Superconducting Niobium S-Band Cavities

Abstract
Recent studies of long-pulse separated beams indicate the need for the development of superconducting rf separators for counter experiments. To achieve this goal we investigated microwave resonators at S-band frequencies manufactured by the three following methods: 1) machining from solid electron-beam-melted niobium, 2) electroforming in molten fused salts by the Linde Company, 3) machining hydroformed cups of commercial grade niobium. Machined parts were electron-beam welded to form microwave cavities. All cavities were heat treated in either the Oak Ridge, Stanford or SLAC high-vacuum, high-temperature furnaces and chemically polished. We investigated and compared both surface resistance and peak rf magnetic field of the above cavities after every step. We measured an improvement factor of ~ 106 at Bˇ = 330 G in a TE011 cavity. The highest rf magnetic field achieved was 464 G in the TM010 mode which corresponds to Ê of ~17 MV/m. In addition, we were looking for peaks in the microwave absorbtion caused by rf magnetic field induced surface states similar to those reported by various researchers on highly polished crystals in low dc magnetic fields. No such anomalies were found.