A Microscopic Assessment About the Fracture of Alumina in the Pillbox-Type RF Window of High-Power Electron Tubes

Alumina is widely used as a vacuum ceramic separator in the pillbox-type output window of high-power electron tubes. The fracture of alumina is one of the most important factors in the failure of these tubes. In this article, the fracture process of alumina in the pillbox-type window of high-power electron tubes is assessed microscopically. The investigation shows the twinning phenomenon in the fractured alumina. The reason for initiating and propagating twins is also investigated. An increase in the alumina temperature decreases the required stress level for generating twins. The overheating condition occurs due to an impedance mismatch in the pillbox-type window. Experimental and numerical analyses are presented to verify the mechanism of mechanical stress due to the overheating condition. The numerical analysis is carried out by the finite-element method (FEM). The result of this study can provide information for proposing a temperature monitoring scheme to turn off the tube before reaching the overheating condition.