James D. Cross

The Effect of Chromium Oxide Coatings on Surface Flashover of Alumina Spacers in Vacuum

An experimental investigation of the effect of chromium oxide coatings upon the surface flashover of high-density alumina in vacuum is reported. It is shown that such coatings improve the withstand voltage for dc, 60-Hz, and impulse voltages by significant amounts. The coatings are simply applied, effective, and durable. It is postulated that the improvement in insulation is due to the elimination of positive surface charging by the use of a coating with a secondary-electron emission coefficient less than unity for all incident electron energies.

Prebreakdown Processes Associated with Surface Flashover of Solid Insulators in Vacuum

A computer simulation of the insulator surface charging in vacuum under the application of a high voltage, is presented for alumina (A1203) insulator and for alumina insulators coated with cuprous oxide (Cu2O) and chromium oxide (Cr2O3). It is shown that the mode of surface charge distribution is dependent on the secondary emission yield of the insulator surface. Mleasurements of insulator- electrode junction fields, using an electronic microforce balance, show the effect of surface coatings on the cathode and anode junction fields. High speed streak photography showing the time development of an arc in the case of coated insulators is presented. The experimental results support the insulator surface charging model which is simulated on a digital computer.

High Speed Photography of Surface Flashover in Vacuum

The method and results of high speed streak photography of surface flashover in vacuum are presented. It is shown that the bright phase of the flashover arc bridges a 12.5 mm gap in 0.15 ns. The streak records indicate that the flashover arc is preceded by an intense electron burst from the cathode-insulator junction.

The Effect of Cuprous Oxide Coatings on Surface Flashover of Dielectric Spacers in Vacuum

An experimental investigation of the effect of a cuprous oxide coating upon the surface flashover of high-density alumina in vacuum is reported. It is shown that such coatings improve the impulse strength of the system and eliminate the conditioning effect observed in the case of uncoated specimens at dc and 60-Hz voltages. An explanation of the observed behavior is given in terms of a surface charging model. It is postulated that the improvement in the insulation by the coatings is due to a reduction in the secondary electron emission yield.

DC Electric-Field Modifications Produced by Solid Insulators Bridging a Uniform-Field Vacuum Gap

The results of measurements of the electric field near the surface of stainless-steel electrodes in vacuum are described. It is found that the field is modified by a solid insulator bridging the interelectrode gap. The extrapolated cathode field is enhanced sufficiently to equal the value considered necessary to initiate break-down in a plane vacuum gap. The measured fields are greater than the applied field and consistent with the accumulation of positive surface charge on the solid insulator. Analysis of the data suggests that the surface charge results from bombardment of the surface by electrons emitted at the insulator-cathode junction.

Circuit modelling of a vacuum gap during breakdown

In this paper several aspects of circuit modelling of a vacuum gap during breakdown are improved or introduced for the first time. More accurate perveance formulae are derived by the method of tracing electron trajectories in the self-consistent electric field calculated by finite element method. The formula for maximum anode current density is also derived by the same method. A practical model of anode heating is proposed by which transient anode temperature is calculated coupled with gap voltage and current, providing more accurate modelling of anode plasma initiation. The circuit model of a vacuum gap during breakdown incorporating all these features is implemented as a subcircuit element in the PSPICE.

Electrical Breakdown at Solid-Liquid Interfaces

Observations of the electrohydrodynamic (EHD) motion of insulating oils in a static electric field, between plane parallel electrodes separated by an insulating support, are reported. A transition from laminar to turbulent flow in the bridged gap is observed near breakdown voltages. Velocity measurements support the observations of the onset of turbulent flow prior to flashoever while photographic observations show the breakdown to be away from the solid. Flashover measurements with increasing pressure indicate that the breakdown at the interface between solid and liquid is a liquid controlled phenomenon. The results are consistent with a liqûid breakdown model dependent upon the formation of a gas bubble of critical size by a gaseous cavitation mechanism in the turbulent flow.

Electric-Field Distortions at Solid-Liquid Dielectric Interfaces

The Kerr electrooptic effect is used to study the static electric-field distortions produced by various solid dielectric materials when introduced between parallel plate electrodes in a chlorobiphenyl liquid insulating medium. The results show that for an Aroclor of commercial purity, the field distortions are independent of the electrical properties of the solid. A weak dependence on the liquid impurity levels is shown. The surface current densities are also independent of the spacer materials. It is suggested that the measured surface current density is due to electrohydrodynamic (EHD) circulation of the liquid near the spacer. The resulting field distortions are discussed in the light of EHD motion of the liquid at the solid-liquid interface.

Electric Field Distortions Produced by Solid Dielectric Spacers Separating Uniform Field Electrodes in Nitrobenzene

The Kerr electrooptic effect is used to study the electric field distortion introduced into a uniform field gap in nitrobenzene by solid dielectric spacers bridging the gap. The results show that the field is distorted by the introduction of the solid, and the degree and location of the distortion depend upon the electrical properties of the solid. The results are compatible with space charge accumulation in the solid dielectric spacer.

High-Speed Photography of Surface Flashover Across High-Density Alumina in Vacuum under Direct and Impulse Voltages

High-speed streak and framing photography of surface flashover across solid insulators in vacuum is reported. Experiments were performed with direct and impulse voltages. It is shown that the initial propagation of a visible arc across an alumina surface in vacuum occurs at a speed greater than 109 cm ¿ s-1 . It is shown that previously observed two-stage arcs under impulse voltages are the result of the interaction between the external supply and the arc impedance.