Coherent Harmonic Generation experiments on UVSOR-II storage ring

Coherent Harmonic Generation (CHG) is of great interest for new Free Electron Laser light sources in the VUV-soft X-ray region. In this configuration, an external laser is focused inside a first undulator. The interaction between the electron beam and the external laser leads to a density modulation resulting in coherent emission with harmonic content in a second undulator. Experiments have been carried out on the UVSOR-II FEL (Okasaki, Japan) seeded with a Ti:Sa laser at 800 nm wavelength, 2.5 mJ, 1 kHz repetition rate and 1.2 ps pulse duration. In this paper, generation of the second, third and fourth harmonics of the fundamental (400, 266 and 200 nm) using planar undulators is reported, together with angular distribution measurement of the second harmonic. We also present coherent generation of the second and third harmonic using helical undulators. These investigations on even harmonic generation with both undulator configurations may provide interesting insight in nonlinear harmonic generation with FELs for next generation FELs, aiming at GW pulse delivery at sub-nm wavelengths.