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- Formation of stationary and transient spots on thermionic cathodes and its preventionPublication . Almeida, Pedro G. C.; Benilov, M. S.; Cunha, M. D.Spots on cathodes of high-pressure arc discharges induced by a rapid increase in the arc current are studied numerically and experimentally. Appearance of stationary and transient spots is analysed in the context of the general pattern of steady-state modes of current transfer to thermionic cathodes and their stability. Transient spots are studied in experiments with COST-529 standard lamps. Modelling and experimental results are in reasonable agreement. A method to prevent formation of transient spots on cathodes of high-pressure arc discharges by means of short negative rectangular current pulses is proposed and validated both numerically and experimentally. Experimental indications are found that the main mechanism of blackening of burners of HID lamps that accompanies appearance of transient cathode spots is evaporation of the cathode material and not sputtering.
- Three-dimensional modeling of self-organization in DC glow microdischargesPublication . Almeida, P. G. C.; Benilov, M. S.; Faria, M. J.Three-dimensional simulations of self-organization in dc glow microdischarges are reported. The results describe a mode with a normal spot and modes with patterns of multiple spots, qualitatively similar to those observed in experiments with microdischarges in xenon.
- Self-consistent modeling of self-organized patterns of spots on anodes of DC glow dischargesPublication . Bieniek, M. S.; Almeida, P. G. C.; Benilov, M. S.Abstract Self-organized patterns of spots on a flat metallic anode in a cylindrical glow discharge tube are simulated. A standard model of glow discharges is used, comprising conservation and transport equations for a single species of ion and electrons, written with the use of the drift-diffusion and local-field approximations, and the Poisson equation. Only processes in the near-anode region are considered and the computation domain is the region between the anode and the discharge column. Multiple solutions, existing in the same range of discharge current and describing modes with and without anode spots, are computed for the first time. A reversal of the local anode current density in the spots was found, i.e. mini-cathodes are formed inside the spots or, as one could say, anode spots operate as a unipolar glow discharge. The solutions do not fit into the conventional pattern of self-organization in bistable nonlinear dissipative systems; In particular, the modes are not joined by bifurcations.
- Multiple solutions in the theory of direct current glow discharges: effect of plasma chemistry and nonlocality, different plasma-producing gases, and 3D modellingPublication . Almeida, P. G. C.; Benilov, M. S.The work is aimed at advancing the multiple steady-state solutions that have been found recently in the theory of direct current (DC) glow discharges. It is shown that an account of detailed plasma chemistry and non-locality of electron transport and kinetic coefficients results in an increase of the number of multiple solutions but does not change their pattern. Multiple solutions are shown to exist for discharges in argon and helium provided that discharge pressure is high enough. This result indicates that self-organization in DC glow microdischarges can be observed not only in xenon, which has been the case until recently, but also in other plasma-producing gases; a conclusion that has been confirmed by recent experiments. Existence of secondary bifurcations can explain why patterns of spots grouped in concentric rings, observed in the experiment, possess in many cases higher number of spots in outer rings than in inner ones.
- Computing DC discharges in a wide range of currents with COMSOL MultiPhysics: time-dependent solvers vs. stationary solversPublication . Almeida, P. G. C.; Benilov, M. S.; Bieniek, M. S.; Cunha, M. D.; Gomes, J. G. L.; Kaufmann, H. T. CostaThe benefits of the usage of stationary over time-dependent solvers of COMSOL Multiphysics in the modelling of DC discharges are explored and demonstrated using as examples glow and high pressure arc discharges; in particular, it is investigated whether time-dependent solvers can be used for a systematic computation of different modes of these discharges. It has been found that most modes of both glow and high-pressure arc discharges cannot be computed in the whole range of their existence by a time-dependent solver. Further, time-dependent solvers are unsuitable for a computation of all the states belonging to the retrograde sections of the current-voltage characteristics of the modes, so the discharge manifests hysteresis, which, in principle, can be observed in the experiment.
