X-ray free electron lasers provide short, polarized, high-power pulses of x-ray radiation, where polarization properties are determined by the undulator magnetic field. We propose a cost-effective method to shape the polarization of FEL radiation on the sample both spatially, on a few-μm scale, and/or temporally on the 100-femtosecond scale. The method is based on coaxial superposition of two coherent radiation pulses with different frequencies emitted in two consequent undulators set to emit radiation with orthogonal polarization states. Its capabilities are demonstrated via numerical simulations for the SASE3 undulator line of the European X-ray Free-Electron Laser. Generation of x-ray free-electron laser pulses with shaped polarization may be interesting for studies of magnetism or ultrafast phenomena and will facilitate further developments of complex light engineering in the soft X-ray range.

Wavefront propagation codes play pivotal roles in the design of optics at synchrotron radiation sources. However, they usually do not account for the stochastic behavior of the radiation field originating from shot noise in the electron beam. We propose a computationally efficient algorithm to calculate a single statistical realization of partially coherent synchrotron radiation fields at a given frequency. This field can be consequently propagated from the source position downstream through an optical beamline to the sample position. The proposed algorithm relies on a method for simulating Gaussian random fields. We initially generate the radiation field as Gaussian white noise and then restrict it in both real and inverse space domains for a given radiation size and divergence. We exploit the assumption of quasi-homogeneity of the source. However, we show that the method is applicable with reasonable accuracy outside of this assumption. The proposed algorithm is consistent with other well-established approaches, and, in addition, it possesses an advantage in terms of computational efficiency. It can be extended to other types of sources that follow Gaussian statistics. Finally, the demonstration of the algorithm is well suited for educational purposes.

The European XFEL is currently the only high-repetition rate hard X-ray free electron laser (FEL) facility in operation worldwide. We significantly improved its capabilities by installing a cascaded Hard X-ray Self-Seeding (HXRSS) system, composed of two single-crystal monochromators. With this system, mJ-level pulses in the photon energy range of 6 -14keV with a bandwidth around 1eV (corresponding to about 1mJ/eV spectral density) were generated. Combined with the burst-mode, multi-MHz repetition rate of the European XFEL accelerator, the cascaded HXRSS setup provides two orders of magnitude higher average spectral brightness than any other FEL facility. At 2.25 MHz repetition rate and photon energies in the 6-7 keV range, we observed for the first time heat-load effects on the HXRSS crystals, substantially altering the spectra of subsequent X-ray pulses. Using the cascaded self-seeding scheme, we successfully reduced this effect to below detection level. These results open up exciting possibilities in a wide range of scientific fields, exploiting the extreme brightness and the narrow bandwidth of HXRSS pulses.

No abstract