Lasers have been used more and more during the last years for high-precision diagnostics of free-electron beams. The basic physical effect which is used for diagnostics is the Compton effect: the laser beam is directed against the electron beam, and some of the laser photons are converted into high-energy photons. The distribution of the high-energy photons is analyzed. The scattered photons have the same direction as the electrons due to relativistic kinematics. Therefore, the scattered photons are an image of the electron beam and can be used to measure the density distribution of the electron beam in a nondestructive manner with extremely high accuracy.1 Using circularly polarized laser light the distribution of the backscattered photons also depends on a possible spin polarization of the electron beam.
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