Development of Semi-rigid Superconducting Coaxial Cables with Clad Central Conductor for Low-Noise Experiments

In this study, semi-rigid coaxial cables with an outer diameter of less than 1 mm were developed for the readout of low-temperature detectors. A bilayer structure comprising a superconductor clad with a normal conductor was used for the central conductor. Structures of this type can exhibit low-pass filtering characteristics and reduce high-frequency noise. To obtain high attenuation along with low thermal conductance using this type of filtering cable, stainless steel (SUS304) was used to clad the NbTi central conductor because it has a high electrical resistivity compared to that of the conventionally used alloy CuNi. The semi-rigid cable considered in this study has a diameter of 0.86 mm and consists of a NbTi central conductor clad with SUS304 with a thickness of 13 \(\upmu \hbox {m}\), a SUS304 outer conductor, and a polytetrafluoroethylene insulator. The low-temperature (2–8 K) thermal conductance of this cable was measured and found to be slightly higher than that of a semi-rigid cable of the same dimensions using SUS304 for the entire central conductor. Additionally, the attenuation measured in the frequency range of 150 MHz to 10 GHz was very large, and the expected low attenuation below approximately 1 GHz due to the superconducting NbTi was not observed. From SEM observation and EDS analysis, it was found that the NbTi region of the central conductor contained grains of approximately \(10~\upmu \hbox {m}\) in size derived from the SUS304, which may have been the cause of the observed large attenuation.