ILTPC Newsletter 12

Limits to Propagation and Deceleration of Positive Streamers

It was shown that positive and negative streamers be-have differently during deceleration and eventual stopping. In both cases, deceleration begins with the loss of a significant portion of the applied potential as a voltage drop across the lengthening channel, which leaves less voltage for the head of the streamer. A pos-itive streamer cannot respond with forward electron drift with this decreasing the head potential, which re-duces ionization and photoelectron production rates in front of the head. The only advancement mechanism for a positive streamer is to decrease the effective head radius leading to a local increase in the electric field. This allows the streamer to continue to develop. How-ever, the decrease in the head radius dramatically re-duces the photoionization efficiency. This forces the streamer to increase the electric field even more. Es-timates show that such a locally enhanced electric field becomes larger than the critical field correspond-ing to the transition of electrons to the runaway mode when the potential of the head decreases to ~1.2 kV (Figure). A pulsed beam of runaway electrons directed into the channel of the decelerating positive streamer is generated. This mechanism accompanying positive streamer deceleration may explain the recorded bursts of X-ray radiation during the propagation of streamers in long discharge gaps and in the atmosphere.

Contact: Dr. Andrey Starikovskiy Princeton University astariko@princeton.edu

Source: A. Yu. Starikovskiy, N. L. Aleksandrov and M. N. Shneider, J. Appl. Phys. 129, 063301 (2021). https://doi.org/10.1063/5.0037669.

Downsizing Magneto-plasma-dynamic (MPD) Thrusters

Magneto-plasma-dynamic (MPD) thrusters are typically powerful (with an average power within the range of kW to hundreds of kW), high-thrust propulsion engines with an electromagnetic Lorentz force J×B that accelerates and expels plasma creating the thrust. We propose an ambitious idea to scale down in power and size the applied-field MPD thruster to fit inside a small satellite. This can be done with a two-stage pulsed (up to ms pulse length), cm-sized, low-power (1–30 W) MPD thruster, with the low-power (~several W) first stage based on microcathode arc thruster (μCAT). In such µCAT-MPD configuration, the magnetized vacuum arc discharge demonstrates an interesting threshold behavior. Parameters such as thrust and the thrust-to-power ratio rapidly jump after a certain dc voltage (~30 V) applied on the accelerating electrode. It was demonstrated that such effect improves the thruster performance by increasing the thrust (from ~2 to ~210 μN), efficiency (from ~1% to 50%), and thrust-to-power ratio (from ~0.5 to ~18 μN/W).

Contacts: Dr. Denis Zolotukhin ZolotukhinDen@gmail.com Prof. Michael Keidar keidar@gwu.edu The George Washington University

Source: Phys. Rev. E 102, No. 2 (2020). https://doi.org/10.1103/PhysRevE.102.021203

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Microwave Plasma Synthesis of High Entropy Transition Metal Diborides

This study explores the boro-carbothermal reduction synthesis of high entropy transition metal diborides via microwave plasma. High entropy diborides are promising materials due to high hardness and resistance to high-temperature oxidation. The use of microwave plasma is a novel approach for high entropy diboride formation that capitalizes on its ability to transfer thermal energy for chemical reactions enabled by low-temperature plasma. Aiming to produce a five-component equimolar diboride, the starting powders consist of metal oxides (HfO₂, TiO₂, ZrO₂, Ta₂O₅, MoO₃) with carbon black blended by high energy ball milling and combined with boron carbide. By annealing the sample using microwave plasma, boro-carbothermal reduction becomes efficient and leads to a single-phase high entropy diboride (Hf_0.2, Zr_0.2, Ti_0.2, Ta_0.2, Mo_0.2)B₂ within a single 1hr plasma process.

Future work includes investigating the role that microwave plasma has in the thermal reduction process and to find ways to achieve the high entropy diboride at lower substrate temperature. This will be done by exploring other plasma feed-gas mixtures, substrate biasing, and by varying initial particle size (via milling). Analysis will focus on microstructure, high-temperature oxidation resistance, and hardness.

Contact: Prof. Shane Catledge University of Alabama at Birmingham catledge@uab.edu

Plasma Enhanced Oxidation (PEO) and Plasma Enhanced Reduction (PER) Toward Surface Engineering of Tissue Scaffolds and Flexible Bioelectronic Materials

An interdisciplinary team at the University of Alabama at Birmingham has developed a new method of surface engineering polymer soft-materials and textiles that has potential to help accelerate bio-integration in tissue scaffolds, applications in plasmonic bioelectronics, and decontaminating personnel protective equipment. The team has utilized plasma enhanced oxidation (PEO) and plasma enhanced reduction (PER) to oxidize/reduce the precursors in the plasma-phase onto the surface of substrate, irrespective of its nature as metallic/nonmetallic. This is a rapid method for the synthesis of silica nanoparticles (SiNp) from a liquid precursor by PEO into dusty plasma before being deposited as nano-particles onto a 3D-printed polymer. Plasma-enhanced CVD of small amount of a liquid precursor, organosilane, for a short exposure time resulted in the generation of dusty plasma of SiNp with a narrow size distribution.

