb-like discharge current waveform consisting of several pulses with high power. In addition, the delay times, consisting of statistical times and formative times in the delayed MPPMS discharges, are experimentally measured with the help of Laue plot analysis. The pressure dependence of delay times observed indicates that the delayed discharge behavior matches the breakdown characteristics well. In the present study, the delayed discharge dynamics of the comb-like discharge current waveform, which can be the origin of deep oscillation magnetron sputtering, are investigated based on measurement of the delay times and the characteristics of discharge current waveforms. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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18 pages, 6519 KiB  
Article
Demonstration of Dynamics of Nanosecond Discharge in Liquid Water Using Four-Channel Time-Resolved ICCD Microscopy
by Václav Prukner, Jiří Schmidt, Petr Hoffer and Milan Šimek
Plasma 2021, 4(1), 183-200; https://doi.org/10.3390/plasma4010011 - 16 Mar 2021
Cited by 9 | Viewed by 2614
Abstract
The microscopic physical mechanisms of micro-discharges produced in liquid waters by nanosecond high-voltage pulses are quite complex phenomena, and relevant coherent experimentally supported theoretical descriptions are yet to be provided. In this study, by combining a long-distance microscope with a four-channel image splitter [...] Read more.
The microscopic physical mechanisms of micro-discharges produced in liquid waters by nanosecond high-voltage pulses are quite complex phenomena, and relevant coherent experimentally supported theoretical descriptions are yet to be provided. In this study, by combining a long-distance microscope with a four-channel image splitter fitted with four synchronised intensified charge-coupled device detectors, we obtained and analysed sequences of microscopic discharge images acquired with sub-nanosecond temporal resolution during a single event. We tracked luminous filaments either through monochromatic images at two specific wavelengths (532 and 656 nm) or through broadband integrated UV–vis–near infrared (NIR) discharge emission. An analysis of the sequences of images capturing discharge filaments in subsequent time windows facilitated the tracking of movement of the luminous fronts during their expansion. The velocity of expansion progressively decreased from the maximum of ~2.3 × 105 m/s observed close to the anode pin until the propagation stopped due to the drop in the anode potential. We demonstrate the basic features characterising the development of the luminous discharge filaments. Our study provides an important insight into the dynamics of micro-discharges during the primary and successive reflected high-voltage pulses in de-ionised water. Full article
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11 pages, 8991 KiB  
Article
Operation of Large RF Driven Negative Ion Sources for Fusion at Pressures below 0.3 Pa
by Dirk Wünderlich, Rudi Riedl, Markus Fröschle, Ursel Fantz and Bernd Heinemann
Plasma 2021, 4(1), 172-182; https://doi.org/10.3390/plasma4010010 - 09 Mar 2021
Cited by 6 | Viewed by 2902
Abstract
The large (size: 1 m × 2 m) radio frequency (RF) driven negative ion sources for the neutral beam heating (NBI) systems of the future fusion experiment ITER will be operated at a low filling pressure of 0.3 Pa, in hydrogen or in [...] Read more.
The large (size: 1 m × 2 m) radio frequency (RF) driven negative ion sources for the neutral beam heating (NBI) systems of the future fusion experiment ITER will be operated at a low filling pressure of 0.3 Pa, in hydrogen or in deuterium. The plasma will be generated by inductively coupling an RF power of up to 800 kW into the source volume. Under consideration for future neutral beam heating systems, like the one for the demonstration reactor DEMO, is an even lower filling pressure of 0.2 Pa. Together with the effect of neutral gas depletion, such low operational pressures can result in a neutral gas density below the limit required for sustaining the plasma. Systematic investigations on the low-pressure operational limit of the half-ITER-size negative ion source of the ELISE (Extraction from a Large Ion Source Experiment) test facility were performed, demonstrating that operation is possible below 0.2 Pa. A strong correlation of the lower pressure limit on the magnetic filter field topology is found. Depending on the field topology, operation close to the low-pressure limit is accompanied by strong plasma oscillations in the kHz range. Full article
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27 pages, 2762 KiB  
Review
Progresses on the Use of Two-Photon Absorption Laser Induced Fluorescence (TALIF) Diagnostics for Measuring Absolute Atomic Densities in Plasmas and Flames
by Kristaq Gazeli, Guillaume Lombardi, Xavier Aubert, Corinne Y. Duluard, Swaminathan Prasanna and Khaled Hassouni
Plasma 2021, 4(1), 145-171; https://doi.org/10.3390/plasma4010009 - 04 Mar 2021
Cited by 21 | Viewed by 4788
Abstract
Recent developments in plasma science and technology have opened new areas of research both for fundamental purposes (e.g., description of key physical phenomena involved in laboratory plasmas) and novel applications (material synthesis, microelectronics, thin film deposition, biomedicine, environment, flow control, to name a [...] Read more.
