Table of contents

The initial stage of the fast electrical breakdown of an air gap with a pin-to-plane electrode geometry is studied on a nanosecond time scale using multi-frame laser probing with an exposure time of 70 ps and spatial resolution as high as 3–4 μm. We find that the gap breakdown is associated with the fast (≲1 ns) formation of a micron-sized (∼10 μm) cathode spot that appears as a plasma with an electron density of ${n}_{e}\approx {10}^{19}\,{\mathrm{cm}}^{-3}$. The spot is then transformed into a spark channel having an electron density of ${n}_{e}\sim {10}^{19}\mbox{--}{10}^{20}\,{\mathrm{cm}}^{-3}$. Within ∼1 ns after the breakdown the dynamics of the highly ionized near-cathode plasma governs the current capacity of the discharge gap as well as the current rise rate.

This paper analyses numerical efforts regarding nanosecond (NS) surface dielectric barrier discharge (SDBD) modelling in atmospheric air. Numerical results of the discharge structure for positive and negative applied voltage pulse polarity, and the features of the physical models and boundary conditions used, are discussed. The results of 2D simulations of the quasi-uniform SDBD mode are presented and compared with the results of other research teams, and with experimental data, to reveal the most appropriate approaches. New results of numerical simulations and analytical estimations of the energy into gas deposition due to NS SDBD driven by a single NS voltage pulse are presented. The problems relating to NS SDBD modelling are discussed.

Copper sulfide (CuS) nanoparticle films with different nanoparticle sizes were fabricated using an inductively coupled plasma (ICP) and a vapor-phase sulfurization method. First, the ICP is applied to a thin copper film to obtain a copper nanoparticle film, based on the plasma–surface interactions through the ion bombardment on the film. The fabrication of the Cu nanoparticles revealed that their size and spatial distribution depend on the discharge mode of the ICP and plasma irradiation time. From the measurements of the plasma density, optical emission spectrum, and ion flux energy distribution function, it was found that the inductive mode of the ICP, compared to the capacitive mode of the ICP, is efficient at fabricating the uniform nanoparticle film due to the optimal plasma potential and high ion flux with narrow ion energy distribution. The Cu nanoparticles are then transformed to CuS nanoparticles through vapor-phase sulfurization. For the hydrophobicity application of the CuS nanoparticle film, the contact angles of the CuS nanoparticle films were measured and compared with those of other thin films, such as SiO₂ and bulk CuS. The contact angle of the CuS nanoparticle film, fabricated through the plasma–surface interactions and sulfurization, was significantly higher than that of other thin films owing to the hydrophobic surface of the CuS with the nanoparticle structure.

An axisymmetric, finite difference frequency domain model is used to study the wave propagation and power absorption in a helicon plasma thruster operating inside a laboratory vacuum chamber. The magnetic field is not purely axial and the plasma beam is cylindrical in the source and divergent in the magnetic nozzle. The influence of the magnetic field strength, plasma density, electron collision frequency and geometry on the wavefields and the power absorption maps is investigated, showing different power deposition patterns. The electromagnetic radiation is not confined to the source region but propagates into the nozzle divergent region, and indeed the power absorption there is not negligible. For the impedance at the antenna, the reactance is rather constant but the resistance is very dependent on operation parameters; optimal parameter values maximizing the resistance are found.

This paper presents the results of the numerical simulation of negative corona discharge in SF₆ at 0.4 Mpa for a point-plane configuration. A 2D axisymmetric model considering three charged species has been investigated with an applied voltage equal to −25 kV. Three drift-diffusion equations coupled with Poisson's equation were solved simultaneously to obtain the temporal and spatial properties of Trichel pulses. The distributions of charged species at different instants of time and the time-dependent total number of charged species have been determined. The characteristics of current pulses with voltages varying from −24 kV to −27 kV were compared and it has been found that the characteristics of the current magnitude and frequency are similar to those in air. The effects of the model coefficients—the ionization coefficient, the attachment coefficient, the mobility of charged species and the secondary emission coefficient—on the characteristics of current pulses have also been studied to investigate the sensitivities of the pulse parameters to these coefficients.

