Abstract

Summary : The CLIO infrared free-electron laser (FEL) is used as an user facility⁽¹⁾ since 1993. CLIO spectral range spans 3 to 50 µm and its peak power is several MW in 0.2 to 5 ps long micropulses. It is based on a dedicated radiofrequency linear accelerator. The repetition rate of the micropulses can be varied from to 32 to 4 ns during 10 µs long macropulses (at up to 50 Hz) and average power up to a few Watts is achievable. About 2400 hours of laser beam time are produced annually, of which 1600 are dedicated to laser users and 800 to FEL physics and optimisation studies : ultrashort FEL pulses, 2-colors operation, undulator step tapering, harmonic generation, self amplified spontaneous emission (SASE) and Compton backscattering. The CLIO infrared free-electron laser (FEL) is used as an user facility⁽¹⁾ since 1993. CLIO spectral range spans 3 to 50 µm and its peak power is several MW in 0.2 to 5 ps long micropulses. It is based on a dedicated radiofrequency linear accelerator. The repetition rate of the micropulses can be varied from to 32 to 4 ns during 10 µs long macropulses (at up to 50 Hz) and average power up to a few Watts is achievable. About 2400 hours of laser beam time are produced annually, of which 1600 are dedicated to laser users and 800 to FEL physics and optimisation studies : ultrashort FEL pulses, 2-colors operation, undulator step tapering, harmonic generation, self amplified spontaneous emission (SASE) and Compton backscattering. The two-colors operation has been demonstrated for the first lime with an FEL<²)by using a very simple scheme : two undulators are used, instead of one, and they arc tuned to different resonnant frequencies. By varying these frequencies (i.e. the undulator magnetic gaps) one can vary independanlly the wavelength of the colors within the optical gain bandwidth. On CLIO, at a given electron energy, the 2-colors wavelengths can be separated by a factor of up to two, when using a hole as the laser cavity output coupler. It was easily verified that the 2 colors are occuring simultaneously during the 10 ps long macropulse. However, the frequency distribution inside the micropulse remained unknown. Recently, we have studied the distribution of the 2 colors within the micropulse, which is only a few picosecond long, by means of sum frequency generation in a Te crystal. The occurcncc of the 2 colors sum frequency show ■ that they are produced simultaneously by the electron beam circulating in the undulators. Therefore the resulting electron beam shape does not look like 2, or more, successive trains of regularly spaced pulses, one for each color; it exhibits a more complicated structure corresponding to the superposition of 2 pulse trains, each being at a different frequency, i.e. a 2-frequency bunching. Autocorrelation spectra of the sum frequency and of each, doubled, single frequency have been recorded in order to better understand the distribution of the different colors within the micropulsc.

© 1996 IEEE