X-ray free electron laser tuning for variable-gap undulators
A tuning method for hard x-ray free electron lasers is presented which combines electron-beam-based alignment (EBBA) with undulator radiation spectrum analysis. This method is conducive to achieving a dispersion-free orbit, finding the undulator field center, and determining the accurate value of the gap distance for the operating undulator K for each undulator segment, and it proves to be highly reliable and robust for free electron laser lasing with variable-gap undulators. The effect of the natural focusing of the undulator on the EBBA is confirmed with measurements and a theoretical analysis. The tilt of an undulator segment subject to ground movement introduces an undulator field change which the electron beam interacts with and can be corrected by K tuning to find a precise compensating gap distance. Lattice matching with a wire-scanner-based Twiss parameter measurement along the undulator line enables the EBBA reference orbit to coincide with the e-beam distribution.