Investigation and Fabrication of a Low Loss and Wideband Single Crystal Diamond Vacuum Window for W-Band Traveling Wave Tube

A vacuum window used for W-band traveling wave tube (TWT) featured with excellent thermal capacity, low insert loss, and wide operation band is investigated in this paper. The window sheet material is upgraded to single crystal diamond. The microwave characteristic simulation, thermal analysis, and the structure design are deeply and successively carried out before the fabrication. The microfabrication of the window frame, the produce and surface treatment of the diamond sheet, the metallization, and the vacuum brazing are demonstrated. According to the experimental test results with a vector network analyzer, the S21 parameters are larger than - 0.8 dB and S11 parameters are lower than - 18 dB in a 25 GHz with more than 25% relative bandwidth, which shows great advantages of the single crystal diamond vacuum window in millimeter wave and even terahertz spectrum.