Annealing effects on breakdown threshold of RF windows

The breakdown threshold of alumina RF-windows depends not only on the bulk characteristics, but also on such surface treatments as polishing and annealing. Sapphire and alumina-ceramic disks, polished and/or annealed, were examined by high-power tests. The polished disk showed a lower threshold value than did the annealed one, probably due to its surface charging at defects mechanically introduced by polishing. The elimination of surface charging is considered to be important.