Development of Free-electron Lasers Using Two "higher Orders" with the Storage Ring Niji-iv

We developed higher-harmonic free-electron laser (FEL) oscillations with the storage ring NIJI-IV from the visible to near-infrared region. Using another “higher order”, that is, higher-diffraction order of the target wavelength of dielectric multilayer mirrors, we realized a lasing on the highest order in the harmonic FELs. Moreover, the FEL wavelength was shortened to the visible region in the NIJI-IV infrared FEL system. We demonstrated that wavelength regions of FEL oscillations could be selected by changing a condition of the optical cavity in the same conditions of the electron beam and insertion device. Our experimental results will give clues to realize a resonator-type FEL in the extreme ultraviolet and X-ray regions.