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Fermi Machine Description

The Undulators and the FEL Process

One undulator around the vacuum chamber

FEL-1 and FEL-2 are required to provide, at all wavelengths, continuously tunable beam polarization ranging from linear-horizontal to circular to linear-vertical. The FEL-1 radiator and the final radiator in FEL-2 have therefore been chosen to be of the APPLE-II, pure permanent magnets type. For the modulator a simple, linearly-polarized configuration is best, due to both its simplicity and because the input radiation seed can be linearly polarized. The wavelength will be tuned by changing the undulator gap at constant electron beam energy. The FEL-1 and FEL-2 radiators consist of 6 and 8 undulator magnets. The magnetic lengths of the individual magnets are 2.34 m (containing 36 periods) for the FEL-1 and first FEL-2 radiators and 2.40 m (48 periods) for the second FEL-2 radiator, respectively. Electromagnetic quadrupoles, high quality beam position monitors, and quadrupole movers are installed in between undulators to correct the electron trajectory.
The accelerator and FEL parameters were defined based on theoretical studies and simulations. A cornerstone has been provided by “start-to-end” simulations, in which the electron beam is tracked from the photocathode, through the linac and all the way through the FEL process. The exhaustive studies carried out included foreseen random perturbations and jitters of accelerator and FEL parameters.
The consequence of orbit displacements from the ideal trajectory in the undulators were simulated. At the wavelength of 10 nm, the FEL process requires the straightness of the electron trajectory in the undulators to stay within 10 µm (rms value over the undulators length). While this requirement is beyond the state-of-the-art of present surveying techniques, realistic simulations show that a combination of the latter and of beam-based-alignment procedures (tested at the Stanford Linear Collider and proposed for the LCLS) will achieve the desired performance.
Last Updated on Thursday, 09 January 2020 18:54