Recent developments in plasma science and technology have opened new areas of research both for fundamental purposes (e.g., description of key physical phenomena involved in laboratory plasmas) and novel applications (material synthesis, microelectronics, thin film deposition, biomedicine, environment, flow control, to name a few). With the increasing availability of advanced optical diagnostics (fast framing imaging, gas flow visualization, emission/absorption spectroscopy, etc.), a better understanding of the physicochemical processes taking place in different electrical discharges has been achieved. In this direction, the implementation of fast (ns) and ultrafast (ps and fs) lasers has been essential for the precise determination of the electron density and temperature, the axial and radial gradients of electric fields, the gas temperature, and the absolute density of ground-state reactive atoms and molecules in non-equilibrium plasmas. For those species, the use of laser-based spectroscopy has led to their in situ quantification with high temporal and spatial resolution, with excellent sensitivity. The present review is dedicated to the advances of two-photon absorption laser induced fluorescence (TALIF) techniques for the measurement of reactive species densities (particularly atoms such as N, H and O) in a wide range of pressures in plasmas and flames. The requirements for the appropriate implementation of TALIF techniques as well as their fundamental principles are presented based on representative published works. The limitations on the density determination imposed by different factors are also discussed. These may refer to the increasing pressure of the probed medium (leading to a significant collisional quenching of excited states), and other issues originating in the high instantaneous power density of the lasers used (such as photodissociation, amplified stimulated emission, and photoionization, resulting to the saturation of the optical transition of interest). Full article
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37 pages, 6312 KiB  
Review
Linear and Nonlinear Plasma Processes in Ionospheric HF Heating
by Spencer Kuo
Plasma 2021, 4(1), 108-144; https://doi.org/10.3390/plasma4010008 - 23 Feb 2021
Cited by 1 | Viewed by 2822
Abstract
Featured observations of high frequency (HF) heating experiments are first introduced; the uniqueness of each observation is presented; the likely cause and physical process of each observed phenomenon instigated by the HF heating are discussed. A special point in the observations, revealed through [...] Read more.
Featured observations of high frequency (HF) heating experiments are first introduced; the uniqueness of each observation is presented; the likely cause and physical process of each observed phenomenon instigated by the HF heating are discussed. A special point in the observations, revealed through the ionograms, is the competition between the Langmuir parametric instability and upper hybrid parametric instability excited in the heating experiments and the impact of the natural cusp at foE (the peak plasma frequency of the ionospheric E region) on the competition. The ionograms also infer the generation of Langmuir and upper hybrid cavitons. Ray tracing theory is formulated. With and without the appearance of large-scale field-aligned density irregularities in the background ionosphere, ray trajectories of the ordinary mode (O-mode) and extraordinary mode (X-mode) sounding pulses are calculated numerically. The results explain the artificial Spread-F recorded by the digisondes in the heating experiments. Parametric instabilities, which are the directly relevant processes to achieve effective heating of the ionospheric F region, are formulated and analyzed. The threshold fields and growth rates of Langmuir and upper hybrid parametric instabilities are derived as the theoretical basis of many radar observations and electron-plasma wave interactions. Harmonic cyclotron resonance interaction processes between electrons and upper hybrid waves are introduced. Formulation and analysis are presented. The numerical results show that ultra-energetic electrons are generated. These electrons enhance airglow at 777.4 nm as well as cause ionization. Physical processes leading to the generation of artificial ionization layers are discussed. The nonlinear Schrodinger equation governing the nonlinear evolution of Langmuir waves and upper hybrid waves are derived and solved. The nonlinear periodic and solitary solutions of the equations are obtained. The localized Langmuir and upper hybrid waves generated by the HF heater form cavitons near the HF reflection layer and near the upper hybrid resonance layer, which induce bumps in the virtual height spread of the ionogram trace similar to that induced by the density cusp at E-F1 transition layer; the down-going Langmuir waves and upper hybrid waves evolve into nonlinear periodic waves propagating along the magnetic field, which backscatter incoherently the sounding pulses to cause downward virtual height spread. Full article
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14 pages, 759 KiB  
Article
Negative Hydrogen and Deuterium Ion Density in a Low Pressure Plasma in Front of a Converter Surface at Different Work Functions
by Sofia Cristofaro, Roland Friedl and Ursel Fantz
Plasma 2021, 4(1), 94-107; https://doi.org/10.3390/plasma4010007 - 18 Feb 2021
Cited by 8 | Viewed by 2768
Abstract
Negative ion sources of neutral beam injection (NBI) systems for future fusion devices like ITER (“The Way” in Latin) rely on the surface conversion of hydrogen (or deuterium) atoms and positive ions to negative ions in an inductively coupled plasma (ICP). The efficiency [...] Read more.
