Third Harmonic Lasing in the Niji-iv Storage Ring Free- Electron Lasers

We report for the first time to our knowledge the experimental achievement of storage ring free-electron laser (FEL) oscillations on the third harmonic in the wide infrared region. The FELs were oscillated in the wavelength regions of 852-881 nm and 1510-1551 nm. The measured linewidth of the third-harmonic FEL was narrower than that of the fundamental FEL owing to the narrower spectral width of the spontaneous emission. The higher harmonic FEL oscillations will be useful for developing an x-ray FEL oscillation with the lower electron-beam energy. INTRODUCTION A higher-harmonic FEL oscillation has been studied for obtaining shorter-wavelength coherent light, such as coherent harmonic generation [1], high gain harmonic generation [2] and echo-enable harmonic generation [3]. The higher-harmonic FEL oscillation is superior to these techniques, which generate higher-harmonic spontaneous emissions, in terms of an average power and stability of the wavelength. Limitation of the FEL wavelength owing to cavity mirrors is regarded as a fault of the higherharmonic FEL oscillation. Recently, however, an x-ray FEL oscillation (X-FELO) was proposed to obtain higher brilliance x-ray compared with the SASE [4]. When the higher-harmonic FEL oscillation is applied to the XFELO, the electron-beam energy for oscillation in the xray region can be decreased. Pioneer studies of the higher-harmonic FELs have been reported [5, 6], and several groups have been already achieved FEL oscillations on the fifth or lower harmonics [7-9]. Although these studies have contributed to the progress in FEL physics, differences between the higherharmonic FEL and fundamental FEL have not been observed sufficiently. Thus, we developed an optical klystron ETLOK-III that was aimed at investigating higher-harmonic FEL oscillations [10]. The thirdharmonic FEL oscillations have been achieved with it in wide wavelength regions of 852-881 nm and 1510-1551 nm [11]. In this article, the experimental results of the third-harmonic FELs are described in detail. STORAGE RING NIJI-IV FEL Although the NIJI-IV dedicated to FEL oscillations is a compact storage ring with a 29.6 m circumference, it has two 7.25 m straight sections [12]. Figure 1 shows the layout of the present NIJI-IV FEL system. The ETLOK-II, a 6.3 m optical klystron, is installed in one of these sections, and we achieved the first FEL oscillations with Long-wavelength FEL section (838 2673 nm) QF