CVD diamond windows studied with low- and high-power millimeter waves

As part of long-pulse high-power gyrotron development low- and high-power millimeter-wave characterization has been performed on bare and brazed chemical vapor deposition (CVD) diamond disks. The dielectric property measurements performed with low-power open resonator studies demonstrate the availability of large area CVD diamond disks fulfilling the requirements for high-power windows. In brazed components, additional surface losses are put to evidence. Their contribution to enhanced dielectric absorption differ characteristically between existing brazing techniques. A particular acid and hydroxide treatment for reducing the observed surface terms was studied for brazed windows at different stages of their integration into gyrotron tubes, such as premounting, onsite, and post-dismantling. Video inspections of the output and transmission windows in high-power tests gave evidence of stationary light emission phenomena from irregularly distributed spot-like centres in the output window. The absence of a similarly stationary phenomenon in all disks studied in transmission rules out typical defect structures such like nondiamond like phase inclusions or specifically terminated surface bonds as a critical source. Local temperature peaks and the presence of only singular light emission events in the transmission windows hint at contamination effects by particles which are only stationary in vacuum conditions.