The team has successfully utilized non-thermal plasma for making super-hydrophilic and blood-friendly material’s surfaces. The process described has advantages over existing methods, in that it is a single-step, greener, and more cost-effective process. In addition, the RF reactor can be an ideal scalable technology that industries can use to produce and modify the surface of various biomedical scaffolds or devices with SiNp. This method can also simultaneously modify the 3D printed PLA scaffolds with SiNp for biomedical applications such as bone-tissue engineering and sterilize them. The team is currently investigating functionalization and attachment of plasmonic metal nanoparticles and titania nanoparticles in the plasma phase onto substrates for flexible bioelectronics and anti-microbial surfaces as well as devising strategies for preparation nanoparticles and 1D nanowires and 2D nanosheets.

Contact: Prof. Vinoy Thomas University of Alabama at Birmingham vthomas@uab.edu

Sources:

• V. Vijayan, ACS Appl. Nano Matls. 3, 7392 (2020).
• B. Tucker, Current Op. Biomed. Engr., 100259 (2021).
• V. Vijayan, J. Matls. Chem. B 8 (14), 2814 (2020).

Effects of Cowling Resistivity in the Weakly Ionized Chromosphere

The physics of the solar chromosphere is complex from both theoretical and modeling perspectives. The plasma temperature from the photosphere to corona increases from ~5,000 K to ~1 million K over a distance of only 10,000 km from the chromosphere and the transition region. Certain regions of the solar atmosphere have sufficiently low temperature and ionization rates to be considered as weakly-ionized. In particular, this is true at the lower chromosphere. As a result, the Cowling resistivity is orders of magnitude greater than the Coulomb resistivity. Ohm’s law therefore includes anisotropic dissipation of currents perpendicular and parallel to the magnetic field lines.

To evaluate the Cowling resistivity, we need to know the external magnetic field strength and need to estimate the neutral fraction as a function of the bulk plasma density and temperature. In this study, we determined the magnetic field topology applying the non-force-free field (NFFF) extrapolation technique to SDO/HMI SHARP vector magnetogram data, and the stratified density and temperature profiles from the Maltby-M umbral core model for sunspots.

We investigated the effects of Cowling resistivity on heating and magnetic reconnection in the chromosphere as the flare-producing active region (AR) 11166 evolves. We analyzed a C2.0 flare emerging from AR 11166 and found a normalized reconnection rate of 0.051, in good agreement with observational and numerical results. We also found a significantly thicker reconnection current sheet of 37 km. The heating due to dissipation of currents by Cowling resistivity is 6-8 orders of magnitude greater than heating due to the dissipation of currents by Coulomb resistivity. Cowling resistivity can be used to trace the time evolution of AR features due to its strong dependence on magnetic field strength (ηC ∝ B²).

Contact: Dr. Mehmet Sarp Yalim The University of Alabama in Huntsville msy0002@uah.edu

Source: M. S. Yalim et al., Astrophys. J. Lett. 899, L4 (2020). https://doi.org/10.3847/2041-8213/aba69a.

Ultra-Fast Processes in Surface Barrier Discharges

Surface barrier discharges (SBD) are widely used sources of non-equilibrium plasma in many applications: plasma-assisted combustion, flow control or surface treatment, to name a few. In this study, we investigated the origin of filamentary patterns in a sinusoidally driven surface barrier discharge at high over-voltage in atmospheric pressure air. Using time-correlated single-photon counting (TCSPC) based optical emission spectroscopy, we revealed ultrafast processes within generated discharges with both polarities of the applied voltage. For negative polarity, we observed initiation of a complex streamer cascade which emerges far from the bare cathode. This event is responsible for long filamentary structure detected by an intensified CCD camera imaging and transfers an exceptionally large electrical charge. It constitutes another, previously unknown, mechanism contributing to the charge-transfer equilibrium in studied periodic discharges. The event is dominated by long positive streamers propagating towards the cathode. It leads to the formation of an intense cathode spot, a critical condition for plasma-transition into a highly ionized state. Our belief is that these measurements and proposed mechanism also help to clarify the issue of initiation of hot-filaments in high-pressure high over-voltage repetitive-pulsed SBDs used for air-flow control or in the field of plasma-assisted combustion.

