Superconducting niobium cavities of improved thermal conductivity

The construction of large scale superconducting accelerators asks for reliable and economical manufacturing procedures for the superconducting cavities. The improvement of thermal conductivity of the niobium used can be of great benefit in that respect. We have built and tested single cell cavities at 500 MHz and 3 GHz from a newly available niobium sheet material of improved thermal conductivity. At 500 MHz a maximum accelerating field of 13.0 MV/m at 4.2 K was achieved. At 3 GHz and 1.5 K as a best result 18.7 MV/m were obtained. A total of 15 cavity tests were carried out and the results clearly exceed field values obtained so far with standard reactor grade niobium at CERN and Wuppertal. Measurements of the thermal conductivity of niobium at helium temperatures are discussed, the temperature dependence of the surface resistance of high purity niobium is given, the observed field and Q limitations and electron loading phenomena are outlined.