Development of an extremely thin-diamond window for terahertz traveling wave tubes

•Fabricated a 60 μm thick diamond for THz traveling wave tube windows•UNCD growth on MCD resulted in surface roughness improvement.•The thin diamond possessed adequate strength for THz TWT windows.•Work provides a novel approach for fabrication of extremely thin diamonds.

This paper describes the design and fabrication of an extremely thin-diamond window for terahertz traveling wave tubes (THz TWTs). To reinforce the mechanical properties of the micro-crystalline diamond (MCD) incorporated with ultra-nano-crystalline diamond (UNCD), a seven-layered composite diamond having a total thickness of 60 μm has been fabricated and studied. SEM studies showed that UNCD growth on MCD resulted in surface roughness improvement via smoothing sharp corners and straight edges as well as filling in valleys of MCD grains. Further comparison studies of the same thickness samples demonstrated that the roughness for the composites were ~ 25-50% lower than those of the MDCs, depending on the number of UNCD layers. More importantly, seams or gaps resulting from irregularly shaped MCD grains were found to be terminated by UNCD growth, which is believed to attribute to the lateral growth mechanism of UNCD. The seven-layered composite diamond film was cut into window disks without lapping and polishing, which were then made into window assemblies. The mechanical testing of the window assembly showed no occurrence of window cracking or other damages due to the mechanical stress from pressure difference, while leak tests found no apparent air leakage resulting from the composite material. These testing results show such a thin composite diamond possesses adequate mechanical properties for the application of THz wave TWT windows. This work provides a novel approach for the fabrication of extremely thin-diamond windows for THz TWTs.

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