Propagation in Superconducting Niobium Rectangular Waveguide in the 100 GHz band

Rectangular waveguides that use superconducting materials for walls are expected to reduce transmission loss compared to normal metal waveguides. However, research on superconducting waveguides has been limited, particularly on the fabrication and measurement of physical models in the mm/sub-mm band. We attempted to fabricate a niobium (Nb) waveguide and measure its propagation. Because Nb is known to be a difficult-to-machine material, we searched for the best cutting tools and machining conditions. Consequently, we fabricated a waveguide with a surface accuracy of 0.3 µm rms using an end mill with a diameter of 1 mm. For the measurement of waveguide propagation, the resonator method was adopted, and the resonance characteristics were measured at room (298 K) and cryogenic (4.8 K) temperatures in the 100 GHz band. The resonance property at 298 K is consistent with the shapes predicted from literature-based conductivity. However, at 4.8 K, that of superconducting Nb is significantly changed. We used electromagnetic analysis to reproduce these properties, and the electrical conductivity and propagation loss were calculated. The conductivity and loss were computed as 1.8×10¹¹ S/m and 0.05 dB/m, respectively. We observed a slight transmission loss and verified the effectiveness of the superconducting waveguides.