Development of a Superconducting RF Crabbing System based on a Quarter Wave Resonator for Ultrashort Pulses at Light Sources

Many lines of research currently ongoing at storage ring light sources would benefit from intense, tunable, few picoseconds x-ray pulses for time-domain experiments. The use of a deflecting cavity has several advantages over other proposals to produce these pulses. A crab cavity does not accelerate the bunch, but instead deflects the head of the bunch relative to the tail. This kind of chirp in the transverse momentum imposes an angle-time correlation on the light emitted from the undulator that can be used for time-domain experiments or manipulated with proper x-ray optics to produce ps-scale x-ray pulses. This SBIR proposal seeks to develop a superconducting crab cavity for this use in light sources. Here, a quarter-wave geometry is proposed that operates in the fundamental resonant mode, unlike the more conventional elliptical cavities that have been used for this application, which operate on a higher-order mode and require dampers for the fundamental mode. Further, the quarter-wave geometry has a very different spectrum of higher-order modes than elliptical shapes. Taking advantage of this sparse mode spectrum, an innovative design is proposed with a large beam pipe to allow all higher-order modes to propagate to absorbers at room temperature outside the cryomodule. The quarter wave structure is also efficient and compact, often relevant considerations for crabbing cavities.