Observation of higher harmonics in the NIJI-IV FEL

Characteristics of higher harmonics from the optical klystron have been investigated to study the feasibility of the coherent harmonic generation (CHG) as a useful method to shorten the FEL wavelength down to the vacuum ultraviolet (VUV) in a small-scale storage ring like the NIJI-IV. Obvious modulation was observed on the 3rd harmonic line at 117 nm with a VUV monochromator. The 5th and 7th harmonics with reasonable intensity were also observed by using a real-time spectrum measurement system. These facts suggest that coherent photons are obtainable at the VUV wavelength down to ∼50 nm through the CHG process by introducing a high power external laser or utilizing the intense intra-cavity optical field in the visible FEL as a source for electron bunching in the optical klystron.