Relationship between Local Contaminated Positions and Breakdown Points in Vacuum under Initial Conditioning Process of Pure Nickel Electrode

Abstract

For focusing electrodes of X-ray tube, a high voltage performance in vacuum is required from an initial conditioning process. In order to obtain a high voltage performance in a vacuum, it is necessary to clarify the breakdown factor and its mechanism for a vacuum electrode gap in the initial conditioning process. In this study, a vacuum breakdown test was performed using pure nickel electrodes that had been polished and heat-treated at various temperatures, and we investigated the relationship between the local contamination point on the electrode surface and the dielectric breakdown point. As a result, it was clarified that the high voltage performance was improved by smoothing the electrode surface using both magnetic polishing and electro polishing. With carefully observing the electrode surface by SEM, it was found that the trace of breakdown events appeared in the local contaminated point after several breakdown shots of the initial stage of conditioning. Moreover, it was found that the vacuum breakdown appeared in the edge of local contaminated area. It was suggested that the interface between the electrode, vacuum, and the local contaminated point was one of the weak point of electrical insulation.