Contact: Prof. Tomáš Hoder Masaryk University hoder@physics.muni.cz

Source: Plasma Sources Sci. Technol. 30, 03LT02 (2021). https://doi.org/10.1088/1361-6595/abe4e2.

Non-thermal Plasma Modulates Cellular Markers Associated with Immunogenicity in a Model of Latent HIV-1 Infection

In this study we investigated non-thermal plasma (NTP) as the basis for an immunotherapy for patients who continue to harbor HIV-1 as latently infected cells despite effective virus suppression by anti-retroviral therapy (ART). If ART is withdrawn, the infection and accompanying disease symptoms reemerge due, in part, to failures of innate and cytotoxic T lymphocyte responses to clear infected cells. Control of HIV-1 infection without ART may be possible if the suboptimal immune responses can be boosted. Our studies demonstrated that application of NTP to J-Lat cells (a model for latently HIV-1 infected T lymphocytes) stimulated viral gene expression, altered the array of peptides displayed on NTP-exposed cells, and increased the cell surface presence of MHC I and co-stimulatory molecules. These effects are vital steps that promote the recognition of infected cells by the host immune system.

NTP exposure of J-Lat cells also stimulated emission of damage-associated molecular patterns (DAMPs), which increase their immunogenicity. DAMPs emitted by NTP-exposed cells included pro-inflammatory interleukin-1β (IL-1β) and interferon-γ (IFN-γ), as well as pro-phagocytic markers calreticulin (CRT) and heat shock proteins (HSP) 70 and 90. The functional relevance of these DAMPs was evident in the enhanced phagocytosis of NTP-exposed J-Lat cells by macrophages. These effects should facilitate a more effective adaptive immune response in vivo. Implementation of our proposed immunotherapeutic method for controlling HIV-1 would rely on ex vivo application of NTP to the patient’s own HIV-1 infected cells to increase their immunogenicity. The cells would then be injected back into the patient as a “personalized vaccine” to stimulate innate immune responses and a robust cytotoxic T lymphocyte response. The envisioned approach would prevent reemergence of infection without the need for ART.

Contacts: Dr. Vandana Miller vam54@drexel.edu Dr. Fred Krebs fck23@drexel.edu Drexel University College of Medicine

Source: https://doi.org/10.1371/journal.pone.0247125.

Generation of Large Volume Plasma Jet in Atmospheric Pressure Air Plasma Torch Implementing Plasma Thruster Action

One of the most important weaknesses of thermal plasma technology is its energy intensive nature. In spite of numerous unique advantages, easy availability of competing technologies at lower cost has caused many thermal plasma industries to shut down permanently. To make these industries competitive and more economic, there is an intensive thrust in the area to develop low cost thermal plasma devices that produce large volume high temperature plasma while consuming low electrical power.

The present development contributes towards this goal by developing a large volume self-propagating plasma jet implementing plasma thruster action in the external plasma jet itself. Specific design features of the torch ensure arc current extension in the external jet that automatically occurs in the thruster action in the tail region through interaction of transverse arc current with the magnetic field produced by the adjacent longitudinal current component. The torch operates with hafnium insert cathode and copper anode. A small compressor takes air from atmosphere into the torch at a flow rate of just 30 slpm to form the large plasma jet. Featured operation at low current (< 200A) and high arc voltage (>150V) avoids use of thick current cables and allows easier handling. The device, currently being used in solid waste management, has tremendous potential in the waste to energy sector.

Contact: Prof. S. Ghorui Homi Bhabha National Institute sghorui@barc.gov.in srikumarghorui@yahoo.com

Source: S. Ghorui, IEEE Transactions on Plasma Science 49, 578-596 (2021). doi:10.1109/TPS.2020.3006023.

Analytical and Experimental Studies of Plasma Thermal-Chemical Instability

The control of instabilities in weakly ionized plasma is of great importance in plasma-assisted combus-tion, fuel reforming, catalysis, and material synthesis. High speed videos of single bursts of a CH4-containing nanosecond pulsed dielectric barrier discharge plasma were captured with and without oxidizer in a transverse gas flow (Fig. 1). The plasma uniformity was significantly modified by fuel oxida-tion as compared to fuel pyrolysis. A possible thermal-chemical instability (TCI) mechanism was proposed to account for the rapid formation of streamers from an originally uniform discharge in a chemically reactive flow. An analytical criterion for the onset of TCI was developed, suggesting the impact of plasma aided chemical reactions on the formation of instability. This thermal-chemical instability was further investigated with quasi-neutral plasma modeling, stability analysis and thermal-chemical mode analysis (TCMA).

