Browsing by Author "Benilov, Mikhail"
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- Ionization layer with collision-free atoms at the edge of partially to fully ionized plasmasPublication . Benilov; Benilov, MikhailAbstract When a hot arc spot has just formed on the cathode surface, e.g. in the course of arc ignition on a cold cathode, a significant part of the current still flows in the glow-discharge mode to the cold surface outside the spot. The near-cathode voltage continues to be high at all points of the cathode surface. The mean free path for collisions between the atoms and the ions within the plasma ball near the spot is comparable to, or exceeds, the thickness of the ionization layer, which is a part of the near-cathode non-equilibrium layer where the ion current to the cathode is generated. The evaluation of the ion current to the cathode surface under such conditions is revisited. A fluid description of the ion motion in the ionization layer is combined with a kinetic description of the atom motion. The resulting problem admits a simple analytical solution. Formulas for the evaluation of the ion current to the cathode for a wide range of conditions are derived and the possibilities of using these formulas to improve the accuracy of existing methods for modeling high-pressure arc discharges in relation to glow-to-arc transitions are discussed.
- Model of non-equilibrium near-cathode plasma layers for simulation of ignition of high-pressure arcs on cold refractory cathodesPublication . Santos, D. F. N.; Almeida, N. A.; Benilova, L. G.; Benilov, M; Santos, Diego; Benilov, MikhailAbstract The introduction of secondary ion-electron emission into an approximate model of non-equilibrium plasma layers on hot (thermionic) cathodes of high-pressure arc discharges allows extending the model to low cathode surface temperatures. Analysis of evaluation results shows that the extended model describes glow-like discharges on cold cathodes and thermionic arc discharges on hot cathodes, as it should. In the course of glow-to-arc transitions on cold cathodes, a transient regime occurs where a hot arc spot has just formed and a significant fraction of the current still flows to the cold surface outside the spot, so that the near-cathode voltage continues to be high. The power input in the near-cathode layer is very high in this regime, and so is the electron temperature in the near-cathode region. The mean free path for collisions between the atoms and the ions in these conditions exceeds the thickness of the layer where the ion current to the cathode is generated. A new method for evaluation of the ion current under such conditions is implemented. The developed model is applicable for cathode surface temperatures below the boiling point of the cathode material and may be used for multidimensional simulations of ignition of high-current arcs on refractory cathodes.
- Numerical and experimental investigation of thermal regimes of thermionic cathodes of arc plasma torchesPublication . Cunha, M. D.; Sargsyan, M. A.; Gadzhiev, M. Kh; Tereshonok, D. V .; Benilov, M. S .; Benilov, Mikhail; Cunha, MárioThe modelling method based on decoupling the simulation of the cathodic part of the arc (the cathode and the near-cathode non-equilibrium plasma layer) from the simulation of the arc on the whole has been extended to cathodes of arc plasma torches, consisting of an insert with a conical tip, made of pure or doped tungsten, and a surrounding water-cooled copper holder. The method was validated by comparison with the experiment, performed on a 200 A DC arc in atmospheric-pressure argon. Standard work function of polycrystalline tungsten of 4.54 eV was used for modelling of pure-tungsten insert and a good agreement with the experiment was found with respect to both the insert tip shape and the temperature distribution in the tip, recorded in the stable operation mode. There are no unambiguous data on the work function for arc cathodes made of doped tungsten, although in situ measurements of the effective work function of cathodes of high-pressure arc discharges provide useful hints. On the other hand, the experiments reported in this work show that the tip temperatures of inserts made of tungsten doped with 1.5% of thorium, or lanthanum, or yttrium, recorded during the stable-mode operation at the arc current of 200 A, vary in a rather narrow range 3100–3200 K. This suggests that the work functions of doped tungsten inserts, operated in the stable mode, are close to each other as well. Indeed, the results of modelling with the same value of the work function of 3 eV give a reasonably good agreement with the experiment in all three cases.