Bunch Compression and Turnaround Loops in the FCC-ee Injector Complex

Abstract

The Future Circular e+ e- Collider (FCC-ee) requires two 180-degree turnaround loops to transport the positron beam from the damping ring to the lower energy section of the linac. In addition bunch compression is required to reduce the RMS bunch length from 5 mm to 0.5 mm, prior to injection into the linac. A dogleg bunch compressor comprised of two triple bend achromat (TBAs) can achieve this compression. Sextupole magnets are incorporated into the bunch compressor design for chromaticity correction as well as optimisation of the second-order longitudinal dispersion, T566, and to linearize the longitudinal phase space distribution. In this paper we present the design of the transport line and the bunch compressor. Measures to limit emittance growth due to coherent synchrotron radiation (CSR) are also discussed, because despite the relatively long bunch length, the large degree of bending required introduces cause for consideration of CSR.