12 research outputs found
Novel opportunities for sub-meV inelastic X-ray scattering at high-repetition rate self-seeded X-ray free-electron lasers
Inelastic X-ray scattering (IXS) is an important tool for studies of
equilibrium dynamics in condensed matter. A new spectrometer recently proposed
for ultra-high-resolution IXS (UHRIX) has achieved 0.6~meV and 0.25~nm
spectral and momentum transfer resolutions, respectively. However, further
improvements down to 0.1~meV and 0.02~nm are required to close the gap
in energy-momentum space between high and low frequency probes. We show that
this goal can be achieved by further optimizing the X-ray optics and by
increasing the spectral flux of the incident X-ray pulses. UHRIX performs best
at energies from 5 to 10 keV, where a combination of self-seeding and undulator
tapering at the SASE-2 beamline of the European XFEL promises up to a
hundred-fold increase in average spectral flux compared with nominal SASE
pulses at saturation, or three orders of magnitude more than possible with
storage-ring based radiation sources. Wave-optics propagation shows that about
~ph/s in a -eV bandwidth can be achieved on the
sample. This will provide unique new possibilities for dynamics studies by IXS.Comment: 17 pages, 14 figure
Transverse coherence properties of X-ray beams in third-generation synchrotron radiation sources
This article describes a complete theory of spatial coherence for undulator
radiation sources. Current estimations of coherence properties often assume
that undulator sources are quasi-homogeneous, like thermal sources, and rely on
the application of the van Cittert-Zernike theorem for calculating the degree
of transverse coherence. Such assumption is not adequate when treating third
generation light sources, because the vertical(geometrical) emittance of the
electron beam is comparable or even much smaller than the radiation wavelength
in a very wide spectral interval that spans over four orders of magnitude (from
0.1 Angstrom up to 10^3 Angstrom). Sometimes, the so-called Gaussian-Schell
model, that is widely used in statistical optics in the description of
partially-coherent sources, is applied as an alternative to the
quasi-homogeneous model. However, as we will demonstrate, this model fails to
properly describe coherent properties of X-ray beams from non-homogeneous
undulator sources. As a result, a more rigorous analysis is required. We
propose a technique, based on statistical optics and Fourier optics, to
explicitly calculate the cross-spectral density of an undulator source in the
most general case, at any position after the undulator. Our theory, that makes
consistent use of dimensionless analysis, allows relatively easy treatment and
physical understanding of many asymptotes of the parameter space, together with
their region of applicability. Particular emphasis is given to the asymptotic
situation when the horizontal emittance is much larger than the radiation
wavelength, and the vertical emittance is arbitrary. This case is practically
relevant for third generation synchrotron radiation sources.Comment: 71 pages, 20 figures - Version accepted for publication in Nuclear
Inst. and Methods in Physics Research,