Frequency pre-tuning of the niobium-sputtered quarter-wave resonator for HIE-ISOLDE project at CERN

Superconducting quarter-wave resonators (QWRs) will be used in the superconducting linac upgrade in the frame of the HIE-ISOLDE project at CERN. The QWRs are made of bulk copper and have their inner surface covered with sputtered niobium. Their resonant frequency is 101.28 MHz at 4.5 K. Each cavity will be equipped with a tuning system to both minimize the forward power and compensate the frequency variations during production and beam operation. After a careful examination of all contributors to the frequency variation, we decomposed them into two components: frequency shift and its uncertainties. A pre-tuning step was subsequently added to the production sequence prior to niobium sputtering to accommodate the frequency shift mainly due to mechanical tolerances during substrate production, substrate surface treatment, niobium sputtering and cooldown process. To this end, the length of the QWR was chosen as a free parameter for the pre-tuning. Consequently the tuning system needs only to compensate the frequency uncertainties and Lorentz force detuning, thus its design has been largely simplified and its production cost was reduced by 80% comparing to its previous version. We have successfully applied this tuning scheme to five HIE-ISOLDE QWRs and the measured tuning error was 2.4±1.9kHz. This is well consistent with our calculations and well recoverable by the current simplified tuning system. It is worth noticing that the pre-tuning method only involves one-time measurement of the cavity׳s resonant frequency and its outer conductor length. This paper focuses on HIE-ISOLDE high-β QWR, but the method can be applied to HIE-ISOLDE low-β QWRs and other variants of QWR-like cavities.