The Measured Time and Frequency Response of a Miniature Superconducting Coaxial Line

A miniature superconducting 52 ohm coaxial transmission line 278 meters long, having a lead outer conductor (0.129 cm id), a teflon dielectric, and a niobium center conductor (0.038 cm), has been measured in both the time and frequency domains. The observed system step response (10% - 90%) times were 1.5 μs at room temperature, 375 ns at 77K, and 255 ps at 4. 2K. The system step response data for 4.2K was processed by a numerical system identification routine to determine the step response of the miniature superconducting line; the processed data gave a 220 ps risetime. Although the observed step responses at 4.2K were smooth transitions, the observed swept frequency (0.1 GHz - 12 GHz) attenuation at 4. 2K was a quasi-periodic function of frequency, which indicated that the nonuniformity of the superconducting line was significant. For example, near 5 GHz and at 4.2K, the attenuation minimum was 1.1 dB while about 0.5 MHz away the attenuation was 5 dB. By making normal conductivity measurements of the Nb conductor and incorporating the results into the two fluid model, the uniform line attenuation for 5 and 10 GHz was calculated with the classical and anomalous limits. The lowest observed attenuation values in the vicinity of 5 and 10 GHz fell between the computed classical and anomalous limit attenuation values.