68 research outputs found
Improvement of the crossed undulator design for effective circular polarization control in X-ray FELs
The production of X-ray radiation with a high degree of circular polarization
constitutes an important goal at XFEL facilities. A simple scheme to obtain
circular polarization control with crossed undulators has been proposed so far.
In its simplest configuration the crossed undulators consist of pair of short
planar undulators in crossed position separated by an electromagnetic phase
shifter. An advantage of this configuration is a fast helicity switching. A
drawback is that a high degree of circular polarization (over 90%) can only be
achieved for lengths of the insertion devices significantly shorter than the
gain length, i.e. at output power significantly lower than the saturation power
level. The obvious and technically possible extension considered in this paper,
is to use a setup with two or more crossed undulators separated by phase
shifters. This cascade crossed undulator scheme is distinguished, in
performance, by a fast helicity switching, a high degree of circular
polarization (over 95%) and a high output power level, comparable with the
saturation power level in the baseline undulator at fundamental wavelength. We
present feasibility study and exemplifications for the LCLS baseline in the
soft X-ray regime
Extension of self-seeding scheme with single crystal monochromator to lower energy < 5 keV as a way to generate multi-TW scale pulses at the European XFEL
We propose a use of the self-seeding scheme with single crystal monochromator
to produce high power, fully-coherent pulses for applications at a dedicated
bio-imaging beamline at the European X-ray FEL in the photon energy range
between 3.5 keV and 5 keV. We exploit the C(111) Bragg reflection
(pi-polarization) in diamond crystals with a thickness of 0.1 mm, and we show
that, by tapering the 40 cells of the SASE3 type undulator the FEL power can
reach up to 2 TW in the entire photon energy range. The present design assumes
the use of a nominal electron bunch with charge 0.1 nC at nominal electron beam
energy 17.5 GeV. The main application of the scheme proposed in this work is
for single shot imaging of individual protein molecules
On quantum effects in spontaneous emission by a relativistic electron beam in an undulator
Robb and Bonifacio (2011) claimed that a previously neglected quantum effect
results in noticeable changes in the evolution of the energy distribution
associated with spontaneous emission in long undulators. They revisited
theoretical models used to describe the emission of radiation by relativistic
electrons as a continuous diffusive process, and claimed that in the asymptotic
limit for a large number of undulator periods the evolution of the electron
energy distribution occurs as discrete energy groups according to Poisson
distribution. We show that these novel results have no physical sense, because
they are based on a one-dimensional model of spontaneous emission and assume
that electrons are sheets of charge. However, electrons are point-like
particles and, as is well-known, the bandwidth of the angular-integrated
spectrum of undulator radiation is independent of the number of undulator
periods. If we determine the evolution of the energy distribution using a
three-dimensional theory we find the well-known results consistent with a
continuous diffusive process. The additional pedagogical purpose of this paper
is to review how quantum diffusion of electron energy in an undulator with
small undulator parameter can be simply analyzed using the Thomson
cross-section expression, unlike the conventional treatment based on the
expression for the Lienard-Wiechert fields
Theoretical computation of the polarization characteristics of an X-ray Free-Electron Laser with planar undulator
We show that radiation pulses from an X-ray Free-Electron Laser (XFEL) with a
planar undulator, which are mainly polarized in the horizontal direction,
exhibit a suppression of the vertical polarization component of the power at
least by a factor , where is the length
of the undulator period and is the FEL field gain length. We illustrate
this fact by examining the XFEL operation under the steady state assumption. In
our calculations we considered only resonance terms: in fact, non resonance
terms are suppressed by a factor and can be
neglected. While finding a situation for making quantitative comparison between
analytical and experimental results may not be straightforward, the qualitative
aspects of the suppression of the vertical polarization rate at XFELs should be
easy to observe. We remark that our exact results can potentially be useful to
developers of new generation FEL codes for cross-checking their results
Scheme to increase the output average spectral flux of the European XFEL at keV
Techniques like inelastic X-ray scattering (IXS) and nuclear resonance
scattering (NRS) are currently limited by the photon flux available at X-ray
sources. At keV, third generation synchrotron radiation sources produce
a maximum of photons per second in a meV bandwidth. In this work we
discuss about the possibility of increasing this flux a thousand-fold by
exploiting high repetition rate self-seeded pulses at the European XFEL. Here
we report on a feasibility study for an optimized configuration of the SASE2
beamline at the European XFEL which combines self-seeding and undulator
tapering techniques in order to increase the average spectral flux at
keV. In particular, we propose to perform monochromatization at keV with
the help of self-seeding, and amplify the seed in the first part of output
undulator. The amplification process can be stopped at a position well before
saturation, where the electron beam gets considerable bunching at the 2nd
harmonic of the coherent radiation. A second part of the output undulator
follows, tuned to the 2nd harmonic frequency, i.e. at keV and is used to
obtain saturation at this energy. One can further prolong the exchange of
energy between the photon and the electron beam by tapering the last part of
the output undulator. We performed start-to-end simulations and demonstrate
that self-seeding, combined with undulator tapering, allows one to achieve more
than a hundred-fold increase in average spectral flux compared with the nominal
SASE regime at saturation, resulting in a maximum flux of order
photons per second in a meV bandwidth
Brightness of Synchrotron radiation from Undulators and Bending Magnets
We consider the maximum of the Wigner distribution (WD) of synchrotron
radiation (SR) fields as a possible definition of SR source brightness. Such
figure of merit was originally introduced in the SR community by Kim. The
brightness defined in this way is always positive and, in the geometrical
optics limit, can be interpreted as maximum density of photon flux in phase
space. For undulator and bending magnet radiation from a single electron, the
WD function can be explicitly calculated. In the case of an electron beam with
a finite emittance the brightness is given by the maximum of the convolution of
a single electron WD function and the probability distribution of the electrons
in phase space. In the particular case when both electron beam size and
electron beam divergence dominate over the diffraction size and the diffraction
angle, one can use a geometrical optics approach. However, there are
intermediate regimes when only the electron beam size or the electron beam
divergence dominate. In this asymptotic cases the geometrical optics approach
is still applicable, and the brightness definition used here yields back once
more the maximum photon flux density in phase space. In these intermediate
regimes we find a significant numerical disagreement between exact calculations
and the approximation for undulator brightness currently used in literature. We
extend the WD formalism to a satisfactory theory for the brightness of a
bending magnet. We find that in the intermediate regimes the usually accepted
approximation for bending magnet brightness turns out to be inconsistent even
parametrically.Comment: 72 pages plus cover, 4 figure
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