Direct current microdischarges in metal vapours of cadmium and zinc occurring during contact separation in a test apparatus related to explosion protection were investigated and compared in detail. Their characterisation was done in a framework that combined a unified nonequilibrium plasma model and a collisional-radiative model (CRM). The plasma model provided the basic plasma parameters (number densities of ground state neutral and singly charged species and their energies, electric field, discharge voltage). Number densities of excited atomic states were obtained on a second stage employing a CRM and the already known plasma parameters. This modelling framework allowed one to obtain spatially and temporally resolved plasma parameters and the population of the excited atomic states in the entire discharge gap during the separation of the electric contacts. The modelling work was supported by electrical measurements, high-speed imaging and optical emission spectroscopy. The experimental findings enabled the calibration of the plasma model with respect to the discharge voltage and a qualitative and to some extent quantitative comparison of computed and measured spectral intensities.

Plasma can remediate nitrogen oxides (NOx) emission from combustion processes. Nitric oxide (NO) oxidation reactions have been studied extensively. However, NO decomposition by plasma in a non-oxidizing environment, the timescales for NO dissociation reactions and their coupling with transport processes, the focus of this work, remains relatively unexplored. We report on axially resolved laser-induced fluorescence measurements of NO densities in a plasma in helium (He) with small admixtures of NO generated in a capillary tube by two outer ring electrodes, where one ring is powered by radio frequency (rf) high voltage. A limited number of chemical reactions describe the He/NO model system, and this description of the plasma chemistry allows for the quantification of the kinetic and transport mechanisms associated with the plasma-mediated NO decomposition. A 1D plug flow model shows the dominant role of electrons in addition to helium metastable species, and atomic nitrogen for NO decomposition, while the effect of metastables is largely counteracted by the NO+ recombination at the capillary wall, yielding a significant source of NO. A 2D reaction transport model assuming a given distribution of short-lived species in the plasma zone is also able to describe the NO recovery experimentally found in the plasma effluent. The observed NO recovery as a result of inhomogeneous NO decomposition by short-lived species and the resulting radial transport underlines the limitations of the widely used plug flow approximation for such systems.

The collection of contributions from the Leibniz Institute for Plasma Science and Technology presents findings from experimental and modelling studies of the physical and chemical processes in non-thermal and thermal plasmas produced by various kinds of plasma sources. The characterisation and properties of micro-scale discharges and suitable gases for switching technologies are explored in view of their applications. Contributions to plasma medicine as an innovative research field focus on cold atmospheric-pressure plasma sources for specific biomedical applications. The development and optimisation of plasma-based processes is considered in works focused on bioeconomy and waste treatment. Finally, a modern concept of research data management for low-temperature plasmas demonstrates the digital workflows being developed to link metadata standards, laboratory notebooks and data repositories.

Microdischarges in cadmium vapour occur in a testing equipment for safety assessment of electric devices for explosion protection. In this work, a unified non-equilibrium model is employed to obtain the plasma properties for a current of 60 mA and gap lengths from 20 up to 160 µm corresponding to conducted experiments. The predicted voltage as a function of the discharge length agrees well with the measured values. The model provides the heat generation relevant to the ignition of a gas mixture.

In this work, we focus on the electrical properties of arcs of length of a few millimeters at a current level of 2 A. The computation is based on a unified non-equilibrium model that resolves the entire inter-electrode region, applies a deformed mesh to simulate the contact opening, and couples the heat transfer to the electrodes. The arcs are burning in atmospheric pressure air at the presence of Cu metal vapour. Experimental findings are used to calibrate the model.

A fluid model of direct current microarcs in copper metal vapour dominated air is developed to resemble the microarcs that occur at low-voltage and low-current conditions in switching devices during a contact separation. The model is capable of predicting the basic plasma parameters, including the electric potential, the temperatures of electrons and heavy particles, the number densities of charged and neutral particles in the spatially resolved inter-electrode region, the heating of the electrodes as well as the release of copper metal vapour from the electrodes. The model predicts a positive anode fall and an increase of the electron temperature in the proximity of the anode.

Low temperature plasma jets at atmospheric pressure with helium and various admixtures are currently studied by modelling and experiments related to plasma catalysis in confined spaces of devices for automotive exhaust treatment. In a first step a mixture of helium and water was investigated. A global model with 45 species and 636 reactions has been accomplished and extended to a plug flow model by converting the temporal evolution of a volume element flowing in the capillary into a spatial distribution. The model comprises rate equations for all species, the electron energy balance and the heat equation. Furthermore, diffusion to the wall, surface reactions and lateral heat flux are taken into account in the framework of the global model. A spatial profile is chosen to account for the inhomogeneous power supply from the RF electrodes. Experimentally, the density of OH radicals and the gas temperature have been determined by means of laser induced fluorescence. The model provides the density profiles over the plug flow distance of all species including ground state neutrals, metastables, and charged particles. Reasonable agreements between calculated and measured results have been obtained for the OH number densities and the gas temperature for different power values. Furthermore, the model delivered maximal values of about $5 \times 10^{18} \mathrm{~m}^{-3}$ for the electron density and about 2.5 eV for the electron mean energy. The dominant production and loss channels of OH radicals have been analyzed for different power cases. The results showing the influence of the H 2 O concentration on the values of the mean electron energy, gas temperature, the number densities of electrons and OH radicals, as well as the chemistry related to their production and loss will be reported on.

