Obtaining picosecond x-ray pulses from fourth generation synchrotron light sources

Abstract

Fourth generation storage ring light sources enabled by multibend achromat lattices and high-gradient magnets can reach unprecedented photon brightness. In this study, we show that, through the two-frequency crab cavity scheme, such machines also offer a unique opportunity to produce intense short x-ray pulses that are ideal for time-resolved user experiments. The short pulses and the high brightness photon beams are simultaneously available at all beamlines in a fully compatible operation mode. Owing to the small momentum compaction factor characteristic in fourth generation storage rings, the vertical emittance contribution due to coupling between the longitudinal and transverse planes by the crab cavities is small, which allows choosing a lower fractional vertical tune and in turn enables reaching the desired beam bunch tilting with a weaker deflecting voltage. However, bunch lengthening by the harmonic cavity is found to drastically increase the vertical emittance, which poses a serious hurdle to the scheme. We propose to use a half-integer harmonic cavity to simultaneously produce bunch lengthening and shortening in the bunch train to facilitate the compatible operation of the normal and short-pulse beams. A concrete case study based on the Advanced Photon Source Upgrade lattice is provided to demonstrate the system configuration and beam performance.