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In this letter, an annular anode is designed for producing arc plasmas with a large non-equilibrium region by using a counterflow cold gas through the annular anode. The coupled mass-momentum-energy exchange processes in an argon arc plasma are studied numerically and experimentally. The counter-injection of the cold argon gas from the center of the anode leads to a steep gradient of the heavy-particle temperature due to the formation of a thin stagnation layer resulting from the interaction of the high temperature plasma with the cold gas; and in particular, a large volume non-equilibrium ‘dark’ plasma region is obtained above the anode surface. The results show that, with the enhancement of the convective heat transfer process in the plasma core region, the fraction of the non-equilibrium region to the whole arc plasma region reaches 92.2% where the heavy-particle temperature can be reduced significantly, e.g. ∼2300 K, while simultaneously, the electron temperature and number density are remained at high levels greater than 8000 K and 2.4 × 1020 m−3, respectively, under the operating condition studied in this letter. This research not only deepens the understanding to the non-equilibrium synergistic transport mechanisms of arc plasmas, but also provides a method for producing a large volume non-equilibrium plasma region so as to promote various existing applications, or even creating new applications in the future.

To gain a better understanding of the mechanism governing arc dynamics in 3D gliding arc discharge (GAD) plasma, the spatio-temporal evolution of GAD was investigated in a reverse vortex flow by a novel reactor with ring (powered electrode, PE) and truncated cone (ground electrode, GE) electrodes. A newly underlying mechanism governing arc evolution in 3D GAD was gained with combination of flow field simulation, synchronous electrical characteristics, intensified charge coupled device images and high-speed photos. The spatio-temporal analysis indicates that, being different from the well-known ignition–gliding–extinction mechanism occurring in traditional GAD, the PE arc root glides continuously in the 3D GAD, but the GE arc root features jumps from the end of the gliding path to the beginning of the next one. By means of the jumping, the arc auto-sheds the caudal part of the arc and a new arc tail is simultaneously generated, rather than rebuilding a new arc channel back to the shortest electrode gap. With this special behavior of caudal autotomy and regeneration, the main part of the arc remains for each jump. This new insight improves the understanding of the discharge mechanism governing arc evolution in 3D GAD and provides a reference for optimization design of gliding arc plasma in a vortex flow.

Recent studies have shown that thermo-field emission is a dominating electron source in microdischarges at the cathode temperature far above room temperature. However, few researches focus on the post-breakdown nature of microdischarges. In order to explore the post-breakdown characteristics of thermo-field emission driven microplasma, a one-dimensional implicit Particle-In-Cell with Monte Carlo Collision method is adopted and updated by considering the thermo-field electron emission to investigate the kinetic characteristics of direct-current argon microdischarges at atmospheric pressure. The fundamental properties of the microplasmas, such as electric field, particle number density, averaged particle temperature and current density, are analyzed in the post-breakdown regime. In addition, the sheath behavior is investigated to further observe how the space charge affects the thermo-field emission. The results indicate that thermo-filed emission driven micro-scale discharges can produce high current density and high-energy particles with low applied voltage of 20V. The impacts of cathode temperature on enhancing the thermo-field emission are more pronounced compared to the applied voltage and the electrode spacing. The electron energy probability function shows a multi-peak distribution.

Numerical simulation of a high-pressure arc discharge has been performed with a self-consistent modelling of most of the components, including the electrodes, and the interactions between them. In particular, the arc column and the cathodic part of the discharge are simulated by means of a two-temperature hydrodynamic model and of a model of nonlinear surface heating, respectively. Simulation results are given for a free-burning arc in atmospheric-pressure argon in the range of arc currents from 10 to 200 A. It is found that the electric power deposited into the near-cathode layer is transported not only to the cathode but also to the arc column, an effect that cannot be described by the local thermodynamic equilibrium (LTE) model. The electron enthalpy transport substantially exceeds the net contribution of thermal conduction by the electrons and heavy particles and is thus the dominating mechanism of energy transfer from the near-cathode layer to the arc column. The predicted gas temperatures along the arc axis in the arc column using the LTE model are much higher than the calculated electron and heavy-particle temperatures (~1000–2000 K or higher) under the same operation conditions using the non-equilibrium model with the consideration of the near-cathode sheath for the cases studied in the present paper. Studies on the influences of the cathode shapes and metal vapour contaminations on the arc characteristics will be conducted in future work.

