Abstract: A gliding arc plasmatron (GAP), which is very promising for purification and gas conversion,
is characterized in nitrogen using optical emission spectroscopy and high-speed photography,
because the cross sections of electron impact excitation of N 2 are well known. The gas
temperature (of about 5500 K), the electron density (up to 1.5 × 10 15 cm −3 ) and the reduced
electric field (of about 37 Td) are determined using an absolutely calibrated intensified charge-
coupled device (ICCD) camera, equipped with an in-house made optical arrangement for
simultaneous two-wavelength diagnostics, adapted to the transient behavior of a GA channel
in turbulent gas flow. The intensities of nitrogen molecular emission bands, N 2 (C–B,0–0) as
well as N +
2 (B–X,0–0), are measured simultaneously. The electron density and the reduced
electric field are determined at a spatial resolution of 30 µm, using numerical simulation and
measured emission intensities, applying the Abel inversion of the ICCD images. The temporal
behavior of the GA plasma channel and the formation of plasma plumes are studied using a
high-speed camera. Based on the determined plasma parameters, we suggest that the plasma
plume formation is due to the magnetization of electrons in the plasma channel of the GAP by
an axial magnetic field in the plasma vortex.

Abstract: Cold atmospheric plasmas (CAPs) have attracted significant interest for their potential benefits in medical applications, including cancer therapy. The therapeutic effects of CAPs are related to reactive oxygen and nitrogen species (ROS and RNS) present in the plasma. The impact of ROS has been extensively studied, but the role of RNS in CAP-treatment remains poorly understood at the molecular level. Here, we investigate the permeation of RNS and ROS across native and oxidized phospholipid bilayers (PLBs) by means of computer simulations. The results reveal significantly lower free energy barriers for RNS (i.e. NO, NO2, N2O4) and O3 compared to hydrophilic ROS, such as OH, HO2 and H2O2. This suggests that the investigated RNS and O3 can permeate more easily through both native and oxidized PLBs in comparison to hydrophilic ROS, indicating their potentially important role in plasma medicine.

Abstract: One of the main issues in plasma chemistry modeling is that the cross sections and rate coefficients are subject to uncertainties, which yields uncertainties in the modeling results and hence hinders the predictive capabilities. In this paper, we reveal the impact of these uncertainties on the model predictions of plasma-based dry reforming in a dielectric barrier discharge. For this purpose, we performed a detailed uncertainty analysis and sensitivity study. 2000 different combinations of rate coefficients, based on the uncertainty from a log-normal distribution, are used to predict the uncertainties in the model output. The uncertainties in the electron density and electron temperature are around 11% and 8% at the maximum of the power deposition for a 70% confidence level. Still, this can have a major effect on the electron impact rates and hence on the calculated conversions of CO2 and CH4, as well as on the selectivities of CO and H2. For the CO2 and CH4 conversion, we obtain uncertainties of 24% and 33%, respectively. For the CO and H2 selectivity, the corresponding uncertainties are 28% and 14%, respectively. We also identify which reactions contribute most to the uncertainty in the model predictions. In order to improve the accuracy and reliability of plasma chemistry models, we recommend using only verified rate coefficients, and we point out the need for dedicated verification experiments.

Abstract: Journal of Physics D: Applied Physics published the first Plasma Roadmap in 2012
consisting of the individual perspectives of 16 leading experts in the various sub-fields of low temperature plasma science and technology. The 2017 Plasma Roadmap is the first update of a planned series of periodic updates of the Plasma Roadmap. The continuously growing interdisciplinary nature of the low temperature plasma field and its equally broad range of applications are making it increasingly difficult to identify major challenges that encompass all of the many sub-fields and applications. This intellectual diversity is ultimately a strength of the field. The current state of the art for the 19 sub-fields addressed in this roadmap demonstrates the enviable track record of the low temperature plasma field in the development of plasmas as an enabling technology for a vast range of technologies that underpin our modern society. At the same time, the many important scientific and technological challenges shared in this roadmap show that the path forward is not only scientifically rich but has the potential to make wide and far reaching contributions to many societal challenges.

