Mechanical reliability of ceramic windows in high frequency microwave heating devices

Abstract

The mechanical reliability was evaluated for the specific alumina and beryllia ceramics now used as microwave windows in the high-power (⩾200 kW) high-frequency (⩾60 GHz) gyrotron tubes being developed for plasma heating in fusion systems. Previous analysis of the stresses generated in the various window configurations and tube operating conditions indicated that significant tensile stresses are generated in the ceramic window by dielectric heating. As a result, we characterized both the static fatigue behaviour in the fluorocarbon fluid used to cool gyrotron windows and the inert strength distributions for these two ceramics. These data were then analysed in order to construct reliability diagrams for these two materials. Such diagrams revealed that the use of these specific ceramic materials will be limited by their time to failure at the tensile stresses imposed on them under the gyrotron operating conditions (60 GHz or greater and ⩾200 kW in a continuous wave (CW), TE₀₂ mode (i.e., radial power distribution in beam exhibits two maxima). The fatigue behaviour and inert strengths of both materials could be improved by increasing their density (>97% of theoretical) and employing a uniform fine (⩽10μm) grain size. Such material improvements would permit significant increases in the mechanical reliability of the gyrotron microwave window.