Abstract
Time-delayed, narrow-band echoes generated by forward Bragg diffraction of an X-ray pulse by a perfect thin crystal are exploited for self-seeding at hard X-ray free-electron lasers. Theoretical predictions indicate that the retardation is strictly correlated to a transverse displacement of the echo pulses. This article reports the first experimental observation of the displaced echoes. The displacements are in good agreement with simulations relying on the dynamical diffraction theory. The echo signals are characteristic for a given Bragg reflection, the structure factor and the probed interplane distance. The reported results pave the way to exploiting the signals as an online diagnostic tool for hard X-ray free-electron laser seeding and for dynamical diffraction investigations of strain at the femtosecond timescale.
Highlights
Hard X-ray free-electron lasers (XFELs) are novel photon sources, which rely on the self-amplified spontaneous emission (SASE) process to obtain peak brightnesses in the soft and hard X-ray regime that are orders of magnitude larger than those achieved with insertion devices at third-generation synchrotron light sources (Margaritondo & Rebernik Ribic, 2011)
The intensity humps in the vertical direction appear only in the latter case, in agreement with the simulation results shown in Figs. 3(d) and (e)
Indirect experimental evidence for the retardation of temporal echoes is given by the fact that self-seeding at an XFEL has been implemented, while for the transverse spatial displacement the hints come from the fact that the trajectory of the electron beam in the downstream undulator section has to be adjusted correspondingly
Summary
Hard X-ray free-electron lasers (XFELs) are novel photon sources, which rely on the self-amplified spontaneous emission (SASE) process to obtain peak brightnesses in the soft and hard X-ray regime that are orders of magnitude larger than those achieved with insertion devices at third-generation synchrotron light sources (Margaritondo & Rebernik Ribic, 2011). The SASE radiation arises from amplification of stochastic noise in the electron bunch. It consists of many longitudinal modes (Wark & Lee, 1999), and exhibits strong shot-to-shot fluctuations of both the mean pulse energy and the pulse spectrum. The relative bandwidth at an XFEL operating in SASE mode is typically of the order of 10À3. Many XFEL experiments require a much narrower bandwidth and excellent spectral stability (Alonso-Mori et al, 2015). These beam properties can be enforced by inserting a monochromator in the X-ray beam path, at the expense of losing a large fraction of the beam intensity
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