“Arclike variable bunch compressors” authored by Peter Williams (ASTeC), Gustavo Perez-Segurana & Ian Bailey (Lancaster U) and Bill Kyle, Sara Thorin & Jonas Bjorklund Svensson (MAX-IV, Sweden) proposes a new type of variable bunch compressor – a crucial component of free-electron lasers. This collaborative work with the MAX-IV facility in Sweden originated in their desire to upgrade their linac to drive an FEL, but has resulted in a more general proposal which is applicable in many contexts. In particular the authors will apply the same techniques to the proposed UK-XFEL.

“Microbunching instability characterization via temporally modulated laser pulses” authored by Alex Brynes & Peter Williams (ASTeC), Andy Wolski (U. Liverpool), Simone DiMitri & others (Elettra Sincrotrone Trieste), and Irwan Setija & others (ASML Netherlands BV) is the fourth paper in a series documenting experimental work carried out on the FERMI linac at Elettra in 2017. This successful collaboration has significantly advanced current knowledge of the physics of the microbunching instability, vital in the design of next generation FEL based light sources. This paper comprises a systematic experimental study where we control the microbunching instability by imposing known modulations via a pulse beating technique, and then benchmarks these measurements against numerical codes and semi-analytic descriptions. We show that modern codes are able to accurately describe the dynamics of the instability, giving confidence in the design of future FEL facilities.