- Transient spots on cathodes of high-pressure arc dischargesPublication . Almeida, P. G. C.; Benilov, M. S.; Cunha, M. D.Experimental and modeling results are reported on transient spots induced on thermionic cathodes by a rapid increase of the arc current and on their prevention.
- Self-organization in dc glow microdischarges in krypton: modelling and experimentsPublication . Zhu, W.; Niraula, P.; Almeida, P. G. C.; Benilov, M. S.; Santos, D. F. N.Self-organized patterns of cathodic spots have been observed in microdischarges operated in xenon, but not in other gases. However, modelling has indicated that it is, in principle, possible to observe the patterns of spots in discharges operated in other gases provided that experimental conditions, in particular pressure, are right. In this work, self-organized patterns of cathodic spots are for the first time observed in dc glow microdischarges operated in a gas other than xenon: krypton. The experiments have been guided by the modelling. According to both the experiment and the modelling, patterns in krypton are similar to those found earlier in xenon, however occur at higher pressures.
- Analysing bifurcations encountered in numerical modelling of current transfer to cathodes of dc glow and arc dischargesPublication . Almeida, P. G. C.; Benilov, M. S.; Cunha, M. D.; Faria, M. J.Bifurcations and/or their consequences are frequently encountered in numerical modelling of current transfer to cathodes of gas discharges, also in apparently simple situations, and a failure to recognize and properly analyse a bifurcation may create difficulties in the modelling and hinder the understanding of numerical results and the underlying physics. This work is concerned with analysis of bifurcations that have been encountered in the modelling of steady-state current transfer to cathodes of glow and arc discharges. All basic types of steady-state bifurcations (fold, transcritical, pitchfork) have been identified and analysed. The analysis provides explanations to many results obtained in numerical modelling. In particular, it is shown that dramatic changes in patterns of current transfer to cathodes of both glow and arc discharges, described by numerical modelling, occur through perturbed transcritical bifurcations of first- and second-order contact. The analysis elucidates the reason why the mode of glow discharge associated with the falling section of the current–voltage characteristic in the solution of von Engel and Steenbeck seems not to appear in 2D numerical modelling and the subnormal and normal modes appear instead. A similar effect has been identified in numerical modelling of arc cathodes and explained.
- Simulation of pre-breakdown discharges in high-pressure air. I: The model and its application to corona inceptionPublication . Ferreira, N. G. C.; Santos, D. F. N.; Almeida, P. G. C.; Naidis, G. V.; Benilov, M. S.A ‘minimal’ kinetic model of plasmachemical processes in low-current discharges in high pressure air is formulated, which takes into account electrons, an effective species of positive ions, and three species of negative ions. The model is implemented as a part of numerical model of low-current quasi-stationary discharges in high-pressure air based on the use of stationary solvers, which offer important advantages in simulations of steady-state discharges compared to standard approaches that rely on time-dependent solvers. The model is validated by comparison of the computed inception voltage of corona discharges with several sets of experimental data on glow coronas. A good agreement with the experiment has been obtained for positive coronas between concentric cylinders in a wide range of pressures and diameters of the cylinders. The sensitivity of the computation results with respect to different factors is illustrated. Inception voltages of negative coronas, computed using the values of the secondary electron emission coefficient of 10−4 –10−3 , agree well with the experimental data. A simplified kinetic model for corona discharges in air, which does not include conservation equations for negative ion species, has been proposed and validated. Modelling of positive coronas in rod-to-plane electrode configuration has been performed and the computed inception voltage was compared with experimental data.
- Quenching thermal instability in the body of a thermionic arc cathodePublication . Almeida, P. G. C.; Benilov, M. S.; Cunha, M. D.; Gomes, J. G. L.The possibility of quenching the instability causing the appearance of spots on thermionic cathodes of high-pressure arc discharges is demonstrated by means of numerical simulations and experimentally. This possibility stems from the fact that the instability is of a thermal nature and therefore slow. While of significant interest by itself, this possibility may be exploited to prevent in real time the formation of cathode spots in arc devices.