SUPERCONDUCTING NIOBIUM SPUTTER-COATED COPPER CAVITIES AT 1500 MHz

A series of copper single-cell cavities were fabricated by hydroforming and sputter coated with Nb. The cavity could be excited both in the fundamental mode at 1500 MHz and in higher-order-modes at 2.5 GHz and 2.8 GHz (quadrupole modes) with low losses in the end flanges. The first results are the following: the lowest residual surface resistance at 1500 MHz was 10 nΩ. It increased quadratically with the frequency. The fundamental low field Q value at 1.8 K was 3 × 10 10 decreasing to 1 × 10 9 at the accelerating gradient of 14 MV/m, the maximum gradient obtained.