Design of liquid-nitrogen-insulated apparatus through unified breakdown field data having different area and volume effects

For designers of high temperature superconducting equipment with liquid nitrogen as a dielectric, an expedient universal curve is sought that incorporates area/volume effects for each member of a specified class of electrode shapes, thus providing the breakdown field of all such liquid nitrogen gaps with any practical sizes of electrodes and gap. Currently a single universal curve for breakdown strength with AC or DC stress is being developed for the class with sphere-sphere, plane-plane and sphere-plane gaps, with three independent parameters: the size of each electrode and gap, all at pressures at or slightly above 100 kPa. The geometrical effects of stressed area/volume are incorporated from most published AC and DC experimental data of the last 50 years, by plotting experimental breakdown field versus new geometrical quantities which replace the traditional abscissa of gap distance, such that all data fall approximately on one normalized universal curve. This avoids the usual difficult task of calculating stressed area and volume geometrical effects on the breakdown field values on the graph ordinate; instead geometrical information replaces the geometrical parameter of gap distance on the graph abscissa. This approach will be discussed with examples. If the method proves reliable, it may be possible to determine design parameters for a broad range of geometries and applications, help unify seemingly disparate breakdown data found in the literature, and provide easily used, practical guidance for designers.