The electrical characteristic of arcs sensitively depends on many factors like electrode material and shape, working gas and gas pressure. Arc sheath voltages and electrode resistance have to be considered in particular for shorter arcs. The arc voltage behaviour is important to the switching performance. But its knowledge also allows to estimate the power consumption of the arc and the heat transferred to the electrodes. Arc voltage models are easy to integrate in power grid simulations and benefitial for the design of arc power sources. Whereas specific arc voltage models are available meanwhile for many examples, there are still knowledge gaps for arcs in a wide range of parameters. This paper provides a review of recently developed electric arc models for high and low voltage switching as well as for welding with the focus on vacuum arcs, short arcs and arcs at low current.

Plasma spray torches are widely used in industrial applications concerning the deposition of protective and functional coatings. The quality and the properties of the produced coatings result from every link on the entire chain. In this work, the processes in the jet plume of a plasma spray torch are analysed and the formation of the coating is simulated. Studies on the plasma properties in the DC torch and in the plume have reported in [1, 2]. The plasma spray torch (F4MB-XL, Oerlikon Metco) is operated with a direct current of 600 A at atmospheric pressure in pure Ar (40 NLPM) and in a mixture $\mathrm{Ar} / \mathrm{H}_{2}$ ($40 / 14 \mathrm{NLPM}$). Al 2 O 3 particles of size (5-120) $\mu \mathrm{m}$ are injected into the plasma jet. The target is placed 12 cm away from and perpendicularly to the jet axis. The powder material is fed by a carrier gas (Ar) with a flow rate of 3.4 NLPM perpendicularly to the torch axis and parallel to the nozzle exit so that it does not affect the plasma properties. The studies performed in pure Ar and in Ar/H 2 mixture show that, the plasma jet is narrower in the mixture and it is characterized by a more rapid decrease of the temperature. In front of the substrate, the temperature of the mixture is about 1750 K and about 320 K higher than that in argon. In particular, a spatially fluctuating heating of the injected particles occurs in the Ar/H 2 mixture. The computed accumulation of droplets on the substrate is found to be less scattered during operation in Ar/H 2 so that coating of larger thickness can be obtained in comparison to pure Ar.

Microdisharges in Cadmium metal vapour occur in a testing equipment for the safety assessment of electric devices for explosion protection, where they represent an ignition source in a hydrogen/air mixture ($\varphi=21 \% \mathrm{H}_{2}$). In this work, electric discharges in a metal vapour of Cadmium released from the cathode during contact opening are studied by means of a unified one-dimensional model [1]. The model employs the drift-diffusion approximation of the electron kinetics, the general species equations for atoms and ions of Cadmium, as well as the heat transfer in the plasma and the electrodes. The electron current density due to a field emission from the cathode is computed applying the transfer matrix method [2]. Results are obtained for electrode gap lengths of ($60-160) \mu \mathrm{m}$ and compared with the voltage measured in related experiments [3]. For a constant electric current of 60 mA, the discharge voltage amounts to about (12-30) V depending on the gap length. The model provides the spatial structure of the discharge: the lengths of the space-charge sheath adjacent to the electrodes, species densities, different temperatures of electrons and heavy particles indicating a deviation from local thermodynamic equilibrium.

The radiative heat transfer in arc plasma models is considered from the point of view of its description in terms of a net emission coefficient, the method of spherical harmonics in its lowest order, and the discrete ordinate method. Net emission coefficients are computed, applying approximate analytical and numerical approaches and a multi-band representation of the spectral absorption coefficient with three kinds of its averaging and two datasets. Self-consistent access to the radiative heat transfer is applied to a two-dimensional axisymmetric model of a free-burning arc in argon at atmospheric pressure. The results obtained from the models employing the net emission coefficient, the method of spherical harmonics, and the discrete ordinate method are compared.

A unified one-dimensional model of an arc plasma in air, between copper electrodes, that includes the change of the gap distance is presented. The occurrence of multiple reversals of the electric field and the anode voltage drop is observed. The evolution of the spatial distribution of the electron and heavy particle temperatures with the gap distance and the opening speed is also studied. The model quantitatively predicts a number of plasma properties under conditions that are relevant to the contact separation in low-voltage switching devices.

The radiation from the arc plasma ignited between the flat ends of copper electrodes in atmospheric pressure air at a direct current of 3.5 A is analyzed to determine the plasma temperature and composition. Side-on spectra in the range (430 – 650) nm are recorded using a CMOS camera connected to a spectrograph. The spectral emission coefficient is evaluated from the spectral radiance, which provides the radiator number density. The excitation temperature is obtained by applying the Boltzmann plot technique to at least two spectral lines and a series of radial positions in the midplane of the arc. Subsequently, the plasma composition is determined. The equation of radiative transfer is solved along lines of sight considered in the experiment. The spectral intensity of Cu I lines is computed and compared with the experimental ones. The arc properties are obtained by assuming axial symmetry and mapping the evaluated values in the midplane.

