Punchthrough of ceramic insulators

A study of punchthrough breakdown in an alumina insulator is presented. The alumina cylinder, containing electrodes at 300 kV in a vacuum of 10/sup -6/ torr, experienced permanent puncture damage from a breakdown process that was unexpected because the fields from the electrodes were well below the breakdown strength of the ceramic. Punchthrough was concluded to be caused by a combination of electric field stress in the ceramic and embedded electrons which formed a volume charge within the ceramic. Calculations were made to determine the minimum electron density necessary to exceed the breakdown strength of the ceramic while in the electric field. The electric fields in the ceramic were determined from computer field mappings of the equipotential contours in the cylinder. The calculated minimum electron charge density, under zero electrode generated field, was 138*10/sup l5/ e/sup -//cm/sup 3/. Color center reactions confirmed that electrons bombarded the ceramic.<>