Recent advances in the production and control of high-brightness electron beams (e-beams) have enabled a new class of intense light sources based on the free electron laser (FEL) that can examine matter atÅngstrom length and femtosecond time scales. The free, or unbound, electrons act as the lasing medium, which provides unique opportunities to exquisitely control the spatial and temporal structure of the emitted light through precision manipulation of the electron distribution. We present an experimental demonstration of light with orbital angular momentum (OAM) generated from a relativistic e-beam rearranged into an optical scale helix by a laser. With this technique, we show that a Gaussian laser mode can be effectively up-converted to an OAM mode in an FEL using only the e-beam as a mode-convertor. Results confirm theoretical predictions, and pave the way for the production of coherent OAM light with unprecedented brightness down to hard x-ray wavelengths for wide ranging applications in modern light sources.

Experimental observation of the microbunching of a relativistic electron beam at the second harmonic interaction frequency of a helical undulator is presented. The microbunching signal is observed from the coherent transition radiation of the electron beam and indicates experimental evidence of a dominantly helical electron beam density distribution. This result is in agreement with theoretical and numerical predictions and provides a proof-of-principle demonstration of proposed schemes designed to generate light with orbital angular momentum in high-gain free-electron lasers.

Short period, high field undulators have been shown to permit operation of x-ray free-electron lasers with short gain-lengths, and at unprecedented short wavelengths. In addition, the reduced beam energy required to reach resonance with a given radiation wavelength in short period undulators could prove useful in advanced inverse free-electron laser (IFEL) seeding schemes for future light sources, such as high-gain harmonic generation and echo-enabled harmonic generation, or in IFEL acceleration. We report here the in situ beam testing of a 9 mm period length cryogenic undulator having undulator parameter near unity in the context appropriate for advanced seeding and acceleration schemes, a linear accelerator. Because of the short period length of the undulator, a 47 MeV high-brightness electron beam could be used to produce near infrared photons via the undulator radiation mechanism. The undulator radiation was observed through a filter and the spectral response of the undulator is compared to simulation. Finally, an 800 nm seed laser was introduced in order to generate an energy modulation via the IFEL mechanism. Resonance between the electron beam and the laser seed was achieved without detailed knowledge of the temperature dependent undulator magnetic field through the observation of the undulator radiation. The energy modulation (and concomitant energy spread increase) of the electron beam was observed both directly in an electron beam spectrometer and indirectly via coherent transition radiation after conversion to a density modulation in a longitudinally dispersive chicane.

No abstract

No abstract