Surface resistivity measurements of superconducting YBCO helices at UHF

The authors describe the measurement of the surface loss resistance (R/sub s/) on helical coils of Y-Ba-Cu-O (YBCO) at frequencies of 507 and 212 MHz. The inductive loading provided by the helix requires a cavity length of less than 6 cm for resonance at these frequencies, minimizing the amount of high-temperature superconductor (HTS) material needed. The sensitivity of the technique-that is the lowest value of R/sub s/ that can be measured with a given accuracy -is much lower than that of the usual cavity endwall substitution method, since nearly 70% of the current flows in the helix so that HTS helix losses dominate the copper wall losses. The observed improvements in cavity Q of factors of five or more compared to copper suggest device applications (e.g. filters) for these YBCO helical structures.<>