Accelerator component materials, such as oxygen-free copper, alumina ceramics and stainless steels to be installed in a vacuum must necessarily be chosen and treated so as not to induce an electrical breakdown. Some R&D work carried out at KEK concerning the breakdown characteristics of several materials and treatments is reported. Hydrogen molecules that are absorbed in the copper bulk as well as surface contamination influence its breakdown strength when used as an electrode. Surface finishing by diamond turning of the copper electrodes has been examined. As for the alumina ceramics used as an electrical insulator, micro voids and sintering additives in the material cause breakdown and surface melting when multipactoring takes place. Oxygen defects in alumina are strongly related to its durability under a high electric field. Outgassing, which may induce breakdown phenomena, have also been measured for some components for in-vacuum use.