In this paper, physical/mathematical models for the three-dimensional, quasi-steady modelling of the plasma flow and heat transfer inside a non-transferred DC arc plasma torch are described in detail. The Steenbeck's minimum principle (Finkelnburg W and Maecker H 1956 Electric arcs and thermal plasmas Encyclopedia of Physics vol XXII (Berlin: Springer)) is employed to determine the axial position of the anode arc-root at the anode surface. This principle postulates a minimum arc voltage for a given arc current, working gas flow rate, and torch configuration. The modelling results show that the temperature and flow fields inside the DC non-transferred arc plasma torch show significant three-dimensional features. The predicted anode arc-root attachment position and the arc shape by employing Steenbeck's minimum principle are reasonably consistent with experimental observations. The thermal efficiency and the torch power distribution are also calculated in this paper. The results show that the thermal efficiency of the torch always ranges from 30% to 45%, i.e. more than half of the total power input is taken away by the cathode and anode cooling water. The special heat transfer mechanisms at the plasma–anode interface, such as electron condensation, electron enthalpy and radiative heat transfer from the bulk plasma to the anode inner surface, are taken into account in this paper. The calculated results show that besides convective heat transfer, the contributions of electron condensation, electron enthalpy and radiation to the anode heat transfer are also important (~30% for parameter range of interest in this paper). Additional effects, such as the non-local thermodynamic equilibrium plasma state near the electrodes, the transient phenomena, etc, need to be considered in future physical/mathematical models, including corresponding measurements.

Three-dimensional (3D) modelling results are presented concerning a direct current (dc) non-transferred arc plasma torch with axisymmetrical geometrical configuration and axisymmetrical boundary conditions. It is shown that the arc is locally attached at the anode surface of the plasma torch, and the heat transfer and plasma flow within the torch are of 3D features. The predicted arc root location at the anode surface and arc voltage of the torch are very consistent with corresponding experimental results.

Electrical characteristics of a 1 cm-long free-burning atmospheric-pressure argon arc are calculated by means of a model taking into account the existence of a near-cathode space-charge sheath and the discrepancy between the electron and heavy-particle temperatures in the arc column. The computed arc voltage exhibits a variation with the arc current I similar to the one revealed by the experiment and exceeds experimental values by no more than approximately 2 V in the current range 20–175 A. The sheath contributes about two-thirds or more of the arc voltage. The LTE model predicts a different variation of the arc voltage with I and underestimates the experimental values appreciably for low currents but by no more than approximately 2 V for I ≳ 120 A. However, the latter can hardly be considered as a proof of unimportance of the space-charge sheath at high currents: the LTE model overestimates both the resistance of the bulk of the arc column and the resistance of the part of the column that is adjacent to the cathode, and this overestimation to a certain extent compensates for the neglect of the voltage drop in the sheath. Furthermore, if the latter resistance were evaluated in the framework of the LTE model in an accurate way, then the overestimation would be still much stronger and the obtained voltage would significantly exceed those observed in the experiment.

In this paper, stable atmospheric-pressure argon, nitrogen or air glow discharges driven by a radio-frequency power supply with water-cooled, bare copper electrodes are obtained by employing a newly-designed plasma generator. Due to the existence of the intensified local electric field by using an insulated tungsten wire between the two bare copper electrodes, which is also verified by the modelling results, the experimental measurements show that stable glow discharges of argon, nitrogen or air can be obtained following a local breakdown process in the vicinity of the tungsten wire under lower applied voltages between the electrodes.