Investigation of a diamond window failure mechanism

A theoretical model for the failure of diamond windows in high power gyrotrons is presented. We tentatively conclude that graphitization of the diamond, as a result of the long pulse operation, is unlikely to cause the diamond window failure. It has been found that diamond windows perform adequately for some time before rapid, destructive failure occurs. This would suggest a failure mechanism that is cumulative for each successive microwave pulse, with the individual contributions being small and non-destructive. We therefore speculate that the RF power is absorbed in thin, graphitic regions adjacent to the grain boundaries in the polycrystalline diamond window. For a sufficiently high temperature increase, the diamond adjacent to the grain boundaries will be converted into graphite. This in turn accelerates the RF power absorption, and ultimately leads to mechanical failure of the window. The failure mechanism proposed is based on a chemical transformation at the grain boundaries.