“Diffraction-free” optical beams in inverse free electron laser accelerators

Abstract “Diffraction-free” optical beams correspond to exact solutions of the wave equation in free space with the remarkable property that they propagate with negligible transverse spreading for distances much larger than the Rayleigh range. The requirement for this to occur is a large aperture. Using a 2D computer code, we find that these optical beams will also propagate with negligible diffraction even when perturbed by the electron beam in an IFEL; indeed they match well the FEL requirement for the accelerator. The numerical simulations are performed for the proposed facility at Brookhaven in which λ s = 10 μ m, B = 1.5 T (linearly tapered l w = 1.31−6.28 cm) and the optical beam power is either 8 × 10 11 W or 2.3 × 10 10 W. Approximately 70% of the electrons constituting a beam of current 5 mA or 15 A, radius 0.14 mm and initial energy of 50 MeV is accelerated at 50 MeV/m.