Failure analysis of ceramic feedthroughs used in traveling-wave tubes

The effective operation of a high-voltage vacuum device, such as a traveling-wave tube (TWT), is dependent on its ability to maintain a high vacuum environment. However, after the brazing process, cracks in thick-wall ceramic shells will cause feedthrough leakage and eventually will impair the operation of a TWT. To investigate the cause of cracking in a thick-wall ceramic shell and to collect reliable design information, the stress field obtained from a shrink-fit solid cylinder can be used as an approximation. The limitation of this approximation can be established. Due to the brittle character of ceramic structures, the theory of linear elastic fracture mechanics can be used to study the ceramic feedthrough structure. With the known stress fields and crack sizes, the stress intensity factors of a typical feedthrough structure can be evaluated. The design curves for allowable crack sizes under different radial pressures can be plotted as well.