The stability analysis indicates that chemical kinetics will modify the bifurcation point and the hystere-sis region of the original thermal-ionization mechanism in the current-voltage curve (CVC) (An example of CVC of air is shown in Fig. 2). From chemical mode analysis, a variety of time scales from milli-second to sub- microsecond and their related kinetics are involved for triggering such instabilities. Kinetic pathways and chemical modes of the fuel oxidation by vibrational and electronically excited species were important during the formation of this thermal-chemical instability. These investigations provide insights and guidance for controlling plasma instability using chemical kinetics.

Contact: Prof. Yiguang Ju Princeton University yju@princeton.edu

Sources:

• A. C. Rousso, Plasma Sources Sci. Technol. (2020). https://doi.org/10.1088/1361-6595/abb7be
• H. Zhong et al., J. Phys. D: Appl. Phys. (2019). https://doi.org/10.1088/1361-6463/ab3d69
• H. Zhong et al., Plasma Sources Sci. Technol. (2021). https://doi.org/10.1088/1361-6595/abde1c

Time-resolved Laser Diagnostics for Studying Non-equilibrium Plasma Chemistry and Dynamics

The time-evolution of rotation-vibration non-equilibrium, electron density and temperature, and radical species concentrations are critical for understanding plasma chemistry. Here, we highlight our recent work in measuring these parameters using in situ laser diagnostics. First, spatially resolved 1-D imaging of both rotational and vibrational temperatures was simultaneously performed using pure rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS). This enables measurement of rotation-vibration non-equilibrium with high temporal and spatial resolution near surfaces such as in plasma catalysis (Fig. a).

Next, in plasma chemistry, O(¹D) and HO₂ are important intermediate species but their reaction kinetics are not well-explored due to spectral interferences. Here we used Faraday rotation spectroscopy for selective and sensitive HO₂ measurements, which further enables direct measurements of the reaction kinetics of O(¹D) with hydrocarbons and oxygenated fuels (Fig. b). Lastly, we have used laser Thomson scattering recently and studied how the electron density and temperature change with CH₄ addition to a He nanosecond-pulsed discharge. This data was then compared with numerical simulations which reproduced the nonlinear trend in electron properties with CH₄ concentration (Fig. c).

Contact: Prof. Yiguang Ju Princeton University yju@princeton.edu

Sources:

• https://doi.org/10.1364/OL.394122
• https://doi.org/10.1364/OE.413063
• https://doi.org/10.1088/1361-6463/ab0598
• https://doi.org/10.1002/kin.21474
• https://doi.org/10.1016/j.combustflame.2019.10.034

A Counter-current Membrane, Micro-packed-bed Dielectric-barrier-discharge Plasmatron

A counter-current membrane, micro-packed-bed dielectric-barrier-discharge plasmatron was designed and characterized. The µ-plasmatron was then applied to study flash chemistry with atmospheric non-thermal plasma.

Gas-liquid two-phase flows within the µ-reactor were described by residence time distribution modeling, to ascertain the extent that axial dispersion would have on the measurement of reaction kinetics with the microplasmas. Microscope images of 2D multiphase flows in the packed bed microplasmatron were also obtained and pieced together, to visualize gas-liquid interfaces in the packed-bed region. Successful formation of methyl radicals was verified in-situ by optical emission spectroscopy, which creates a new experimental method for the study of plasma catalytic reactions.

Direct addition of methyl radicals to the metal center of a homogeneous cobalt catalyst was confirmed by characteristic Raman shifts. These findings underpin the exciting prospects of methane plasmas in organometallic synthesis, which remains a vastly unexplored area of chemical catalysis. In the context plasma physics, our results demonstrate that “flash chemistry”, or reactions that occur in short time scales, can be studied with multiphase microplasmatrons, where transport limitations in conventional reactor designs could be expected to influence the overall rate of reaction.

Contact: Prof. Ryan L. Hartman New York University ryan.hartman@nyu.edu

Source: J. Phys. D. Appl. Phys. 54, 194003 (2021).https://doi.org/10.1088/1361-6463/abe488.

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Sustainable Gas Conversion by Gliding Arc Plasmas: A New Modelling Approach for Reactor Design Improvement

Plasma technology is gaining increasing interest for sustainable gas conversion applications. In this paper, we presented a new modelling approach for the design of a gliding arc plasma reactor, revealing the fluid dynamics, arc behavior and plasma chemistry by solving a combination of five complementary models. This approach allows one to efficiently evaluate the performance of a plasma reactor and indicate possible design improvements before actually building it. We demonstrate the capabilities of this method for plasma-based NO_x formation in a rotating gliding arc reactor. The model demonstrates the importance of the vortex flow and the presence of a recirculation zone in the reactor, as well as the formation of hot spots in the plasma near the cathode pin and the anode wall that are responsible for most of the NO_x formation. The model also reveals the underlying plasma chemistry and the vibrational non-equilibrium that exists due to the fast cooling during each arc rotation. Good agreement is reached with experimental measurements.

