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In the Etude d'un LaSer Accordable linear accelerator, electron bunches consist of trains of picosecond pulses extracted from a photocathode by a drive laser system. The fluctuations of the mean intensity of pulse trains at the output of the laser system are around 3% rms. A feed-forward stabilization system that reduces these fluctuations to better than 0.7% rms for periods of 5 min is presented.

Spectral broadening is expected in free-electron lasers (FEL) due to sideband generation and nonlinear mode coupling. For high enough electron current, the FEL beam presents an asymptotic regime characterized by a broad and turbulent spectrum. Theoretical investigations have shown that this regime also exhibits a strong efficiency enhancement and a universal low brightness. This paper reports experimental evidence for these new scaling laws using the rf linac-based ELSA FEL. PACS numbers: 41.60.Cr, 52.35.Mw, 52.35.Ra A Compton free-electron laser (FEL) provides physics issues that are typical of a wide range of plasma physics problems where electrons and electromagnetic waves are coupled in a highly nonlinear way. Extensive numerical and theoretical investigations [1,21 lead to the following scenario for the FEL spectral behavior. Because of the synchrotron motion of the electrons, a sideband mode appears after saturation of the fundamental line [3]. Then, nonlinear mode coupling mechanisms, such as difference frequency generation [1], result in an effective efficiency

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