Intense FEL light pulses with a length of three optical cycles produced at zero detuning of an optical cavity length

Nishimori, Nobuyuki; Nagai, Ryoji

We have observed sustained saturation in a Free-Electron Laser (FEL) oscillator at zero detuning of an optical cavity for the first time, and confirmed that the optical pulse length at zero detuning is 250 fs FWHM at wavelength of 22 um. The optical length corresponds to three optical cycles, which is the fewest cycles ever achieved at FELs in the world. The peak FEL power reaches 0.8 GW with the single optical pulse energy of 200 uJ. The sustained lasing at zero detuning is not allowed in conventional FEL oscillator theories and the physical mechanism is under investigation.