Microbunching instability study in a linac-driven free electron laser spreader beam line

Suppression of the microbunching instability is a priority for free electron lasers (FELs) whose goal is production of intense coherent radiation with very narrow spectral bandwidth. The instability can have large gain in multibend switchyard lines that connect the accelerator to multiple undulator lines. This study provides practical guidelines for switchyard optics designs that are largely immune to microbunching growth. A case study of the FERMI FEL switchyard is illustrated, resulting in an improved optics design that provides a unity microbunching gain while preserving the electron beam brightness. The analytical computation of the gain for the improved optics is supported by simulations and by experimental results