Design and development of an innovative system for Nb3Sn thin films for SRF elliptical cavities

The development of Nb3Sn films on copper as coatings for accelerating cavities is a viable option to potentially cover the specifications of next generation particle accelerators, such as the Future Circular Collider (FCC), while taking care of the optimization of key aspects like energy consumption. The successful development of a Nb3Sn on copper (Nb3Sn/Cu) scalable prototype would allow for the operation of the accelerator’s Superconducting Radio Frequency (SRF) system at 4.5 K, resulting in a reduction of the needed cryogenic power by a factor 3 with respect to what is normally required for bulk niobium (Nb) cavities, operated at 2 K. Nowadays, the niobium-coated copper (Nb/Cu) technology, developed with Physical Vapor Deposition (PVD) techniques, is a well-established approach for the production of SRF cavities, whose effectiveness is demonstrated by particle accelerators such as the Large Electron-Positron collider (LEP-II) and the Large Hadron Collider (LHC) for elliptical cavities coated via Direct Current Magnetron Sputtering (DCMS), and the Acceleratore Lineare Per Ioni (ALPI) and the Isotope Separator On Line Device (ISOLDE) for quarter-wave resonators coated via DC sputtering. As for these precursors, the development of Nb3Sn/Cu cavities can be pursued via DCMS. At Istituto Nazionale di Fisica Nucleare - Laboratori Nazionali di Legnaro (INFN-LNL) an optimized recipe for Nb3Sn films deposited via DCMS has been established on small flat samples via the refinement of the deposition parameters based on the analysis of the film properties. In this framework, the next challenge lies in the scalability of the coating recipe to an elliptical 1.3 GHz cavity prototype. The experimental work within this project focuses on the design of a new coating system for the deposition via DCMS of Nb3Sn films on elliptical 1.3 GHz copper cavities. The system features a fixed rectangular magnetron sputtering source hosting a stoichiometric Nb3Sn commercial target, around which the cavity to be coated will axially rotate to produce a homogeneous coating. Magnetic simulations were conducted to determine the best configuration for the magnets, including their material and shape, in order to optimize the Nb3Sn thin film sputtering process. The dedicated vacuum system and magnetron were also designed and produced within the context of this study. The successful operation of the finalized system would contribute to validating the feasibility of Nb3Sn/Cu via DCMS technology for 1.3 GHz cavities in the development of next-generation particle accelerators that require such cavities and paving the path for the scalability of the technology toward larger machines operating at lower frequency, such as the FCC-ee.