Negative ion sources of neutral beam injection (NBI) systems for future fusion devices like ITER (“The Way” in Latin) rely on the surface conversion of hydrogen (or deuterium) atoms and positive ions to negative ions in an inductively coupled plasma (ICP). The efficiency of this process depends on the work function of the converter surface. By introducing caesium into the ion source the work function decreases, enhancing the negative ion yield. In order to study the isotope effect on the negative ion density at different work functions, fundamental investigations are performed in a planar ICP laboratory experiment where the work function and the negative ion density in front of a sample can be simultaneously and absolutely determined. For work functions above 2.7 eV, the main contribution to the negative hydrogen ion density is solely due to volume formation, which can be modeled via the rate balance model YACORA H, while below 2.7 eV the surface conversion become significant and the negative ion density increases. For a work function of 2.1 eV (bulk Cs), the H density increases by at least a factor of 2.8 with respect to a non-caesiated surface. With a deuterium plasma, the D density measured at 2.1 eV is a factor of 2.5 higher with respect to a non-caesiated surface, reaching densities of surface produced negative ions comparable to the hydrogen case. Full article
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23 pages, 865 KiB  
Review
Super Transition Arrays: A Tool for Studying Spectral Properties of Hot Plasmas
by Jean-Christophe Pain
Plasma 2021, 4(1), 42-64; https://doi.org/10.3390/plasma4010002 - 08 Jan 2021
Cited by 8 | Viewed by 2161
Abstract
For the theoretical study of X and extreme-UV spectra of ions in plasmas, quantum mechanics brings more detailed results than statistical physics. However, it is impossible to handle individually the billions of levels that must be taken into account in order to properly [...] Read more.
For the theoretical study of X and extreme-UV spectra of ions in plasmas, quantum mechanics brings more detailed results than statistical physics. However, it is impossible to handle individually the billions of levels that must be taken into account in order to properly describe hot plasmas. Such levels can be gathered into electronic configurations or superconfigurations (groups of configurations) and the corresponding calculations rely on appropriate statistical methods, for local or non-local thermodynamic equilibrium plasmas. In this article we present the basic principles of the Super-Transition-Array approach as well as its practical implementation. During the last decades, calculations performed with the SCO code (Superconfiguration Code for Opacity) have been compared to opacity measurements. The code includes static screening of ions by plasma and is well suited for studying plasma density effects (for example pressure ionization) on opacity and equation of state. The recently developed SCO-RCG code (Superconfiguration Code for Opacity combined with Robert Cowan’s “G” subroutine) combines statistical methods from SCO and fine-structure (detailed-level-accounting) calculations using subroutine RCG from Cowan’s code. SCO-RCG enables us to obtain very detailed spectra and to significantly improve the interpretation of experimental spectra. The Super-Transition-Array formalism is still the cornerstone of several opacity codes, and new ideas are emerging, such as the Configurationally Resolved-Super-Transition-Array approach or the extension of the Partially Resolved-Transition-Array concept to the superconfiguration method. Full article
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41 pages, 20558 KiB  
Review
Piezoelectric Direct Discharge: Devices and Applications
by Dariusz Korzec, Florian Hoppenthaler and Stefan Nettesheim
Plasma 2021, 4(1), 1-41; https://doi.org/10.3390/plasma4010001 - 28 Dec 2020
Cited by 26 | Viewed by 8753
Abstract
The piezoelectric direct discharge (PDD) is a comparatively new type of atmospheric pressure gaseous discharge for production of cold plasma. The generation of such discharge is possible using the piezoelectric cold plasma generator (PCPG) which comprises the resonant piezoelectric transformer (RPT) with voltage [...] Read more.
