Investigation of the Surface Resistance of a Niobium-Cavity at S-Band

Measurements of the RF-residual surface resistance of superconducting niobium have been carried out between 2-4 GHz. The experiments show that also for type II superconductors like niobium the losses due to frozen-in magnetic flux can be described by Rres¿¿2/3 (anomalous skin-effect). For negligible frozen-in flux the residual losses are proportional to ¿1.7±0.2 like in the case of lead, indicating that a general mechanism seems to be responsible for these residual losses. By electrochemical oxidation we have achieved fields up to 325 G in the TM011-mode.