Nb Sputtered Quarter Wave Resonators for the HIE-ISOLDE

The HIE-ISOLDE superconducting linac will be based on quarter wave resonators (QWRs), made by niobium sputtering on copper. The operating frequency at 4.5 K is 101.28 MHz and the required performance for the high beta cavity is 6 MV/m accelerating field for 10 W maximum power dissipation. These challenging specifications were recently met at CERN at the end of a vigorous development program. The paper reports on the progress of the cavity RF performance with the evolution of the sputtering process; it equally illustrates the parallel R&D which is on-going at CERN and at INFN in the quest for even higher performances.