Non-thermal plasma has been proposed as a promising technique for the reformation of fossil fuel into cleaner fuel. Despite recent achievements in the economic and technical development of plasma-assisted reforming techniques, a better understanding of thermochemistry and electron-induced chemistry is required to optimize the performance of the relevant systems. We study the relative importance of electron-induced chemistry and thermochemistry for C₅H₁₂ (model gasoline) activation and products formation using a temperature-controlled dielectric barrier discharge (DBD) reactor. Important mechanical insight is obtained from the comparison between high-temperature and low-pressure conditions under similar reduced field intensities. In a tested range of background temperatures (303 < T < 623 K), we found that the conversion of C₅H₁₂ in the DBD depended only on electron-induced chemistry for the Ar/C₅H₁₂ and He/C₅H₁₂ mixtures, while it was affected by both electron-induced chemistry and thermochemistry for the N₂/C₅H₁₂ mixture. However, the consecutive reactions from the initiation to the product always depended upon the corresponding thermochemistry for a given temperature. Due to an enhanced electron density and electron energy in the Ar/C₅H₁₂ and He/C₅H₁₂ DBD, increased C₅H₁₂ conversion was observed when compared to N₂/C₅H₁₂. More importantly, cleaner products were produced from the Ar/C₅H₁₂ and He/C₅H₁₂ DBD because He and Ar were not involved in the product formation reaction pathways. We also found that extensive thermochemical involvement is favorable for enhancing the performance of plasma-assisted C₅H₁₂ reforming. Our findings may not only be useful for a greater understanding of the plasma-assisted reforming process but also helpful in designing a cost-effective plasma reformer.

Spokes, identified as the source of anomalous across-B field transport during the high-power impulse magnetron sputtering (HiPIMS) discharge, have been considered as a result of plasma instability. This paper investigates the evolution of the coupling between two azimuthal waves in HiPIMS plasma by revisiting the dispersion relation. A coupling-induced wave model is proposed for the origin of the spoke. The ion sound wave can be strongly coupled with a Doppler-shifted electron Bernstein wave, inducing a long-scale (cm size) electric field oscillation waving at the frequency difference. The spoke appears to be a collective behavior of ion rearrangement guided by the difference frequency wave along the azimuthal direction. The direct comparison of spoke characteristics, such as rotating velocity, frequency and dynamic features, given by this model with experimental and simulation results from the literature shows a good qualitative agreement. Moreover, the evolution of spoke behavior following on from the model are presented, including rotating velocity and mode number along the racetrack. Their variation tendency matches well with experimental findings, from the literature.

The results of the calculations of electron transport and rate coefficients in oxidized methane:O_2, propane:O₂ and H₂:O₂ mixtures were presented for different temperatures and reduced electric fields E/N from 1 to 3 × 10³ Td. The calculations were made for various oxidized fuel fractions. It was shown that fuel oxidation led to a drastic decrease in the average electron energy, electron drift velocity and attachment coefficient for E/N < 60 Td. At higher reduced electric fields, the effect of fuel oxidation on the average electron energy and electron drift velocity was small, whereas the attachment coefficient increased with increasing oxidation degree. In addition, in the hydrocarbon-containing mixtures for E/N ∼ 30 Td, the electron drift velocity varied nonmonotonously with the oxidation degree increase. The critical reduced electric field at which the average rate of electron production in the mixture was equal to the average attachment rate increased with fuel oxidation due to the increased attachment rate and decreased detachment rate. The effect of intermediate species on the electron transport and rate properties in partially oxidized mixtures was small. The calculated results were used to self-consistently simulate fuel oxidation and plasma characteristics for high-voltage nanosecond repetitive discharges in combustible mixtures. Zero-dimensional simulation in a H₂:O₂ mixture showed that the reduced electric field at the instant when the deposited energy peaked was close to the critical values of E/N and increased with increasing oxidized fuel fraction.

