Abstract
It was demonstrated that harmonic order in free-electron laser (FEL) oscillations could be switched by adjusting the dispersive gap of the optical klystron ETLOK-III in the storage ring NIJI-IV. The effective gains for the fundamental and third-harmonic FEL oscillations were evaluated and it was confirmed that the FEL oscillated at the order of the harmonic with the higher effective gain. The ratio between the effective gain for the fundamental and that for the third harmonic was controlled by the dispersive gap. It was also demonstrated that a spectral measurement of the FEL-based Compton scattering X-ray beam was effective for directly observing the switching of the harmonic order. These results contribute to the development of higher-harmonic FEL oscillations suppressing the fundamental FEL oscillation in the extreme ultraviolet and X-ray regions.
Highlights
The use of higher-harmonic oscillations in free-electron lasers (FELs) is an effective technique for extending the lasing wavelength to the shorter-wavelength region (Colson, 1981)
Using the infrared FEL device in the storage ring NIJI-IV, we demonstrated that the order of the harmonic in the FEL oscillation could be switched by adjusting the dispersive gap of the optical klystron without changing the characteristics of the optical cavity
The effective gains for the fundamental and third-harmonic FEL oscillations were evaluated, and the condition that both effective gains were equalized was derived as a curve on a twodimensional graph with axes of the electron-beam current per bunch and dispersive gap
Summary
The use of higher-harmonic oscillations in free-electron lasers (FELs) is an effective technique for extending the lasing wavelength to the shorter-wavelength region (Colson, 1981). To select the order of the harmonic in the resonator-type FEL devices, conventionally, an aperture or etalon was inserted into the optical cavity (Benson & Madey, 1989; Hajima et al, 2001; Kubarev et al, 2011). These additional elements controlled the effective gain for each harmonic FEL. Using a simpler approach, even one optical klystron can make the effective gain of the higher-harmonic FEL larger than that of the fundamental FEL by controlling the dispersive section As these techniques do not affect the optical cavity, it is simple to switch the oscillation harmonics. The switching of the order of the harmonic FEL by controlling the dispersive gap is demonstrated
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