Measurements of Fel Polarization at Fermi

We report detailed quantitative characterization of different polarization states of a single-pass, externallyseeded FEL operating with variable polarization undulators in the VUV spectral range. The experiment has been performed at FERMI FEL-1 operated in the 52–26 nm wavelength range. Three different, independent polarimeter setups, installed at the end of experimental beamlines, have been used to characterize the four “pure” polarization states: horizontal, vertical, right-circular and left-circular. The impact of downstream transport optics upon the radiation polarization has been assessed; at longer wavelengths, dichroism effects lead to a nonnegligible ellipticity for an originally circularly polarized state. The results from the different polarimeter setups validate each other and allow a cross-calibration of the instruments.