Contact: Prof. Dr. Annemie Bogaerts University of Antwerp annemie.bogaerts@uantwerpen.be

Source: Sustain. Energy Fuels 5, 1786-1800 (2021). https://doi.org/10.1039/D0SE01782E.

Transport of Gaseous Hydrogen Peroxide and Ozone into Bulk Water vs. Electrosprayed Aerosol

Production and transport of reactive species through plasma–liquid interfaces play a significant role in multiple applications in biomedicine, environment, and agriculture. Experimental investigations of the transport mechanisms of typical air plasma reactive species: hydrogen peroxide (H₂O₂) and ozone (O₃) into water are presented. Solvation of gaseous H₂O₂ and O₃ from an airflow into water bulk vs. electrosprayed microdroplets was compared, while changing the water flow rate and applied voltage, during different treatment times and gas flow rates. The solvation rate of H₂O₂ and O₃ increased with the treatment time and the gas–liquid interface area. The total surface area of the electrosprayed microdroplets was larger than that of the bulk, but their lifetime was much shorter.

We estimated that only microdroplets with diameters below ~40 µm could achieve saturation by O₃ during their lifetime. The saturation by H₂O₂ was unreachable due to its fast depletion from air. In addition to the short-lived flying electrosprayed microdroplets, the longer-lived microdroplets collecting at the reactor bottom and walls substantially contributed to H₂O₂ and O₃ solvation in water. The experimental results were compared with simple theoretical considerations. We experimentally demonstrated and theoretically explained that Henry′s law coefficient is not the only important parameter determining the solvation of highly and lowly soluble (H₂O₂ and O₃) gaseous species into the water. This study contributes to a better understanding of the gaseous H₂O₂ and O₃ transport into water and will potentially lead to design optimization of the water spray and plasma-liquid interaction systems.

Contact: Zdenko Machala Comenius University Bratislava, Slovakia machala@fmph.uniba.sk

Source: Water 13, 182 (2021). https://doi.org/10.3390/w13020182.

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A Tutorial Review on the LXCat Project: Status and Perspectives

Non-equilibrium low temperature plasma (LTP) modeling requires very large amounts of data. Accessibility, sourcing, traceability and evaluation of data are therefore key components for reliable and reproducible modeling of LTP. In a recently published, open-access, tutorial review [1], an overview is given of the data types present in the various datasets curated by contributors to the LXCat project and available tools on the website. A brief history of the LXCat project and its current status are given. The review is mainly focused on the electron component of plasmas for LTP modeling; best practices are given for accessing data on the LXCat website and referencing it; and some of the common pitfalls in data usage for plasma modeling are discussed. Properly referencing the data is also critical for traceability as well as for the survival of open-access data sharing projects. The policy for data reuse and referencing is described with some additional new details for more clarity. This review should be useful for new users as well as more experienced users. A YouTube video tutorial for first-time users of LXCat is provided complimentary to the paper.

A (successful) database is not only about the data but also about the tools that are provided for analyzing the data and how the data is interfaced with a website or online calculation tools. The LXCat team appreciates very much the feedback from the LTP community and always seeks new contributions/contributors (e.g. data, tools,…). The present design and implementation of LXCat have served the community well for many years. However, as the complexity of the datasets has grown, it has become clear that further improvements to the backend may substantially improve the LXCat user experience. In particular, ongoing work is focused on enabling more timely improvements and additions to LXCat. In the past year the LXCat team has identified several bottlenecks, extracted a set of specifications for a new version of LXCat, and prototyped these ideas. In this paper, a flavor of the results from this work is provided. Feedback and involvement from members of the LTP community are critical for the next steps of the project and we look forward to new input!

Contacts: Prof. Emile Carbone INRS, University of Quebec emile.carbone@inrs.ca The LXCat team info@lxcat.net

[1] Carbone, E., Graef, W., Hagelaar, G., Boer, D., Hopkins, M. M., Stephens, J. C., Yee, B. T., Pancheshnyi, S., van Dijk, J.; Pitchford, L., “Data Needs for Modeling Low-Temperature Non-Equilibrium Plasmas: The LXCat Project, History, Perspectives and a Tutorial”, Atoms 9, 16 (2021). https://doi.org/10.3390/atoms9010016.

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