The radiative heat transfer in arc plasma models of a free-burning arc and a plasma torch in atmospheric pressure argon is taken into account in a self-consistent way. This is realized by the P1 method for solving the equation of radiative transfer and the multi-band approximation that considers the division of the emitted spectrum into a number of spectral bands. Net emission coefficients are evaluated by solving the equation of radiative transfer in three dimensions in an isothermal cylindrical plasma. The arc plasma parameters of the free-burning arc and the plasma torch obtained accounting for radiative transport have been compared with those from the temperature-dependent net emission coefficient for a radius of 1 mm. The results show that in general, the models applying the net emission coefficient provide results in the arc core close to that using the P1 method. The discrepancy is stronger in the arc periphery and near walls, where the P1 method predicts absorption of radiation.

Electric arcs generated by transient lightning-type surge currents in protection devices for low voltage appliances are studied by optical emission spectroscopy and spectra simulations. A surge pulse amplitude of 5kA and 8/20µs shape are applied. The arc radiation is recorded by a 3/4m spectrometer and a high-speed camera equipped with metal interference filters for the O I (777nm) and Hα (656nm) lines. Absolute calibration was realized using a tungsten strip lamp. The arc images indicate a non-symmetrical shape. To determine the plasma properties, accompanying simulation solving the equation of radiative transfer for a given pressure and a temperature profile was carried out assuming local thermodynamic equilibrium. Line broadening due to the Stark effect is taken into account. The computed and measured spectra are compared. The conditions are varied until the measured and computed spectra match.

The lifetime of tungsten cathodes used in plasma spray torches is limited by processes leading to a loss of cathode material. It was reported in the literature that the mechanism of their erosion is the evaporation. A model of the ionization layer of a cathode is developed to study the diffusive transport of evaporated tungsten atoms and tungsten ions produced due to ionization by electron impact in a background argon plasma. It is shown that the Stefan–Maxwell equations do not reduce to Fick law as one could expect for the transport of diluted species, which is due to significant diffusion velocities of argon ions. The ionization of tungsten atoms occurs in a distance of a few micrometers from the cathode surface and leads to a strong sink, which increases the net flux of tungsten atoms far beyond that obtained in absence of tungsten ions. This shows that the tungsten ions are driven by the electric field towards the cathode resulting in no net diffusive flux and no removal of tungsten species from the ionization layer even if convection is accounted for. A possible mechanism of removal is found by extending the model to comprise an anode. The extended model resolves the inter-electrode region and provides the plasma parameters for a current density corresponding to the value at the center of the cathode under typical arc currents of 600 A and 800 A. The presence of the anode causes a reversal of the electric field on the anode side, which pulls the ions away from the ionization layer of the cathode. The net flux of tungsten ions can be further fortified by convection. This model allows one to evaluate the loss of cathode material under realistic operating conditions in a quantitative agreement with measured values.

The work is concerned with the effect of a spatially fluctuating heating of Al2O3 particles with diameters of 5–120 μm during a plasma spray process. A plasma jet is generated in a mixture of Ar (40 NLPM) and H2 (14 NLPM) and in pure Ar at an electric current of 600 A. The tracing of the injected particles in the plume region of the plasma jets is considered in the framework of a three-dimensional model taking into account a turbulent fluid flow. It is shown that the heat source for the injected particles exhibits a well pronounced spatially fluctuating structure due to the enhancement of the thermal conductivity resulting from dissociation and ionization of the molecular gas in the temperature range of 2500–4000 K and 13,000–14,000 K, respectively. During their travel towards the substrate, the particles are therefore repeatedly heated in the gas mixture in contrast to the case of pure argon. Particles injected in the gas mixture reach the substrate with a higher average temperature and velocity.

This work presents results of a self-consistent modelling analysis on microwave plasma generated in Ar–O 2 mixtures at a frequency of 2.45 GHz at atmospheric pressure. The study focuses on how the plasma properties are influenced by the increase of the oxygen fraction in the gas mixture. The oxygen admixture is increased from 1% up to 95% in mass for values of the input microwave power of 1 and 1.5 kW. The results show that for a power of 1 kW and gradually increasing the oxygen admixture from 1% to 25% the electron density drops by a factor of more than four due to the energy lost by the electrons due to dissociation of oxygen molecules and the gas heating. An analysis of the number densities of species produced in the Ar–O 2 plasma is presented. Oxygen admixtures of above 50% are considered along with an increase of the input microwave power in order to supply the discharge with electron number density values of the order of 10 19 m −3 . Gas temperatures above 3700 K are obtained in the plasma core along with a strong production of oxygen atoms with a number density of the order of 10 23 m −3 .