Optical measurements on the Boeing free electron laser ring resonator experiment

Abstract The Boeing free electron laser is presently operating with a stable semiconfocal ring optical cavity surrounding a 5 m wiggler. The resonator consists of two grazing-incidence reflective telescopes each formed by a grazing-incidence hyperboloid and its companion off-axis paraboloid. The resonator is closed by two flats: one used for dynamic jitter correction and the other used to outcouple some of the circulating beam. It is critical to lasing that the alignment of the resonator be maintained, the focus of the optical mode be at the midpoint of the wiggler, and the wavefront quality of the propagating beam be maintained. Measurements of the focus position, power loss, and wavefront quality are made before operation of the electron accelerator to diagnose whether the conditions required for lasing exist.