• E. Hemsing, A. Marinelli, P. Musumeci, J.B. Rosenzweig, R. Tikhoplav
  UCLA, Los Angeles, California
• A. Gover
  University of Tel-Aviv, Faculty of Engineering, Tel-Aviv
• S. Reiche
  PSI, Villigen

The resonant harmonic interaction of an electron beam (e-beam) with an EM input field in a helical undulator is explored. The e-beam is coupled to the input radiation field at frequency harmonics through transverse gradients in the EM field, and helical micro-bunching of the e-beam is shown to occur naturally at the higher harmonics with the injection of a simple gaussian laser mode onto a cylindrically symmetric e-beam. This approach is under investigation as a method to generate a strongly pre-bunched e-beam seed for superradiant emission of light that carries orbital angular momentum in a downstream free-electron laser.