Stability of Bragg reflectors under megahertz heat load at XFELs.

Modern X-ray free-electron laser (XFEL) sources can deliver photon pulses with millijoule pulse energies and megahertz repetition rate. As shown by the simulations in this work, for particular cases the dynamical heat load effects for Bragg reflectors could cause problems at these facilities. These problems would be underestimated if only quasi-static thermoelastic simulations are considered. Nevertheless, for the sake of simplicity the quasi-static approach is a common choice for estimating heat load effects. To emphasize the relevance of dynamical thermoelastic effects, the response to the partial absorption of an X-ray pulse, as provided by a saturated X-ray free-electron laser oscillator (XFELO) in a single crystal diamond with a thickness of 100 µm and lateral dimensions in the millimetre range, is discussed in this work. The outcome of the dynamic thermoelastic simulations indicates a clear dominance regarding the strain value reached, which is present for consecutive X-ray matter interactions with megahertz repetition rate.

Mapping few-femtosecond slices of ultra-relativistic electron bunches.

Free-electron lasers are unique sources of intense and ultra-short x-ray pulses that led to major scientific breakthroughs across disciplines from matter to materials and life sciences. The essential element of these devices are micrometer-sized electron bunches with high peak currents, low energy spread, and low emittance. Advanced FEL concepts such as seeded amplifiers rely on the capability of analyzing and controlling the electron beam properties with few-femtosecond time resolution. One major challenge is to extract tomographic slice parameters instead of projected electron beam properties. Here, we demonstrate that a radio-frequency deflector in combination with a dipole spectrometer not only allows for single-shot extraction of a seeded FEL pulse profile, but also provides information on the electron slice emittance and energy spread. The seeded FEL power profile can be directly related to the derived slice emittance as a function of intra-bunch coordinate with a resolution down to a few femtoseconds.