Cryogenic RF Characterization of Superconducting Materials at SLAC With Hemispherical Cavities

For the characterization of superconducting radiofrequency (SRF) materials, SLAC has commissioned a second-generation, X-band cavity cryostat for the rapid analysis of either thin-film coatings or bulk samples. With this test cryostat one can measure the sample’s surface resistance, critical temperature, and magnetic quenching field. The system operates at a frequency of 11.4 GHz, at temperatures between room temperature and 4 K, and utilizes two interchangeable hemispherical cavities (one copper, one niobium) that can accommodate 2”-diameter (50.8 mm) samples on the flat surface. With the niobium cavity one can resolve surface resistances down to 0.7 (about one tenth of bulk niobium), while with the copper cavity one can measure quenching fields up to 360 mT (about twice that of bulk niobium).