Results of the free electron laser oscillation experiments on the ACO storage ring

A storage ring free-electron laser oscillator has been operated above threshold at a visible wavelength λ ≃ 6 500 Å. This laser was obtained with electrons of 166 MeV circulating in the storage ring ACO. The light emitted by the electrons passing through an optical klystron (a modified version of an undulator) is stored in a high-Q optical cavity. First some below laser threshold phenomena are reported. Then the spectral and macro-temporal structure of the laser are described. The average power is shown to be consistent with the limit imposed by the heating of the electron beam (Renieri's limit). Finally a factor of 100 enhancement in the peak power, over the Renieri's limit, has been obtained by driving the laser with an external low frequency generator.