Abstract: Microdischarges are an important type of plasma discharge that possess several unique characteristics, such as the presence of a stable glow discharge, high plasma density and intense excimer radiation, leading to several potential applications. The intense and controllable gas heating within the extremely small dimensions of microdischarges has been exploited in microthruster technologies by incorporating a micro-nozzle to generate the thrust. This kind of microthruster has a significantly improved specific impulse performance compared to conventional cold gas thrusters, and can meet the requirements arising from the emerging development and application of micro-spacecraft. In this paper, we performed a self-consistent 2D particle-in-cell simulation, with a Monte Carlo collision model, of a microdischarge operating in a prototype micro-plasma thruster with a hollow cylinder geometry and a divergent micro-nozzle. The model takes into account the thermionic electron emission including the Schottky effect, the secondary electron emission due to cathode bombardment by the plasma ions, several different collision processes, and a non-uniform argon background gas density in the cathode-anode gap. Results in the high-pressure (several hundreds of Torr), high-current (mA) operating regime showing the behavior of the plasma density, potential distribution, and energy flux towards the hollow cathode and anode are presented and discussed. In addition, the results of simulations showing the effect of different argon gas pressures, cathode material work function and discharge voltage on the operation of the microdischarge thruster are presented. Our calculated properties are compared with experimental data under similar conditions and qualitative and quantitative agreements are reached.

Abstract: Here, we present routes to increase CO2 conversion into CO using an atmospheric pressure dielectric-barrier discharge. The change in conversion as a function of simple plasma parameters, such as power, flow rate, but also frequency, on-and-off power pulse, thickness and the chemical nature of the dielectric, wall and gas temperature, are described. By means of an in-depth electrical characterization of the discharge (effective plasma voltage, dielectric voltage, plasma current, number and lifetime of the microdischarges), combined with infrared analysis of the walls of the reactor, optical emission spectroscopy for the gas temperature, and mass spectrometry for the CO2 conversion, we propose a global interpretation of the effect of all the experimental parameters on the conversion and efficiency of the reaction.

Abstract: We present a 3D and 2D Cartesian quasi-neutral plasma model for a low current argon gliding arc discharge, including strong interactions between the gas flow and arc plasma column.
The 3D model is applied only for a short time of 0.2 ms due to its huge computational cost. It mainly serves to verify the reliability of the 2D model. As the results in 2D compare well with those in 3D, they can be used for a better understanding of the gliding arc basic characteristics. More specifically, we investigate the back-breakdown phenomenon induced by an artificially controlled plasma channel, and we discuss its effect on the gliding arc characteristics. The
back-breakdown phenomenon, or backward-jump motion of the arc, as observed in the experiments, results in a drop of the gas temperature, as well as in a delay of the arc velocity with respect to the gas flow velocity, allowing more gas to pass through the arc, and thus increasing the efficiency of the gliding arc for gas treatment applications.

Abstract: Electrical breakdown by the application of an electric field occurs more easily in hot gases than in cold gases because of the extra electron-species interactions that occur as a result of dissociation, ionization and excitation at higher temperature. This paper discusses some overlooked physics and clarifies inaccuracies in the evaluation of the effective ionization coefficients and the critical reduced breakdown electric field of CO2 at elevated temperature, considering the influence of excited states and ion kinetics. The critical reduced breakdown electric field is obtained by balancing electron generation and loss mechanisms using the electron energy distribution function (EEDF) derived from the Boltzmann transport equation under the two-term approximation. The equilibrium compositions of the hot gas mixtures are determined based on Gibbs free energy minimization considering the ground states as well as vibrationally and electronically excited states as independent species, which follow a Boltzmann distribution with a fixed excitation temperature. The interaction cross sections between electrons and the excited species, not reported previously, are properly taken into account. Furthermore, the ion kinetics, including electron–ion recombination, associative electron detachment, charge transfer and ion conversion into stable negative ion clusters, are also considered. Our results indicate that the excited species lead to a greater population of high-energy electrons at higher gas temperature and this affects the Townsend rate coefficients (i.e. of electron impact ionization and attachment), but the critical reduced breakdown electric field strength of CO2 is only affected when also properly accounting for the ion kinetics. Indeed, the latter greatly influences the effective ionization coefficients and hence the critical reduced breakdown electric field at temperatures above 1500 K. The rapid increase of the dissociative electron attachment cross-section of molecular oxygen with rising vibrational quantum number leads to a larger electron loss rate and this enhances the critical reduced breakdown electric field strength in the temperature range where the concentration of molecular oxygen is relatively high. The results obtained in this work show reasonable agreement with experimental results from literature, and are important for the evaluation of the dielectric strength of CO2 in a highly reactive environment at elevated temperature.

