Gamma rays produced by intra-cavity inverse Compton scattering of a storage ring free-electron laser

A collimated tunable gamma-ray beam has been generated by inverse Compton scattering of an additional positron bunch with the Super-ACO storage ring free-electron laser (FEL) positioned inside the optical resonator in the UV range (345-355 nm). This specific novel set-up ensures a natural transverse overlap and synchronization. The tunable gamma-ray beam produced covers the energy range from a few MeV up to 35 MeV, the maximum energy achievable with a FEL. The number of scattered gamma rays is relatively large (5000 000 photons/s at 800 MeV and 2000 000 photons/s at 700 MeV, i.e. the greatest value for a FEL). Furthermore, a significant number of high-energy gamma rays have also been produced by scattering of the spontaneous radiation stored in the optical cavity. The scattering is performed from a positron bunch outside the undulator, demonstrating for the first time the possibility of optimizing the yield and/or the spectrum by alteration of the stored beam without alteration of the FEL.