High fields in superconducting Nb3Sn accelerating structures

Several X-band three-cell and nine-cell accelerating structures were fabricated from bulk niobium and coated with a Nb3Sn layer by the vapour diffusion technique. To determine the quality of the Nb3Sn layer the temperature dependent surface impedance was measured in the range 25 K\geq T \geq 1.5K at low field level. From this data material parameters as Tc, Δ/kTc, λ0and ρnwere calculated. The high field behaviour of the cavities was investigated for temperatures 4.2 K\geq T \geq 1.5K. Maximum surface fields of 45 mT were reached corresponding to an accelerating field of Eacc= 6 MV/m in the π/2-mode. The results of low and high field measurements with respect to the fabrication and treatment of the Nb3Sn-layer are discussed.