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Modelling self-organization on electrodes of DC glow discharges
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Abstract(s)
O trabalho apresentado nesta dissertaªo refere-se modelaªo de padrıes de autoorganizaªo de densidade de corrente em elØtrodos de descargas DC luminescentes. Padrıes de manchas andicas foram modelados de forma auto-consistente pela primeira vez e os fenmenos nas manchas foram investigados. As soluıes que descrevem as manchas foram encontradas num intervalo de correntes com mœltiplas soluıes. Foi descoberta uma inversªo da densidade de corrente local do nodo no centro de cada uma das manchas, isto Ø, mini-cÆtodos sªo formados dentro das manchas; poder-se-ia dizer, as manchas do nodo funcionam como uma descarga luminescente unipolar. As soluıes nªo se enquadram no padrªo convencional de auto-organizaªo em sistemas dissipativos nªo-lineares biestÆveis; por exemplo, as transiıes de um modo para outro nªo se realizam atravØs de bifurcaıes. Padrıes auto-organizados de manchas catdicas em descargas luminescentes foram modelados na camada de plasma junto ao cÆtodo, numa geometria igual utilizada na maioria das experiŒncias descritas na literatura. O efeito da geometria da cmara de descarga nas manchas foi investigado. Os padrıes de manchas modelados sªo idŒnticos aos observados nas experiŒncias e similares aos calculados na conguraªo de elØtrodos planos em paralelo. Uma tentativa foi feita para modelar quantitativamente a descarga DC luminescente com manchas catdicas. Uma descriªo detalhada desta modelaªo, a mais precisa deste fenmeno atØ data, Ø apresentada. Em geral, os padrıes calculados sªo semelhantes aos observados nas experiŒncias, mas as CVC sªo qualitativamente diferentes. CenÆrios de transiıes entre modos com diferentes padrıes de manchas em elØtrodos de descargas DC luminescentes e em cÆtodos de descargas de arco sªo investigados. No caso de transiıes entre padrıes em cÆtodos de descargas DC luminescentes, foram encontradas as transiıes observadas nas experiŒncias que podem estar diretamente relacionados a bifurcaıes de soluıes estacionÆrias, e as bifurcaıes correspondentes foram modeladas. Os padrıes encontrados na modelaªo numØrica estªo em conformidade com os observados no decurso das transiıes nas experiŒncias. No caso dos cÆtodos de descargas de arco, mostra-se que qualquer transiªo entre diferentes modos de transferŒncia de corrente estÆ relacionada a uma bifurcaªo de soluıes estacionÆrias.
In this work self-organized patterns of current density on electrodes of dc glow discharges were modelled. Patterns of anodic spots were modelled self-consistently for the rst time and their physics was investigated. The solutions describing the spots were found to exist in a region of current with multiple solutions. A reversal of the local anode current density in the middle of each of the spots was discovered, i.e. mini-cathodes are formed inside the spots or, as one could say, the anode spots operate as a unipolar glow discharge. The solutions do not t into the conventional pattern of self-organization in bistable nonlinear dissipative systems e.g. the modes are not joined by bifurcations. Self-organized patterns of cathodic spots in glow discharges were computed in the cathode boundary layer geometry, which is the one employed in most of the experiments reported in the literature. The e⁄ect that the geometry of the vessel has on the spots was investigated. The computed spot patterns are the same as the ones observed in the experiment and similar to the ones computed in the parallel plane electrode conguration. An unsuccessful attempt was made to quantitatively model DC glow discharge with cathodic spots, an account of this, the most accurate modelling of the phenomenon yet performed, is given. In general, the computed patterns are observed in the experiment but the CVC are qualitatively di⁄erent. Scenarios of transitions between modes with di⁄erent patterns of spots on electrodes of dc glow discharges and cathodes of arc discharges are investigated. In the case of transitions between patterns on dc glow cathodes, those transitions reported in the experiments that may be directly related to bifurcations of steady-state solutions are found and the corresponding bifurcations are computed. Patterns found in the numerical modelling conform to those observed in the course of the transitions in the experiment. In the case of cathodes of arc discharges, it is shown that any transition between di⁄erent modes of current transfer is related to a bifurcation of steady-state solutions.
In this work self-organized patterns of current density on electrodes of dc glow discharges were modelled. Patterns of anodic spots were modelled self-consistently for the rst time and their physics was investigated. The solutions describing the spots were found to exist in a region of current with multiple solutions. A reversal of the local anode current density in the middle of each of the spots was discovered, i.e. mini-cathodes are formed inside the spots or, as one could say, the anode spots operate as a unipolar glow discharge. The solutions do not t into the conventional pattern of self-organization in bistable nonlinear dissipative systems e.g. the modes are not joined by bifurcations. Self-organized patterns of cathodic spots in glow discharges were computed in the cathode boundary layer geometry, which is the one employed in most of the experiments reported in the literature. The e⁄ect that the geometry of the vessel has on the spots was investigated. The computed spot patterns are the same as the ones observed in the experiment and similar to the ones computed in the parallel plane electrode conguration. An unsuccessful attempt was made to quantitatively model DC glow discharge with cathodic spots, an account of this, the most accurate modelling of the phenomenon yet performed, is given. In general, the computed patterns are observed in the experiment but the CVC are qualitatively di⁄erent. Scenarios of transitions between modes with di⁄erent patterns of spots on electrodes of dc glow discharges and cathodes of arc discharges are investigated. In the case of transitions between patterns on dc glow cathodes, those transitions reported in the experiments that may be directly related to bifurcations of steady-state solutions are found and the corresponding bifurcations are computed. Patterns found in the numerical modelling conform to those observed in the course of the transitions in the experiment. In the case of cathodes of arc discharges, it is shown that any transition between di⁄erent modes of current transfer is related to a bifurcation of steady-state solutions.
Description
Keywords
Interação plasma-cátodo Auto-organização Descargas luminescentes Manchas no ânodo Cátodos termiónicos Manchas catódicas Plasma-electrode interaction Self-organization Glow discharges Cathodic spots Anodic spots Physics . Faculdade de Ciências Exatas e da Engenharia