ESTUDO DOS MODOS DE TRANSFERÊNCIA DE CORRENTE AOS CÁTODOS DAS DESCARGAS DE GÁS

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Publications

Multiple solutions in the theory of dc glow discharges

Publication . Almeida, P. G. C.; Benilov, M. S.; Faria, M. J.

Multiple steady-state solutions existing in the theory of dc glow discharges are computed for the first time. The simulations are performed in 2D in the framework of the simplest self-consistent model, which accounts for a single ion species and employs the drift–diffusion approximation. Solutions describing up to nine different modes were found in the case where losses of the ions and the electrons due to diffusion to the wall were neglected. One mode is 1D, exists at all values of the discharge current, and represents in essence the well-known solution of von Engel and Steenbeck. The other eight modes are axially symmetric, exist in limited ranges of the discharge current, and are associated with different patterns of current spots on the cathode. The mode with a spot at the centre of the cathode exhibits a well pronounced effect of normal current density. Account of diffusion losses affects the solutions dramatically: the number of solutions is reduced, a mode appears that exists at all discharge currents and comprises the Townsend, subnormal, normal and abnormal discharges. The solutions that exist in limited current ranges describe patterns, and these patterns seem to represent axially symmetric analogues of the 3D patterns observed in dc glow microdischarges in xenon.

2010Journal article

Open access

Study of stability of dc glow discharges with the use of Comsol Multiphysics software

Publication . Almeida, P. G. C.; Benilov, M. S.; Faria, M. J.

Stability of different axially symmetric modes of current transfer in dc glow discharges is investigated in the framework of the linear stability theory with the use of Comsol Multiphysics software. Conditions of current-controlled microdischarges in xenon are treated as an example. Both real and complex eigenvalues have been detected, meaning that perturbations can vary with time both monotonically and with oscillations. In general, results given by the linear stability theory confirm intuitive concepts developed in the literature and conform to the experiment. On the other hand, suggestions are provided for further experimental and theoretical work.

2011Journal article

Open access

Three-dimensional modeling of self-organization in DC glow microdischarges

Publication . 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.

2011Journal article

Open access

Formation of stationary and transient spots on thermionic cathodes and its prevention

Publication . 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.

2008Journal article

Open access

Analysing bifurcations encountered in numerical modelling of current transfer to cathodes of dc glow and arc discharges

Publication . 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.

2009Journal article

Open access