Free-electron laser dynamics in storage ring with a low emittance and momentum compaction factor configuration

Free-electron laser (FEL) operations in a storage ring operating in a low emittance and reduced momentum compaction factor setting are here presented with reference to the experimental case of Super-ACO. The new working point of the storage ring has been optimized by renouncing to the standard double achromatic feature of the magnetic optics. This makes possible the laser dynamics to benefit from high particle density conditions together with a particular regime of beam dynamics whose peculiarity is to correlate the longitudinal and horizontal oscillations in the laser interaction region. In such a new configuration, the Super-ACO two-bunches dynamics has been experimentally characterized and analyzed in comparison with analogous experimental results corresponding to standard machine operations. For a low emittance and a reduced momentum compaction factor, the calculated small signal gain of the laser results to be enhanced because of the increased particle density. Passing from the nominal to the low emittance setting of the storage ring, variations of the measured beam life-time have been observed by reason of either the increased particle density or the effect of the laser heating. Useful indications about the dependence of the laser saturation mechanism on the modified regime of the beam dynamics can be obtained from the analysis of the beam life-time. In spite of the dependence of the beam orbit on the energy spread in the laser interaction region, the typical non-equilibrium distribution of the laser intensity—the so-called detuning curve—has been observed also in correspondence of the new operation point of the storage ring. Finally, the average laser power measured at the equilibrium has been analyzed and theoretically interpreted.