Control of the Polarization of a Vacuum-Ultraviolet, High-Gain, Free-Electron Laser

Abstract

International audienceThe two single-pass, externally seeded free-electron lasers (FELs) of the FERMI user facility are designed around Apple-II-type undulators that can operate at arbitrary polarization in the vacuum ultraviolet-to-soft x-ray spectral range. Furthermore, within each FEL tuning range, any output wavelength and polarization can be set in less than a minute of routine operations. We report the first demonstration of the full output polarization capabilities of FERMI FEL-1 in a campaign of experiments where the wavelength and nominal polarization are set to a series of representative values, and the polarization of the emitted intense pulses is thoroughly characterized by three independent instruments and methods, expressly developed for the task. The measured radiation polarization is consistently >90% and is not significantly spoiled by the transport optics; differing, relative transport losses for horizontal and vertical polarization become more prominent at longer wavelengths and lead to a non-negligible ellipticity for an originally circularly polarized state. The results from the different polarimeter setups validate each other, allow a cross-calibration of the instruments, and constitute a benchmark for user experiments

Polarization Characterization of Soft X-Ray Radiation at FERMI FEL-2

“…As expected, the absence of correlation between the degree of polarization of the two stages and the intensities confirms that two polarizations are independent and can be tuned separately. Results also suggest, as already reported [19,28], that measured polarization fluctuations are mostly related to instrumental statistical uncertainty of the polarimeter and not associated with a real fluctuation of the degree of polarization. One feature of FERMI FEL resides in the used undulator type, the variable polarization undulators of Apple-II type [22], which permits switching from one state to another state of polarization in a fast and reliable way.…”

“…The degree of circular polarization can be deduced upon assuming that the radiation is fully polarized. This assumption is reasonable in the case of FERMI FELs since it has already been demonstrated, by measuring the linear polarization, that the degree of polarization is clearly above 90% [28]. The degree of circular polarization (P circ ), without distinction between left and right, can be evaluated by the formula P circ = 1 − P 2 lin .…”

“…In our previous work [28], we extensively compared the merits of different instruments, namely, an optical, a fluorescence, and an eTOF polarimeter. We preferred the latter instrument over the other two because it permits non-invasive, single-shot measurements over a large wavelength range.…”

“…The purity of the polarization is a key parameter for the success of users' experiments. To satisfy this requirement, a systematic characterization of the polarization state of the FERMI FEL-1, whose wavelength ranges from 100 to 20 nm, was conducted a few years ago [28] and has shown a very good degree of polarization, consistently above 90%. More recently, a second FEL line (FERMI FEL-2) has been commissioned to produce radiation down to 4 nm and is now available for users' operation [21].…”

Ultrafast spin-switching of a ferrimagnetic alloy at room temperature traced by resonant magneto-optical Kerr effect using a seeded free electron laser

“…21,22 Compared to the commonly used self-amplified spontaneous emission (SASE) scheme, FERMI that uses a HGHG seeding scheme has excellent longitudinal coherence and spectral purity and is also offering multiple polarized pulses. 23 Furthermore, since FERMI is a laser-seeded FEL, the same IR laser, intrinsically synchronized with the FEL pulses, can be used for practically jitter-free time resolved experiments. 24 In the present measurements, we used as a probe 80-100 fs FERMI pulses tuned to 23.6 nm (52.5 eV), generated with repetition rate of 10 Hz, and as a pump a 780 nm IR laser.…”