Cold-Test of Transverse Input-Output Microwave Circuit Components for a High-Power W-Band Gyro-TWT

The input/output microwave circuit of a W-band gyrotron traveling-wave tube (gyro-TWT) with a predicted output pulse power of more than 300 kW and a bandwidth of about 8% is under investigation. A distinctive feature of this circuit is the transverse (relative to the axial velocity of the electrons in the gyro-TWT) extraction of the output power and its compatibility for feeding and coupling the microwaves out through one oversized barrier window. The whole circuit consists of components inside and outside the tube vacuum volume. The key elements of the inner circuit are a waveguide mode converter forming the ring-like distribution of the radiation from the incoming TE
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mode and a 45° inclined mirror with a hole enabling a lossless transport of the gyro-TWT electron beam to the collector. Aluminum prototypes of these components have been fabricated and cold-tested in the frequency range of 90-102 GHz. The measured 2D mode patterns and level of the cross-polarization are found to be in good agreement with the simulations.