Scaling up atmospheric pressure dielectric barrier discharges is a challenging prerequisite to expanding their industrial applications. In this study, we present the results of large-volume dielectric barrier discharges (DBDs) with a parallel plate electrode configuration having an electrode surface of 50 × 200 mm² and an interelectrode gap of 5 mm. Highly dense filamentary DBD plasma was stably generated over a large area using pre-ionization electrodes embedded in the one of electrodes. We show that the role of the pre-triggering system is crucial in producing a densely distributed filamentary discharge over the electrode surface, especially at the main discharge voltage values of 14.0–18.0 kVp-p and pre-ionization voltage of 7.0–10.0 kVp-p at the same frequency, ranging from 1 to 4 kHz. By tuning the phase difference between the pre-ionization and main discharge voltages to around 180 degrees, both discharge current amplitude and duration time per period of main DBDs are maximized. The present results indicate that such a large-volume DBD is extremely versatile and suited for a wide range of industrial applications.

The interaction of deuterium atoms on a cesiated surface and the formation of hydrogen isotopologue molecules via the Eley–Rideal mechanism is studied using the computational set-up recently adopted in simulations of the same reactions for H atoms. The probability for scattering and adsorption processes on the surface as well as the mechanism underlying the reaction is shown for D atoms impinging on the surface in the same dynamical conditions previously used for H atoms. The isotopic effect in molecule formation is highlighted by considering the formation of D₂ and HD molecules; this latter is obtained by exchanging either the incoming or pre-adsorbed H atom with a D isotope. Collisional data have been determined for two different adsorption sites on the surface. The recombination probabilities and coefficients for D₂ and HD and the probabilities of other competitive surface processes have been determined. Internal states of isotopic molecules were solved and compared with those of H₂ molecules formed on the same surface showing that the heteronuclear molecules are vibrationally more excited. A strong isotope mass effect emerged in the collision of the D atom enhancing the probabilities for adsorption/desorption processes. Interestingly, a D atom impinging on the pre-adsorbed H atom on the surface increases the recombination probability, which remains low nonetheless.

Cross-sections for dissociative recombination and electron-impact vibrational excitation of the ${\mathrm{BF}}_{2}^{+}$ molecular ion are computed using a theoretical approach that combines the normal modes approximation for the vibrational states of the target ion and use of the UK R-matrix code to evaluate electron–ion scattering matrices for fixed geometries of the ion. Thermally-averaged rate coefficients are obtained from the cross-sections for temperatures in the 10–3000 K range.

A numerical simulation of a technical radio frequency inductively coupled plasma was made with three coils and coupled power in the range of 2–10 kW. The generator frequency varied from 1.76 to 13.56 MHz. The mechanisms of forming a vortex on the upstream side of the discharge and the influences of coupled power, generator frequency and flow rate of central gas on the vortex dimensions were studied. Special attention was paid to investigate two different kinds of vortex flow pattern—Benard and toroidal—as well as the vortex intensity under those two kinds of flow pattern. A critical flow rate of central gas, above which the flow pattern would transform from a Benard to a toroidal vortex, was found on the basis of force balance between the dynamic head of central cool gas and magneto-hydrodynamic and Stokes drag forces of plasma. The influences of coupled power, generator frequency, diameter of the injection probe and the compositions of central gas on the critical flow rate of the central gas were discussed. The dependences of the vortex intensity and maximum vorticity on the coupled power under different generator frequencies and flow rates of central gas were established.

This is the second of two papers presenting the study of vibrational energy exchanges in non-equilibrium CO₂ plasmas in low-excitation conditions. The companion paper addresses a theoretical and experimental investigation of the time relaxation of ∼70 individual vibrational levels of ground-state CO${}_{2}(X{}^{1}{{\rm{\Sigma }}}^{+})$ molecules during the afterglow of a pulsed DC glow discharge, operating at pressures of a few Torr and discharge currents around 50 mA, where the rate coefficients for vibration–translation (V–T) and vibration–vibration (V–V) energy transfers among these levels are validated (Silva et al 2018 Plasma Sources Sci. Technol.27 015019). Herein, the investigation is focused on the active discharge, by extending the model with the inclusion of electron impact processes for vibrational excitation and de-excitation (e-V). The time-dependent calculated densities of the different vibrational levels are compared with experimental data obtained from time-resolved in situ Fourier transform infrared spectroscopy. It is shown that the vibrational temperature of the asymmetric stretching mode is always larger than the vibrational temperatures of the bending and symmetric stretching modes along the discharge pulse—the latter two remaining very nearly the same and close to the gas temperature. The general good agreement between the model predictions and the experimental results validates the e-V rate coefficients used and provides assurance that the proposed kinetic scheme provides a solid basis to understand the vibrational energy exchanges occurring in CO₂ plasmas.

