Superconducting Semi-Rigid Cable as Low-Pass Filter for Cryogenic Readout

Abstract

We are developing thin semi-rigid cables with outer diameters less than 1 mm, for accurate readout of cryogenic applications. The center conductor is separated from seamless metal outer conductor by dielectric material, polytetrafluoroethylene (PTFE). In order to suppress heat penetration through the cables into cryogenic stage, we used low thermal conductivity normal conducting alloys or superconductors as the material for both center and outer conductors. And to cut high frequency noise components in readout, we adopted bilayer structure in the center conductor, where superconducting wire with a diameter of 178 μm made of niobium–titanium (NbTi) was clad in normal conducting alloy, cupronickel (CuNi) of 13 μm thickness. We also used CuNi pipe with an outside diameter of 0.86 mm as the outer conductor. From the results of thermal conductance and attenuation below the critical temperature of NbTi ( ∼9 K), we confirmed that this kind of semi-rigid cable can work as a low-pass filter used at cryogenic temperature. The −3 dB cutoff frequency of 1 m length cable was observed at ∼500 MHz, above which attenuation became significant because of the skin effect and large electrical resistivity of CuNi cladding.