Superconducting Low-beta Nb3Sn Cavity for ATLAS and Future Ion Accelerators
Abstract
We report on a Nb3Sn-coated low-beta superconducting radio frequency (SRF) cavity intended for accelerating ions. We aim to apply the cavity in ATLAS, our Argonne National Laboratory user facility for nuclear physics studies with ion beams in the energy range of 5-20 MeV/u. The Nb3Sn-coated cavity, a 145 MHz quarter-wave optimized for ions moving with velocity β=v/c=0.08 exhibits an order-of-magnitude reduction in radiofrequency (RF) losses into helium at 4.4\,K compared to a superconducting niobium (Nb) cavity at the same frequency and temperature. Experimentally measured fields are among the highest to date for any Nb3Sn-coated cavity, reaching a peak surface magnetic field of 105 mT. We also present a practical solution to the problem of cavity frequency tuning. Tuning by mechanical deformation has been a challenge with Nb3Sn due to its brittle nature, however, using a set of techniques tailored to the properties of thin-film Nb3Sn on Nb, we can repeatably tune the cavity to the ATLAS master clock frequency after it is cooled, while maintaining the excellent performance characteristics. The same Nb3Sn cavity technology offers broad benefits for future ion accelerators.
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