Nb3Sn coating of high purity Nb cavities

The authors have developed a modified vapor diffusion technique using specially constructed UHV furnaces to produce uniform Nb₃ Sn layers on 1- and 3-GHz accelerator cavities without losing the high thermal conductivity of the Nb wall. In a first experiment with a single-cell 3-GHz cavity of medium Nb purity (residual resistivity ratio, RRR=156) they have achieved E_acc=10 MV/m with an 0.6-μm-thick Nb₃Sn layer. This is so far the highest accelerating field for their Nb₃Sn cavities, but it is low compared to the 25 MV/m achieved in the same cavity before coating. Possible reasons for the reduced breakdown field for Nb₃Sn are discussed on the basis of the local thermal breakdown model