Appl. Phys. Lett. 100, 091110 (2012)

Erik Hemsing and colleagues from the University of California Los Angeles in the USA have presented evidence of the helical microbunching of charge within an electron beam. Previous theory suggested that such beams could be realized in principle, but an experimental demonstration was lacking until now. The ability to induce microbunching may ultimately lead to the development of high-gain, high-mode-generation free-electron lasers capable of emitting light with orbital angular momentum. For this to be possible, the electrons must be arranged with a spiral period equal to the wavelength of light emitted from the free-electron laser. Theory suggested this could be achieved by tuning the laser such that it imparts energy to the electrons through a second-harmonic interaction, which can have an azimuthal dependence on the laser mode profile. The researchers passed a short 3 ps electron bunch (enery of around 12 MeV) from the Neptune Inverse Free Electron Laser through a 1.9-cm-period helical undulator together with a 10.6 μm, 100 ps pulse from a CO2 laser. They measured the coherent transition radiation emitted from the electron beam by focusing the output into a liquid-nitrogen-cooled mercury cadmium telluride detector. The data confirmed the existence of microbunching and gave a measure of the process efficiency.