Free-electron laser diagnosis by polarization analysis of Thomson backscattered light

The polarization properties of backscattered radiation from a relativistic electron beam are analyzed. It is shown that the polarization state of the backscattered radiation is changed with respect to that of the incoming radiation whenever a transverse component is present in the velocity distribution of the electron beam. Changes of polarization properties are evaluated in order to serve as a diagnostic tool for electron-beam parameters relevant to the performance of free-electron lasers, namely the beam emittance and the wiggler parameter. A measurable dimensionless "depolarization parameter" is introduced which directly measures these two parameters.