​Energy Recovery Linacs (ERLs) reuse the kinetic energy of an accelerated electron bunch by transferring it to a freshly injected bunch. This mechanism enables a large reduction in the RF power required in comparison to linacs, without resorting to the lower quality inherent in stored beams. ERLs are therefore recognised as prime candidates for the next generation of electron colliders and free-electron laser light sources, promising higher luminosity and average power respectively.

Unlike storage rings and linacs, the topology and ordering of acceleration, beam manipulation, and deceleration sections in an ERL with multiple passes is not strongly constrained – meaning many machine configurations are possible. However only a small subset of these possible configurations produce a "self-consistent longitudinal match", i.e. where the correct bunch lengths and energy spreads are simultaneously satisfied at all points in the machine, whilst still fulfilling the energy recovery criterion.

In a paper just published by Gustavo Pérez-Segurana (CI PhD student – Lancaster), Ian Bailey (Lancaster Physics) and Peter Williams (ASTeC Accelerator Physics) in Physical Review – Accelerators and Beams^[1], a detailed study of self-consistent longitudinal matches for both collider and FEL applications is explored for the first time.

One hitherto unrealised conclusion is that choosing common return transport beamlines severely restricts the availability of a self-consistent match, particularly when synchrotron radiation losses are important. Future facilities based on ERLs must therefore be designed carefully in order to exhibit a self-consistent match.

1. Pérez-Segurana G, Bailey I, Williams P. Construction of self-consistent longitudinal matches in multipass energy recovery linacs. Physical Review. 2022;25(2):021003. doi:10.1103/PhysRevAccelBeams.25.021003