Vacuum requirements in high power microwave tubes

Abstract

During recent years microwave tubes for low and medium power have been replaced by solid state devices. However, the generation of microwaves at high power is still only possible by the use of high-vaccum electron tubes. For reliable operation and sufficiently long live times severe requirements on vacuum conditions have to be fulfilled. To avoid poisoning of the electron emitter material, high voltage and rf breakdowns, multipactoring and beam instabilities caused by ion oscillations, the pressure in the tubes should not exceed 10 −5 Pa. In many cases the tubes are used without any additional pumps or with very small pumps. To preserve the good vacuum conditions over long periods, only very low degassing rates and very low leaking rates (10 −12 Pa m 3 s −1) are required. The operation of electron tubes under extreme conditions as for example high voltage, high power load in cavity and window, high beam power at the collector and high temperature of the electron emitter restricts the possibilities of the usable material. The combination of many different materials (dispersion strengthened copper, stainless steel, ceramics for the window, rf attenuation ceramics in the beam tunnel, HV isolation ceramics, etc.) require special joining and sealing techniques.