Investigation of a broadband double-disc diamond window unit

In this paper we investigate the potential causes of failure of a diamond window unit employed in the experimental tokamak ASDEX Upgrade in Garching, Germany. The window unit features a double-disc configuration that allows for a more complex transmission spectrum than with a single disc and is used in conjunction with step-tunable gyrotrons for electron-cyclotron heating of fusion plasmas. The window unit failed during high-power operation as a consequence of a crack in one of the two diamond discs. The event that initiated the crack is unknown. Two possible reasons have been investigated in detail: high stress on the diamond due to bad quality of the brazing and due to localized power depositions. The first is investigated by computer tomography imaging of the brazing, the second by means of a custom ray-tracing code that uses a “hybrid” approach that accounts for the waves' phase, thus reproducing the effects of interference.