• E. Hemsing, A. Marinelli, P. Musumeci, J.B. Rosenzweig, D. Schiller
  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.