Abstract: CO2 conversion into value-added products has gained significant interest over the few last years, as the greenhouse gas concentrations constantly increase due to anthropogenic activities. Here we report on experiments for CO2 conversion by means of a cold atmospheric plasma using a cylindrical flowing dielectric barrier discharge (DBD) reactor. A detailed comparison of this DBD ignited in a so-called burst mode (i.e. where an AC voltage is applied during a limited amount of time) and pure AC mode is carried out to evaluate their effect on the conversion of CO2 as well as on the energy efficiency. Decreasing the duty cycle in the burst mode from 100% (i.e. corresponding to pure AC mode) to 40% leads to a rise in the
conversion from 16–26% and to a rise in the energy efficiency from 15 to 23%. Based on a detailed electrical analysis, we show that the conversion correlates with the features of the microfilaments. Moreover, the root-mean-square voltage in the burst mode remains constant as a function of the process time for the duty cycles <70%, while a higher duty cycle or the usual pure AC mode leads to a clear voltage decay by more than 500 V, over approximately 90 s, before reaching a steady state regime. The higher plasma voltage in the burst mode yields a higher electric field. This causes the increasing the electron energy, and therefore their
involvement in the CO2 dissociation process, which is an additional explanation for the higher CO2 conversion and energy efficiency in the burst mode.

Abstract: Although CO2 conversion by plasma technology is gaining increasing interest, the
underlying mechanisms for an energy-efficient process are still far from understood. In this work, a reduced non-equilibrium CO2 plasma chemistry set, based on level lumping of the vibrational levels, is proposed and the reliability of this level-lumping method is tested by a self-consistent zero-dimensional code. A severe reduction of the number of equations to be solved is achieved, which is crucial to be able to model non-equilibrium CO2 plasmas by 2-dimensional models. Typical conditions of pressure and power used in a microwave plasma for CO2 conversion are investigated. Several different sets, using different numbers of lumped groups, are considered. The lumped models with 1, 2 or 3 groups are able to reproduce the gas temperature, electron density and electron temperature profiles, as calculated by the full model treating all individual excited levels, in the entire pressure range investigated. Furthermore, a 3-groups model is also able to reproduce the shape of the vibrational distribution function (VDF) and gives the most reliable prediction of the CO2 conversion. A strong influence of the vibrational excitation on the plasma characteristics is observed. Finally, the limitations of the lumped-levels method are discussed.

Abstract: In this computational study, a gliding arc plasma reactor with a reverse-vortex flow stabilization is modelled for the first time by a fluid plasma description. The plasma reactor operates with argon gas at atmospheric pressure. The gas flow is simulated using the k-ε Reynolds-averaged Navier–Stokes turbulent model. A quasi-neutral fluid plasma model is used for computing the plasma properties. The plasma arc movement in the reactor is observed, and the results for the gas flow, electrical characteristics, plasma density, electron temperature, and gas temperature are analyzed.

