Seamless Quarter Wave Resonators for HIE ISOLDE

The HIE-ISOLDE post accelerator consists of 4 cryomodules with 5 niobium-coated Quarter Wave Resonators (QWR) each. The standard manufacturing technique was to machine the inner and outer conductor separately, to shrink-fit the 2 pieces and to apply an electron beam welding at the interface. Due to microcracks, observed on some of the cavities around the welds, we took the decision to explore the possibility of a seamless design. First cavities became available in late 2017 and were then cold-tested in the vertical cryostat. These seamless coated quarter wave resonators have shown some of the highest Q-values of all HIE-ISOLDE cavities in the acceptance tests. Furthermore, we studied the cavity performance with different compensations of the earth magnetic field and different temperature gradients upon cool down. These tests have demonstrated record-breaking RF surface fields for the Nb/Cu technology. This paper reviews the design and fabrication and reports on the cold tests results of seamless quarter wave resonators. Possible future applications are briefly illustrated.