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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.
- 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.
- Investigation of modes of current transfer to cathodes of glow and arc dischargesPublication . Almeida, Pedro Jorge Gomes Camacho de; Benilov, MikhailThis work is dedicated to modelling of di⁄erent modes of current transfer to cathodes of glow discharges and to experimental investigation of formation of transient spots on thermionic arc cathodes. Multiple steady-state solutions in the theory of DC glow discharge have been found for the rst time. The modelling was performed in the framework of the basic model of glow discharges and also in the framework of a more detailed model. The basic model comprises equations of conservation of a single ion species and the electrons, transport equations for the ions and the electrons written in the local approximation, and the Poisson equation. The detailed model takes into account atomic and molecular ions, atomic excited states, excimers, and non-locality of electron transport and kinetic coe¢ cients. These multiple solutions describe modes with a normal spot as well as modes with patterns of several spots similar to those observed in experiments with microdischarges in xenon. It is shown that modes with more than one spot can, in principle, occur not only in xenon but also in other plasma-producing gases. Formation of transient spots on thermionic cathodes was studied in experiments with COST529 standard HID lamps. There is a generally good agreement between experiment and results of numerical modelling. A possibility of prevention of appearance of transient spots by means of a brief reduction of the arc current shortly after the initial current increase is proposed and justied. It is shown 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.
- Computing Different Modes on Cathodes of DC Glow and High-Pressure Arc Discharges: Time-Dependent Versus Stationary SolversPublication . Almeida, Pedro G. C; Benilov, Mikhail S.; Cunha, Mário D.; Gomes, José G. L.Complex behavior can appear in the modeling of gas discharges even in apparently simple steady-state situations. Time-dependent solvers may fail to deliver essential information in such cases. One of such cases considered in this work is the 1D DC discharge. The other case is represented by multiple multidimensional solutions existing in the theory of DC discharges and describing modes of current transfer with different patterns of spots on the cathodes. It is shown that, although some of the solutions, including those describing beautiful self organized patterns, can be computed by means of a time-dependent solver, in most examples results of time-dependent modeling are at best incomplete. In most examples, numerical stability of the time dependent solver was not equivalent to physical stability.
- 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.
- A simple model of distribution of current over cathodes of vacuum circuit breakersPublication . Cunha, Mário D.; Wenzel, Norbert; Almeida, Pedro G. C.; Hartmann, Werner; Benilov, Mikhail S.There are several hundreds of spots operating simultaneously on cathodes of vacuum arcs in high-power vac uum circuit breakers. In this work, the spot distribution along the contact surface is simulated by means of an approach that is based on the concept of surface density of spots and represents a natural alternative to tracing individual spots. An equation governing the evolution of the surface density of the spots or, equivalently, the distribution of macroscopic (averaged over individual spots) current density over the cathode is obtained by generalizing the concept of random walk of a single cathode spot in low-current vacuum arcs. The model relies on empirical parameters characterizing individual spots (the diffusion coefficient of the random motion of cathode spots and the velocity of drift superimposed over the random motion), which may be taken from experiments with low-current arcs, and does not involve adjustable parameters. The model is simple and physically transparent and correctly reproduces the trends observed in the experiments under conditions where the cathode arc attachment is diffuse. The distribution of the macroscopic current density on the cathode, given by the model, represents the boundary condition that is required for existing numerical models of vacuum arcs in high-power vacuum circuit breakers.
- 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.