Process of manufacturing an electrical insulating material with a vacuum high break down voltage

Process for manufacturing an electrical insulator (5) immersed in an intense electrical field, especially between two electrodes (1 and 2) of an electron tube. The insulator (5) is a crystalline body whose free surfaces (7) under vacuum (6) have been treated so as to reduce or eliminate crystallisation defects. The treatment is controlled by measuring a property, especially an optical or mechanical property, of the free surfaces. The breakdown voltage can be multiplied by three or four in relation to conventional insulators and can approach the breakdown voltage of the vacuum.