First Cavity Design Studies for the Bessy-vsr Upgrade Proposal

Recently HZB proposed an upgrade of the 3 generation synchrotron light source BESSY II allowing simultaneous long and short pulse operation [1]. For this scheme superconducting higher harmonic cavities of the fundamental 500MHz at two frequencies need to be installed in the BESSY II storage ring. Given an appropriate choice of the higher harmonics the resulting gradient leads to a beating effect of the effective longitudinal focusing voltage at the stable fix points resulting in different bunch lengths in subsequent buckets. This project places stringent requirements on the cavity performance, as high accelerating fields, excellent HOM damping capabilities and high reliability as they will operate in a 300 mA 24/7 user facility. In this paper we describe the requirements for the cavity design and first design steps.