Mechanical Design of the VSR Cavity: An Elliptical 4-Cell 1.5 GHz SRF Cavity With Strong Waveguide HOM Damping for High-Current Accelerators

The mechanical design of an elliptical 1.5 GHz superconducting RF (SRF) 4-cell cavity with strong waveguide higher order mode (HOM) damping is presented. It was developed in the VSR (Variable Storage Ring) project framework to serve as part of an upgrade for the synchrotron light source BESSY II operating at beam currents of up to 300 mA. This poses a great design challenge, demanding both: a system capable of extracting very high HOM powers (above 2 kW per cavity), the stresses arising during operation, and an overall cavity length of less than 1 m to fit into a given magnet lattice. As a result, a compact HOM waveguide-loaded cavity has been developed, fulfilling all the required criteria. The most remarkable feature of the cavity are the five waveguide extensions. The paper describes the mechanical solutions adopted in the layout of the cavity with its subgroups. Special attention is given to numerical studies aiming for high pressure resistance whilst allowing for required tuning. A standardized pressure vessel code was not followed. Instead, a dedicated quality control procedure for manufacturing including ancillary material testing and welding qualification, was developed to allow for peak pressures of 3.5 bar(a) of the cooling medium. This paper shows the successful mechanical design solutions developed to allow for the operation of such cavities in demanding operational regimes featuring both high gradients and strong beam currents. At time of writing, the first prototype is mechanically completed, and the first RF tests have been performed. Relevant manufacturing experiences which led to late-stage design modifications are included in this paper.