|
内容記述 |
In a free-electron laser (FEL) oscillator with a linear undulator, odd harmonics appear as either spontaneous emission or as linear and nonlinear harmonics associated with the FEL pulse evolution. Additionally, asymmetric misalignment of the electron beam or the resonator can lead to off-axis gain, generating even harmonics. We conducted an experiment at KU-FEL, operated at 5 µm, to measure the 6th- and 7th- harmonic spectra using a spectrometer equipped with an intensified CCD (ICCD). By setting the ICCD gate width to 30 ns, individual pulses were isolated from the FEL pulse train with a repetition rate of 29.75 MHz. Scanning the gate timing enabled us to capture the time evolution of the harmonic spectrum within the FEL macro pulse. The results revealed that the 6th- and 7th-harmonic spectra broaden as the macropulse intensity rises, with interference observed between the two harmonics. This spectral broadening correlates with the shortening of the FEL pulse during amplification, consistent with the characteristics of pulse evolution in the superradiant regime. Such interference between adjacent-order harmonics has also been observed in high-harmonic generation from solid-state targets and, in some cases, utilized to measure the carrier-envelope phase (CEP) of optical pulses. Based on the results of our FEL harmonic measurements and numerical simulations, we will discuss the potential for CEP measurement using FEL harmonics, highlighting the implications for further developments in FEL-based attosecond X-ray sources via high-harmonic generation. |
