UCLA, Los Angeles, California
ENEA C.R. Frascati, Frascati (Roma)
PSI, Villigen
Narrow bandwidth FEL operation is one of the main challenges for future FEL facilities. Several schemes have been proposed to obtain a narrower bandwidth than that achievable with self amplified spontaneous emission starting from shot-noise. In this work the properties of two such schemese (high gain harmonic generation and self-seeding) are investigated and compared taking into account several non-ideal beam effects, with particular attention paid to the effect of bandwidth broadening associated to non-linear longitudinal phase-space. The comparison between the two schemes has been carried out with numerical simulations performed with GENESIS 1.3 and PERSEO FEL codes using both an idealized beam and realistic beams obtained with start to end simulations.
UCLA, Los Angeles, California
An electron beam that is subject to the typical FEL microbunching instability is microbunched longitudinally in both density and velocity, according to the shape of the ponderomotive phase bucket. Higher-order three-dimensional microbunching geometries can be created if the e-beam interacts either with a more complicated resonant field structure, or at higher harmonics of the fundamental resonance. At harmonics inside a helical wiggler, the e-beam interacting with an axisymmetric gaussian laser field becomes microbunched into a helix, or combination of twisted helices, depending on the harmonic number. The twisted e-beam can then be used to emit coherent light with orbital angular momentum in a downstream radiator. An experimental effort to explore the principles of this interaction is discussed.
UCLA, Los Angeles, California
Recent observation of coherent optical transition radiation in FEL injectors has provoked considerable theoretical activity. As this phenomena is provoked by microbunching at or near the level of the mean interparticle spacing in the beam, any description of it should resolve the beam at the particle level. Here we present first simulations that fulfill this requirement, based on a three-dimensional code in which fields are found in the beam frame using Fourier methods. The simulations take into account acceleration, which serves to freeze relative longitudinal motion, transverse focusing, and downstream bending motion. Comparisons are made between code predictions and experimental results, as well as with theoretical models.