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Advanced modeling of plasma-cathode interaction in vaccum and unipolar arcs
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Abstract(s)
Este trabalho Ø dedicado modelaªo da interacªo plasma-cÆtodo em descargas de arco em vÆcuo e em descargas de arco unipolares. Pela primeira vez foi desenvolvido um modelo numØrico detalhado de manchas catdicas solitÆrias em descargas de arco em vÆcuo. O modelo leva em conta todos os mecanismosrelevantesdosfenmenosfsicosdemanchascatdicas: obombardeamento dasuperfciedocÆtodoporiıesprovenientesdeumplasma prØ-existente; avaporizaªo do material do cÆtodo na mancha, a ionizaªo deste material vaporizado e a interacªo do plasma produzido com o cÆtodo; desenvolvimento do efeito de Joule no interior do cÆtodo; fusªo do material do cÆtodo e movimento do metal fundido sob o efeito da pressªo exercida pelo plasma e da fora de Lorentz; a deformaªo da superfcie fundida do cÆtodo; a formaªo de crateras e jactos de metal fundido; a ejecªo de gotas. Os resultados da modelaªo permitem identicar as diferentes fases da vida de uma mancha solitÆria. A emissªo de electrıes da superfcie do cÆtodo e o transporte de calor por convecªo sªo os mecanismos dominantes de arrefecimento na mancha catdica, limitando deste modo a temperatura mÆxima possvel no cÆtodo. A formaªo de crateras na superfcie do cÆtodo ocorre sem explosıes, seguida da formaªo de um jacto de metal fundido e da ejecªo de uma gota. Os resultados da modelaªo sªo concordantes com estimativas efectuadas para diferentes mecanismos de erosªo do cÆtodo, com base nos dados experimentais relativos erosªo em cÆtodos de cobre de descargas de arco em vÆcuo. A fase inicial de uma descarga de arco unipolar em condiıes relevantes para a fusªo nuclear em reactores tokamak foi investigada no mbito do modelo detalhado de manchas catdicas solitÆrias em descargas de arco em vÆcuo. Mais concretamente, a interacªo de um uxo intenso de energia com uma placa de tungstØnio imersa num plasma de hØlio e a correspondente transferŒncia de corrente foi estudada em condiıes baseadas em experiŒncias laboratoriais. Uma vez que o arco Ø de natureza unipolar, a transferŒncia de corrente fora da mancha Ø tida em consideraªo no modelo utilizado e a variaªo da diferena de potencial entre o plasma e a placa Ø avaliada a partir da condiªo de corrente total nula transferida para a placa a cada instante. Os resultados da modelaªo revelam a formaªo de uma cratera, mas sem a formaªo de um jacto de metal fundido ou a ejecªo de uma gota. A modelaªo Ø realizada para diferentes condiıes e demonstra-se que Ø necessÆrio ter em consideraªo a limitaªo da corrente terminica de electrıes pela banha de carga de espao.
This work is dedicated to modeling of the plasma-cathode interaction in vacuum and in unipolar arcs. A detailed numerical model of individual cathode spots in vaccum arcs has been developed for the rst time. The model takes into account all the relevant mechanisms of the physics of cathode spots: the bombardment of the cathode surface by ions coming from a pre-existing plasma cloud; vaporization of the cathode material in the spot, its ionization, and the interaction of the produced plasma with the cathode; Joule heat generation in the cathode body; melting of the cathode and motion of the molten metal under the e⁄ect of the plasma pressure and the Lorentz force; the change in shape of the molten cathode surface; the formation of craters and liquid-metal jets; the detachment of droplets. The simulation results allow the identication of the di⁄erent phases of life of an individual spot. Electron emission cooling and convective heat transfer are dominant mechanisms of cooling in the spot, limiting the maximum temperature of the cathode. Craters are formed on the surface without explosions, followed by the the formation of a liquid-metal jet and the ejection of a droplet. The modeling results conform to estimates of di⁄erent mechanisms of cathode erosion derived from the experimental data on the net and ion erosion of copper cathodes in vacuum arcs. The initial stage of unipolar arcing in fusion-relevant conditions was investigated in the framework of the detailed model of cathode spots in vacuum arcs. In particular, the interaction of an intense heat ux with and current transfer to a tungsten metal plate immersed in a helium background plasma is studied in conditions based on experiments. Since the arc is unipolar, the model is supplemented with an account of current transfer outside the arc attachment and the potential di⁄erence between the plasma and the plate is evaluated from the condition of the net current transferred to the plate being zero at each moment. The simulation results reveal the formation of a crater, but no jet formation or droplet detachment. Simulations are performed for di⁄erent sets of conditions, and it is found that in order for the developed model to be applicable to real experimental situations, space-charge limited thermionic electron emission must be considered.
This work is dedicated to modeling of the plasma-cathode interaction in vacuum and in unipolar arcs. A detailed numerical model of individual cathode spots in vaccum arcs has been developed for the rst time. The model takes into account all the relevant mechanisms of the physics of cathode spots: the bombardment of the cathode surface by ions coming from a pre-existing plasma cloud; vaporization of the cathode material in the spot, its ionization, and the interaction of the produced plasma with the cathode; Joule heat generation in the cathode body; melting of the cathode and motion of the molten metal under the e⁄ect of the plasma pressure and the Lorentz force; the change in shape of the molten cathode surface; the formation of craters and liquid-metal jets; the detachment of droplets. The simulation results allow the identication of the di⁄erent phases of life of an individual spot. Electron emission cooling and convective heat transfer are dominant mechanisms of cooling in the spot, limiting the maximum temperature of the cathode. Craters are formed on the surface without explosions, followed by the the formation of a liquid-metal jet and the ejection of a droplet. The modeling results conform to estimates of di⁄erent mechanisms of cathode erosion derived from the experimental data on the net and ion erosion of copper cathodes in vacuum arcs. The initial stage of unipolar arcing in fusion-relevant conditions was investigated in the framework of the detailed model of cathode spots in vacuum arcs. In particular, the interaction of an intense heat ux with and current transfer to a tungsten metal plate immersed in a helium background plasma is studied in conditions based on experiments. Since the arc is unipolar, the model is supplemented with an account of current transfer outside the arc attachment and the potential di⁄erence between the plasma and the plate is evaluated from the condition of the net current transferred to the plate being zero at each moment. The simulation results reveal the formation of a crater, but no jet formation or droplet detachment. Simulations are performed for di⁄erent sets of conditions, and it is found that in order for the developed model to be applicable to real experimental situations, space-charge limited thermionic electron emission must be considered.
Description
Keywords
Plasma-cathode interaction Vacuum arc discharges Unipolar arcs Cathode spots Interação plasma-cátodo Descargas de arco em vácuo Descargas unipolares Manchas catódicas Physics . Faculdade de Ciências Exatas e da Engenharia