Protection Considerations of High Power Vacuum Tubes Against Reflected Power Fault

The ceramic fracture in output RF1 window is one of the most important failure factors in high power vacuum tubes. Fast protection systems are used to protect the tube. These protection systems are activated with some measured variables such as the level of backward RF power. On the other hand, a fast protection system can be activated with noise. Therefore, a tradeoff must be done between the response time of the protection system and its sensitivity to the noise. There are many methods to determine the response time of the protection system. The level of allowable power loss in the tube failure process is one of the conventional figures of merit for protection response time determination. However, these methods are conservative and the resulted response time is usually less than the critical time which is necessary for protecting the tube. In this paper, an experimental investigation is presented about the relation between the response time of the tube protection system and the crack growth in the ceramic of RF window. It is shown that the twin growth in ceramic can be a reliable figure of merit for the response time calculation of the tube protection system.