The piezoelectric direct discharge (PDD) is a comparatively new type of atmospheric pressure gaseous discharge for production of cold plasma. The generation of such discharge is possible using the piezoelectric cold plasma generator (PCPG) which comprises the resonant piezoelectric transformer (RPT) with voltage transformation ratio of more than 1000, allowing for reaching the output voltage >10 kV at low input voltage, typically below 25 V. As ionization gas for the PDD, either air or various gas mixtures are used. Despite some similarities with corona discharge and dielectric barrier discharge, the ignition of micro-discharges directly at the ceramic surface makes PDD unique in its physics and application potential. The PDD is used directly, in open discharge structures, mainly for treatment of electrically nonconducting surfaces. It is also applied as a plasma bridge to bias different excitation electrodes, applicable for a broad range of substrate materials. In this review, the most important architectures of the PDD based discharges are presented. The operation principle, the main operational characteristics and the example applications, exploiting the specific properties of the discharge configurations, are discussed. Due to the moderate power achievable by PCPG, of typically less than 10 W, the focus of this review is on applications involving thermally sensitive materials, including food, organic tissues, and liquids. Full article
(This article belongs to the Special Issue Piezoelectric Direct Discharge)
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8 pages, 1272 KiB  
Article
Plasma Activation as a Powerful Tool for Selective Modification of Cellulose Fibers towards Biomedical Applications
by Olivia Mauger, Sophia Westphal, Stefanie Klöpzig, Anne Krüger-Genge, Werner Müller, Joachim Storsberg and Jörg Bohrisch
Plasma 2020, 3(4), 196-203; https://doi.org/10.3390/plasma3040015 - 16 Nov 2020
Cited by 6 | Viewed by 2479
Abstract
Cellulosic substrates are known for their biocompatibility, non-cytotoxicity, hypoallergenicity and sterilizability. It is therefore desirable to have a bundle of methods to equip them with tailored properties such as affinity profiles for various applications. In the case of highly swelling materials such as [...] Read more.
Cellulosic substrates are known for their biocompatibility, non-cytotoxicity, hypoallergenicity and sterilizability. It is therefore desirable to have a bundle of methods to equip them with tailored properties such as affinity profiles for various applications. In the case of highly swelling materials such as cellulose sponges, “dry” functionalization using plasma activation is the method of choice. The purpose of the study was to adapt low-pressure plasma technology for targeted cellulose modification. Using plasma (pre-) treatment combined with gaseous reactants like O2, ethylene oxide or silane, three different cellulose modifications were obtained and characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Swelling measurements and bacterial adhesion tests revealed distinctive material properties compared to educt. The development of these non-aqueous methods demonstrated an effective procedural route towards modified cellulosic materials for usage in wound dressing, micro patterned assays or bacterial filtration. Full article
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9 pages, 3434 KiB  
Article
Polymerization of Solid-State Aminophenol to Polyaniline Derivative Using a Dielectric Barrier Discharge Plasma
by Ketao Chen, Meijuan Cao, Eileen Feng, Karl Sohlberg and Hai-Feng Ji
Plasma 2020, 3(4), 187-195; https://doi.org/10.3390/plasma3040014 - 30 Oct 2020
Cited by 3 | Viewed by 2784
Abstract
We present a method to prepare polyaminophenol from solid-state aminophenol monomers using atmospheric dielectric barrier discharge (DBD) plasma. The polymerizations of o-aminophenol and m-aminophenol are studied. The polymers were analyzed via Fourier-Transform inferred spectroscopy (FTIR) and ultraviolet-visible (UV-vis) spectroscopy. The kinetics [...] Read more.
