Strategic Project - UI 219 - 2011-2012

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Organizational Unit

Publications

Quenching thermal instability in the body of a thermionic arc cathode

Publication . 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.

2013Journal article

Open access

Comparing two non-equilibrium approaches to modelling of a free-burning arc

Publication . Baeva, M.; Uhrlandt, D.; Benilov, M. S.; Cunha, M. D.

Two models of high-pressure arc discharges are compared with each other and with experimental data for an atmospheric-pressure free-burning arc in argon for arc currents of 20–200 A. The models account for space-charge effects and thermal and ionization non-equilibrium in somewhat different ways. One model considers space-charge effects, thermal and ionization non-equilibrium in the near-cathode region and thermal non-equilibrium in the bulk plasma. The other model considers thermal and ionization non-equilibrium in the entire arc plasma and space-charge effects in the near-cathode region. Both models are capable of predicting the arc voltage in fair agreement with experimental data. Differences are observed in the arc attachment to the cathode, which do not strongly affect the near-cathode voltage drop and the total arc voltage for arc currents exceeding 75 A. For lower arc currents the difference is significant but the arc column structure is quite similar and the predicted bulk plasma characteristics are relatively close to each other.

2013Journal article

Open access

Stability of very-high pressure arc discharges against perturbations of the electron temperature

Publication . Benilov, M. S.; Hechtfischer, U.

We study the stability of the energy balance of the electron gas in very high–pressure plasmas against longitudinal perturbations, using a local dispersion analysis. After deriving a dispersion equation, we apply the model to a very high–pressure (100 bar) xenon plasma and find instability for electron temperatures, Te, in a window between 2400 K and 5500-7000 K, depending on the current density (106 –108 A/m2 ). The instability can be traced back to the Joule heating of the electron gas being a growing function of Te, which is due to a rising dependence of the electron atom collision frequency on Te. We then analyze the Te range occurring in very high–pressure xenon lamps and conclude that only the near-anode region exhibits Te sufficiently low for this instability to occur. Indeed, previous experiments have revealed that such lamps develop, under certain conditions, voltage oscillations accompanied by electromagnetic interference, and this instability has been pinned down to the plasma-anode interaction. A relation between the mechanisms of the considered instability and multiple anodic attachments of high-pressure arcs is discussed.

2012Journal article

Open access

Space-resolved modeling of stationary spots on copper vacuum arc cathodes and on composite CuCr cathodes with large grains

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

A self-consistent space-resolved numerical model of cathode spots in vacuum arcs is realized on the computational platform COMSOL Multiphysics. The model is applied to the investigation of stationary spots on planar cathodes made of copper or composite CuCr material with large ( 20 µm) chromium grains. The modeling results reveal a well defined spot with a structure, which is in agreement with the general theory of stationary cathode arc spots and similar to that of spots on cathodes of arcs in ambient gas. In the case of CuCr contacts with large chromium grains, spots with currents of the order of tens of amperes on copper coexist with spots on chromium with currents of the order of one or few amperes. The main effect of change of the cathode material from copper to chromium is a reduction of thermal conductivity of the cathode material, which causes a reduction of the radius of the spot and a corresponding reduction of the spot current.

2013Journal article

Open access

Modeling cathode spots in vacuum arcs burning on multi-component contacts

Publication . Benilov, M. S.; Benilova, L. G.; Cunha, M. D.; Hartmann, W.; Lawall, A.; Wenzel, N.

A self-consistent space-resolved numerical model of cathode spots in vacuum arcs is developed on the basis of the COMSOL Multiphysics software. The model is applied to cathode spots on copper-chromium (CuCr) contacts of vacuum interrupters. In the limiting case of large grains, the main effect of change in cathode material from Cu to Cr is the reduction of thermal conductivity of the cathode material, which causes a reduction of spot radius and spot current. Hence, the model indicates that spots with currents of the order of tens of amperes on Cu coexist with spots on Cr with currents between one and two amperes. The parameters of spots on small Cr grains of the order of 10 µm size are rather close to those of spots on pure Cu, whereas the parameters for spots on medium-size Cr grains of around 20 µm are quite different from those of spots on both pure Cu and pure Cr. The power flux is directed from the cathode into the plasma, i.e., it is the cathode that heats the plasma – and not the other way round. What maintains the spot is a substantial Joule heating inside the cathode bulk. About 70 percent of the heat is generated in the grain and 30 percent in the surrounding copper. One may hypothesize that such grains are highly unstable, leading to explosive-like behavior with a consequent additional loss of cathode material, and a severe limitation in spot lifetime.

2012Conference object

Open access