Sustained saturation in a free-electron laser oscillator at perfect synchronism of an optical cavity

The first observation of sustained saturation in a free-electron laser (FEL ) oscillator at perfect syn chronism of an optical cavity is presented. A s imultaneous measurement of FEL power and absolute detuning length of the op tical cavity (SL) has clearly shown that the FEL efficiency becomes maximum at deltaL = 0.0 +/-: 1 mum, although it has been considered that only a tr ansient state exists at deltaL = 0 due to the well-known laser lethargy eff ect. The observed efficiency detuning curve is well reproduced by our numer ical simulation including a small shot-noise effect.