Increasing useful FEL undulator aperture, beam current and power of an electron beam propagating in a longitudinal magnetic field

Transverse inhomogeneity of undulator magnetic field prevents using large diameter electron beams for FEL because of longitudinal velocity spread. A method of beam conditioning was proposed by A.M. Sessler et al. [1]. The method is rather complex and expensive, so it is most suitable for large FEL facilities to obtain unique ultrashort-wavelength radiation. When an electron beam is transported in a longitudinal magnetic field, a simpler method of beam conditioning becomes possible. One can equalize β| across the beam by imparting supplementary cyclotron velocities to inner electrons with a profile Δv[1 − (r/ro)²], where r and ro are current radial coordinate and beam outer radius, respectively. Related devices to achieve β|= const. are described and discussed as well as the influence of beam space charge.