Nb Coated HIE-ISOLDE QWR Superconducting Accelerating Cavities: From Process Development to Series Production

The new HIE-ISOLDE accelerator at CERN requires the production of 32 superconducting cavities (20 high- β and 12 low- β ) in order to increase the energy of the rare isotope beam delivered to the experiments. The Quarter Wave Resonators (QWR) cavities (0.3m diameter and 0.9m height for the high- β ) are made of OFE 3D-forged copper and are coated by DC-bias diode sputtering with a thin superconducting layer of niobium. Following a preliminary process development phase, the series production of the high-beta cavities has started. A summary of the production process is presented, with a focus on the coating recipe developed to meet the HIE- ISOLDE specifications (operation at 4.5 K with an accelerating field of 6 MV/m at 10W RF losses, Q 0 =4.7x10 8 and an average surface resistance of 65 n Ω ) and on the niobium layer characteristics. The RF performances of cavities produced with the coating baseline recipe are reviewed.