Abstract: This work examines the properties of a dielectric barrier discharge (DBD) reactor, built for CO2 decomposition, by means of electrical characterization, optical emission spectroscopy and gas chromatography. The discharge, formed in an electronegative gas (such as CO2, but also O2), exhibits clearly different electrical characteristics, depending on the surface conductivity of the reactor walls. An asymmetric current waveform is observed in the metaldielectric (MD) configuration, with sparse high-current pulses in the positive half-cycle (HC) and a more uniform regime in the negative HC. This indicates that the discharge is operating in two alternating regimes with rather different properties. At high CO2 conversion regimes, a conductive coating is deposited on the dielectric. This so-called coated MD configuration yields a symmetric current waveform, with current peaks in both the positive and negative HCs. In a double-dielectric (DD) configuration, the current waveform is also symmetric, but without current peaks in both the positive and negative HC. Finally, the DD configuration with conductive coating on the inner surface of the outer dielectric, i.e. so-called coated DD, yields again an asymmetric current waveform, with current peaks in the negative HC. These different electrical characteristics are related to the presence of the conductive coating on the dielectric wall of the reactor and can be explained by an increase of the local barrier capacitance available for charge transfer. The different discharge regimes affect the CO2 conversion, more specifically, the CO2 conversion is lowest in the clean DD configuration. It is somewhat higher in the coated DD configuration, and still higher in the MD configuration. The clean and coated MD configuration, however, gave similar CO2 conversion. These results indicate that the conductivity of the dielectric reactor walls can highly promote the development of the high-amplitude discharge current pulses and subsequently the CO2 conversion.

Abstract: In this work we have analyzed the properties of a gliding dc discharge in argon at atmospheric pressure. Despite the usual designation of these discharges as ‘gliding arc discharges’, it was found previously that they operate in two different regimes—glow and arc. Here we analyze the differences in both regimes by means of two dimensional fluid modeling. In order to address different aspects of the discharge operation, we use two models—Cartesian and axisymmetric in a cylindrical coordinate system. The obtained results show that the two types of discharges produce a similar plasma column for a similar discharge current. However, the different mechanisms of plasma channel attachment to the cathode could produce certain differences in the plasma parameters (i.e. arc elongation), and this can affect gas treatments applications.

Abstract: In this study we report on a 2D fluid model of a gliding arc discharge in argon. Despite the 3D nature of the discharge, 2D models are found to be capable of providing very useful information about the operation of the discharge. We employ two modelsan axisymmetric and a Cartesian one. We show that for the considered experiment and the conditions of a low current arc (around 30 mA) in argon, there is no significant heating of the cathode surface and the discharge is sustained by field electron emission from the cathode accompanied by the formation of a cathode spot. The obtained discharge power and voltage are relatively sensitive to the surface properties and particularly to the surface roughness, causing effectively an amplification of the normal electric field. The arc body and anode region are not influenced by this and depend mainly on the current value. The gliding of the arc is modelled by means of a 2D Cartesian model. The arcelectrode contact points are analysed and the gliding mechanism along the electrode surface is discussed. Following experimental observations, the cathode spot is simulated as jumping from one point to another. A complete arc cycle is modelled from initial ignition to arc decay. The results show that there is no interaction between the successive gliding arcs.

Abstract: Modelling at the molecular or atomic scale can be very useful for obtaining a better insight in plasma medicine. This paper gives an overview of different atomic/molecular scale modelling approaches that can be used to study the direct interaction of plasma species with biomolecules or the consequences of these interactions for the biomolecules on a somewhat longer time-scale. These approaches include density functional theory (DFT), density functional based tight binding (DFTB), classical reactive and non-reactive molecular dynamics (MD) and united-atom or coarse-grained MD, as well as hybrid quantum mechanics/molecular mechanics (QM/MM) methods. Specific examples will be given for three important types of biomolecules, present in human cells, i.e. proteins, DNA and phospholipids found in the cell membrane. The results show that each of these modelling approaches has its specific strengths and limitations, and is particularly useful for certain applications. A multi-level approach is therefore most suitable for obtaining a global picture of the plasma–biomolecule interactions.