Institute for Plasmas and Nuclear Fusion

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Organizational Unit

Publications

Bifurcations in the theory of current transfer to cathodes of dc discharges and observations of transitions between different modes

Publication . Bieniek, M. S.; Santos, D.; Almeida, P. G. C.; Benilov, M. S.

General scenarios of transitions between different spot patterns on electrodes of dc gas discharges and their relation to bifurcations of steady-state solutions are analyzed. In the case of cathodes of arc discharges, it is shown that any transition between different modes of current transfer is related to a bifurcation of steady-state solutions. In particular, transitions between diffuse and spot modes on axially symmetric cathodes, frequently observed in the experiment, represent an indication of the presence of pitchfork or fold bifurcations of steady-state solutions. Experimental observations of transitions on cathodes of dc glow microdischarges are analyzed and those potentially related to bifurcations of steady-state solutions are identified. The relevant bifurcations are investigated numerically and the computed patterns are found to conform to those observed in the course of the corresponding transitions in the experiment.

2018-02-20Journal article

Open access

Self-consistent modeling of self-organized patterns of spots on anodes of DC glow discharges

Publication . 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.

2018Journal article

Open access

Computing Different Modes on Cathodes of DC Glow and High-Pressure Arc Discharges: Time-Dependent Versus Stationary Solvers

Publication . 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.

2017Journal article

Open access

Modelling cathode spots in glow discharges in the cathode boundary layer geometry

Publication . Bieniek, M. S.; Almeida, P. G. C.; Benilov, M. S.

Self-organized patterns of cathode spots in glow discharges are computed in the cathode boundary layer geometry, which is the one employed in most of the experiments reported in the literature. The model comprises conservation and transport equations of electrons and a single ion species, written in the drift-diffusion and local-field approximations, and Poisson’s equation. Multiple solutions existing for the same value of the discharge current and describing modes with different configurations of cathode spots are computed by means of a stationary solver. The computed solutions are compared to their counterparts for plane-parallel electrodes, and experiments. All of the computed spot patterns have been observed in the experiment.

2016Journal article

Open access

Modeling spots on composite copper-chromium contacts of vacuum arcs and their stability

Publication . Benilov, Mikhail S.; Cunha, Mário D.; Hartmann, Werner; Kosse, Sylvio; Lawall, Andreas; Wenzel, Norbert

Cathode spots on copper–chromium contacts of vacuum interrupters are simulated by means of a self-consistent space-resolved numerical model of cathode spots in vacuum arcs developed on the basis of the COMSOL Multiphysics software. Attention is focused on spots attached to Cr grains in the Cu matrix in a wide range of values of the ratio of the grain radius to the radius of the spot. In the case where this ratio is close to unity, parameters of spot are strongly different from those operating on both pure-copper and pure-chromium cathodes; in particular, the spot is maintained by Joule heat generation in the cathode body and the net energy flux is directed from the cathode to the plasma and not the other way round. An investigation of stability has shown that stationary spots are stable if current controlled. However, under conditions of high power circuit breakers, where the near-cathode voltage is not affected by ignition or extinction of separate spots, the spots are unstable and end up either in explosive-like behavior or in destruction by thermal conduction. On the other hand, spots live significantly longer-up to one order of magnitude-if the spot and grain sizes are close; else, typical spot lifetimes are of the order of 10 µs. This result is very interesting theoretically and may explain the changes in grain size occurring in the beginning of the lifetime of contacts of high-power current breakers. A sensitivity study has shown that variations in different aspects of the simulation model produce quantitative changes but do not affect the results qualitatively.

2015Journal article

Open access