We present a method to prepare polyaminophenol from solid-state aminophenol monomers using atmospheric dielectric barrier discharge (DBD) plasma. The polymerizations of o-aminophenol and m-aminophenol are studied. The polymers were analyzed via Fourier-Transform inferred spectroscopy (FTIR) and ultraviolet-visible (UV-vis) spectroscopy. The kinetics of the polymerization reactions were investigated by using UV-vis and the polymerization was found to be first-order for both o-aminophenol and m-aminophenol. The resulting polymer film exhibits a conductivity of 1.0 × 10−5 S/m for poly-o-aminophenol (PoAP) and 2.3 × 10−5 S/m for poly-m-aminophenol (PmAP), which are two orders more conductive than undoped (~10−7 S/m) polyaniline (PANI), The PoAP has a quinoid structure and the PmAP has an open ring keto-derivative structure. The process provides a simple method of preparing conductive polyaminophenol films. Full article
(This article belongs to the Special Issue Dielectric Barrier Discharges)
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14 pages, 323 KiB  
Article
Two-Parametric, Mathematically Undisclosed Solitary Electron Holes and Their Evolution Equation
by Hans Schamel
Plasma 2020, 3(4), 166-179; https://doi.org/10.3390/plasma3040012 - 30 Sep 2020
Cited by 5 | Viewed by 2488
Abstract
The examination of the mutual influence of the two main trapping scenarios, which are characterized by B and D and which in isolation yield the known sech4 (D=0) and Gaussian (B=0) electron holes, show [...] Read more.
The examination of the mutual influence of the two main trapping scenarios, which are characterized by B and D and which in isolation yield the known sech4 (D=0) and Gaussian (B=0) electron holes, show generalized, two-parametric solitary wave solutions. This increases the variety of hole solutions considerably beyond the two cases previously discussed, but at the expense of their mathematical disclosure, since ϕ(x), the electrical wave potential, can no longer be expressed analytically by known functions. Therefore, they belong to a variety with a partially hidden mathematical background, a hitherto unexplored world of structure formation, the origin of which is the chaotic individual particle dynamics at resonance in the coherent wave particle interaction. A third trapping scenario Γ, being independent of (B, D) and representing the perturbative trapping scenarios in lowest order, provides a broad, continuous band of associated phase velocities v0. For structures propagating near CSEA=1.307, the slowelectronacousticspeed, a Generalized Schamel equation is derived: φτ+[AB158φ+Dlnφ]φxφxxx=0, which governs their evolution. A is associated with the phase speed and τ:=CSEAt and φ:=ϕ/ψ0 are the renormalized time and electric potential, respectively, where ψ is the amplitude of the structure. Full article
36 pages, 1229 KiB  
Article
Ion Acceleration in Multi-Fluid Plasma: Including Charge Separation Induced Electric Field Effects in Supersonic Wave Layers
by Ross Burrows
Plasma 2020, 3(3), 117-152; https://doi.org/10.3390/plasma3030010 - 21 Aug 2020
Viewed by 2487
Abstract
The need to understand the process by which particles, including solar wind and coronal ions as well as pickup ions, are accelerated to high energies (ultimately to become anomalous cosmic rays) motivate a multi-fluid shock wave model which includes kinetic effects (e.g., ion [...] Read more.
The need to understand the process by which particles, including solar wind and coronal ions as well as pickup ions, are accelerated to high energies (ultimately to become anomalous cosmic rays) motivate a multi-fluid shock wave model which includes kinetic effects (e.g., ion acceleration) in an electromagnetically self-consistent framework. Particle reflection at the cross-shock potential leads to ion acceleration in the motional electric field and thus anisotropic heating and pressure in the shock layer, with important consequences for the multi-fluid dynamics. This motivates development of a multi-fluid model of solar wind electrons and ions treated as fluid, coupled self-consistently with a small population of ions (e.g., pickup ions) dynamically treated as individual particles. Consideration of both the time dependent and steady state regimes, indicate that such a multi-fluid approach is necessary for resolving the, Debye scale, particle reflecting cross-shock potential and subsequent dynamics. To study charge separation effects in narrow, supersonic wave layers we consider a reduction of the system to the steady state for cold ions and hot electrons and find two types of solitary waves inherent to the reduced two-fluid system in this limiting case. Full article
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11 pages, 1474 KiB  
Article
Electrical Properties of Reversed-Polarity Ball Plasmoid Discharges
by Scott E. Dubowsky, Amber N. Rose, Nick G. Glumac and Benjamin J. McCall
Plasma 2020, 3(3), 92-102; https://doi.org/10.3390/plasma3030008 - 29 Jun 2020
Cited by 2 | Viewed by 3220
Abstract
Ball plasmoid discharges are a unique type of atmospheric-pressure plasma discharge with a lifetime on the order of a hundred milliseconds without attachment to a power source. These discharges are generated by a moderate current pulse over the surface of an aqueous electrolyte, [...] Read more.
