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Seeded Free Electron Lasers (FELs) - an external laser source is injected inside the undulator to perform energy modulation of the electron beam - deliver intense, short duration, short wavelength light pulses with a high temporal coherence degree. Since the polarization of the radiation depends on the undulator magnetic field, the FELs operated with adjustable undulators are unique candidates for providing flexible polarized light of high quality, required for probing symmetry properties of matter. Experiments have been performed on the UVSOR-II storage ring FEL in the seeded Coherent Harmonic Generation (CHG) scheme operated with helical undulators. The electron beam stored at 500 MeV is seeded using a 800 nm Ti:Sa laser, allowing radiation at 400 nm (second harmonic). A quarter-wavelength plate allows the tuning of the polarization of the seeded laser from linear to elliptic or circular. We show that the FEL radiation intensity can be significantly improved via the seed polarization matching. Other parameters of optimization are also presented, such as the focussing mode. The experimental results are analysed using a simple 1D analytical model and the 3D numerical code GENESIS.
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