PRODUCTION OF COHERENT X-RAYS WITH A FREE-ELECTRON LASER BASED ON OPTICAL

The interaction between high-brilliance electron beams and counter-propagating laser pulses produces X-rays via Thomson back-scattering. If the laser pulse is long and intense enough, the electrons of the beam can bunch on the scale of the emitted X-ray wavelength and collective effects can occur. These effects give rise to the FEL instability and the system behaves like a free-electron laser based on an optical undulator. Coherent X-rays can be irradiated, with a bandwidth much thinner than that of the corresponding incoherent emission. We analyse with a 3D code the transverse effects in the emission and give a generalized form of the Pellegrini criterion which is validated on numerical evidence