We report on Raman scattering and optical emission spectroscopy (OES) measurements in recombining atmospheric pressure plasmas of air and nitrogen. An inductively coupled plasma torch is used to create an equilibrium plasma, which is then forced to rapidly recombine by flowing through a water-cooled tube. For all conditions, temperature measurements are performed using OES and Raman scattering at the exit of tubes of varying lengths. The density of atomic nitrogen is also determined. Evidence of strong chemical nonequilibrium is found in a number of cases. For these cases, we observe that the rotational temperatures measured with OES differ from those measured with Raman scattering, and that the atomic nitrogen density is elevated with respect to equilibrium. A power balance analysis confirms that a large fraction of gas enthalpy is stored in the non-recombined nitrogen atoms. For cases where the plasma remains in equilibrium, we perform numerical simulations using the Eilmer3 computational fluid dynamics (CFD) code. Eilmer3 does not predict the observed drop in gas temperature measured using Raman scattering and OES. Prior efforts by the CFD community have also failed to correctly predict this temperature drop. The results presented in this paper are therefore intended as validation test cases for CFD simulations.

Metal pin-to-dielectric-covered electrode arrangements can be considered as a combination of corona discharge and dielectric barrier discharge. In the current study, the discharge is operated in dry air at atmospheric pressure. Sinusoidal voltage is applied to the dielectric-covered hemispherical electrode, and the metal pin electrode is grounded. Using an ICCD camera and a current probe (Rogowski coil), images and current pulses of single microdischarges (MDs) are recorded simultaneously. In addition, a time-correlated single photon counting technique is used to record the spatio-temporally resolved development of the MDs. The appearance and properties of the MDs in the two polarities of the applied voltage differ significantly. In this contribution, only the results of the negative half-cycle, i.e. anodic pin, are presented. In this half-cycle, for the voltage amplitude being applied, two MDs appear in each applied voltage cycle. The first MDs leave a positive charge on the surface of the dielectric, which has considerable influence on the propagation and properties of the second MDs, namely slower propagation of the cathode directed streamer in the dielectric vicinity, further expansion of the plasma on the dielectric surface and longer presence of the bulk plasma in the gap.

We calculate the rarefaction of Ar background gas in front of a magnetron caused by sputtered target material atoms (so called 'sputtering wind effect') during pulsed sputtering. For a detailed analysis, we use a three-dimensional particle simulation using the direct simulation Monte Carlo (DSMC) method. We compare the results with those of two volume-averaged (V-A) models describing the same problem but with significantly lower computational demands. The comparison is made for three values of flux of sputtered atoms (expressed as 'sputtering current' in the units of amperes) and for three target materials (Zr, Al and C) sputtered from a circular target of $5\,\mathrm{cm}$ diameter placed in a chamber with realistic geometry. Ar rarefaction is more pronounced for target atoms with higher mass, but the difference between the three target materials is surprisingly small. The region where Ar is significantly rarefied extends much further from the target than a typical extent of the high-density plasma confined by the magnetic field. The V-A models provide good approximation of the time evolution of the target material density in front of the target compared to the DSMC simulation. However, the presented V-A models underestimate (in all but one case) the magnitude of Ar rarefaction during the pulse-on time and also predict faster return to equilibrium during the pulse-off time comared to the DSMC simulation.

In this work, we present an analytic model for the interfacial behavior of hydroxyl radicals (OH_aq) and solvated electrons (${{{\rm{e}}}^{-}}_{{\rm{aq}}}$) delivered into an aqueous solution by an atmospheric pressure low-temperature plasma. The model yields simple scaling laws for the interfacial concentration and average penetration depth into the solution in terms of a few key operational parameters. Notably, the interfacial flux of free radicals is shown to be a key parameter, and the penetration of radicals into solution is shown to decrease as the radical flux increases. Additionally, the model sets upper limits for the average penetration on the order of 100 nm and interfacial concentration on the order of 1 mM (10¹⁷ cm⁻³).