Ball plasmoid discharges are a unique type of atmospheric-pressure plasma discharge with a lifetime on the order of a hundred milliseconds without attachment to a power source. These discharges are generated by a moderate current pulse over the surface of an aqueous electrolyte, and some consider the spherical plasmoid that results to bear some resemblance to ball lightning. This article presents the first analysis of the electrical properties of ball plasmoid discharges in a reversed-polarity configuration, i.e., with the central electrode serving as the anode rather than as the cathode. These experiments demonstrate that ball plasmoids can indeed be generated with either electrode polarity with similar observable properties. These results are contrary to what has previously been discussed in the literature and raise additional questions regarding formation mechanisms of ball plasmoids. Analysis of images and electrical measurements collected at various discharge energies show that two distinct processes occur during discharges with our circuitry and in this reversed-polarity configuration: the formation of spark channels between the anode and electrolyte, and the generation of streamers and a jet from the surface of the anode. Full article
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16 pages, 1325 KiB  
Article
Spatially-Resolved Spectroscopic Diagnostics of a Miniature RF Atmospheric Pressure Plasma Jet in Argon Open to Ambient Air
by Florent P. Sainct, Antoine Durocher-Jean, Reetesh Kumar Gangwar, Norma Yadira Mendoza Gonzalez, Sylvain Coulombe and Luc Stafford
Plasma 2020, 3(2), 38-53; https://doi.org/10.3390/plasma3020005 - 01 Apr 2020
Cited by 8 | Viewed by 3888
Abstract
The spatially-resolved electron temperature, rotational temperature, and number density of the two metastable Ar 1 s levels were investigated in a miniature RF Ar glow discharge jet at atmospheric pressure. The 1 s level population densities were determined from optical absorption spectroscopy (OAS) [...] Read more.
The spatially-resolved electron temperature, rotational temperature, and number density of the two metastable Ar 1 s levels were investigated in a miniature RF Ar glow discharge jet at atmospheric pressure. The 1 s level population densities were determined from optical absorption spectroscopy (OAS) measurements assuming a Voigt profile for the plasma emission and a Gaussian profile for the lamp emission. As for the electron temperature, it was deduced from the comparison of the measured Ar 2 p i 1 s j emission lines with those simulated using a collisional-radiative model. The Ar 1 s level population higher than 10 18 m 3 and electron temperature around 2.5 eV were obtained close to the nozzle exit. In addition, both values decreased steadily along the discharge axis. Rotational temperatures determined from OH(A) and N 2 (C) optical emission featured a large difference with the gas temperature found from a thermocouple; a feature ascribed to the population of emitting OH and N 2 states by energy transfer reactions involving the Ar 1 s levels. Full article
(This article belongs to the Special Issue Low Temperature Plasma Jets: Physics, Diagnostics and Applications)
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11 pages, 6563 KiB  
Article
Taylor State Merging at SSX: Experiment and Simulation
by Michael Brown, Kaitlin Gelber and Matiwos Mebratu
Plasma 2020, 3(1), 27-37; https://doi.org/10.3390/plasma3010004 - 17 Mar 2020
Cited by 3 | Viewed by 3639
Abstract
We describe experiments and simulations of dynamical merging with two Taylor state plasmas in a Swarthmore Spheromak Experiment (SSX) device. Taylor states are formed by magnetized plasma guns at opposite ends of the device. We performed experiments with Taylor states of both senses [...] Read more.
We describe experiments and simulations of dynamical merging with two Taylor state plasmas in a Swarthmore Spheromak Experiment (SSX) device. Taylor states are formed by magnetized plasma guns at opposite ends of the device. We performed experiments with Taylor states of both senses of magnetic helicity (right-handed twist or left-handed twist). We present results of both counter-helicity merging (one side left-handed, the other right-handed) and co-helicity merging (both sides left-handed). Experiments show significant ion heating, consistent with magnetic reconnection. We suggest that the merged, warm state could be a suitable target for future magneto-inertial fusion experiments. Magnetohydrodynamic simulations of these experiments reveal the structure of the final relaxed, merged state. Full article
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