401 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
Radiation by Moving Charges
It is generally accepted that the dynamics of relativistic particles in the
lab frame can be described by taking into account the relativistic dependence
of the particles momenta on the velocity, with no reference to Lorentz
transformations. The electrodynamics problem can then be treated within a
"single inertial frame" description. To evaluate radiation fields from moving
charged particles we need their velocities and positions as a function of the
lab frame time t. The relativistic motion of a particle in the lab frame is
described by Newton's second law corrected for the relativistic dependence of
the particle momentum on the velocity. In all standard derivations the
trajectories in the source part of the usual Maxwell's equations are identified
with the trajectories calculated by using the "corrected" Newton's
second law. This way of coupling fields and particles is considered correct. We
argue that this procedure needs to be changed by demonstrating a
counterintuitive: the results of conventional theory of radiation by
relativistically moving charges are not consistent with the principle of
relativity. The trajectory of a particle in the lab frame consistent with the
usual Maxwell's equations, is found by solving the dynamics equation in
manifestly covariant form, with the proper time used to parameterize the
particle world-line in space-time. We find a difference between the "true"
particle trajectory calculated or measured in the conventional
way, and the covariant particle trajectory calculated by
projecting the world-line to the lab frame and using t to parameterize the
trajectory curve. The difference is due to a choice of convention, but only
is consistent with the usual Maxwell's equations: therefore,
a correction of the conventional synchrotron-cyclotron radiation theory is
required
Scheme for generation of fully-coherent, TW power level hard X-ray pulses from baseline undulators at the European X-ray FEL
The most promising way to increase the output power of an X-ray FEL (XFEL) is
by tapering the magnetic field of the undulator. Also, significant increase in
power is achievable by starting the FEL process from a monochromatic seed
rather than from noise. This report proposes to make use of a cascade
self-seeding scheme with wake monochromators in a tunable-gap baseline
undulator at the European XFEL to create a source capable of delivering
coherent radiation of unprecedented characteristics at hard X-ray wavelengths.
Compared with SASE X-ray FEL parameters, the radiation from the new source has
three truly unique aspects: complete longitudinal and transverse coherence, and
a peak brightness three orders of magnitude higher than what is presently
available at LCLS. Additionally, the new source will generate hard X-ray beam
at extraordinary peak (TW) and average (kW) power level. The proposed source
can thus revolutionize fields like single biomolecule imaging, inelastic
scattering and nuclear resonant scattering. The self-seeding scheme with the
wake monochromator is extremely compact, and takes almost no cost and time to
be implemented. The upgrade proposed in this paper could take place during the
commissioning stage of the European XFEL, opening a vast new range of
applications from the very beginning of operations. We present feasibility
study and examplifications for the SASE2 line of the European XFEL.Comment: 23 pages, 20 figure
Brightness of synchrotron radiation from wigglers
According to literature, while calculating the brightness of synchrotron
radiation from wigglers, one needs to account for the so called
`depth-of-field' effects. In fact, the particle beam cross section varies along
the wiggler. It is usually stated that the effective photon source size
increases accordingly, while the brightness is reduced. Here we claim that this
is a misconception originating from an analysis of the wiggler source based on
geometrical arguments, regarded as almost self-evident. According to
electrodynamics, depth-of-field effects do not exist: we demonstrate this
statement both theoretically and numerically, using a well-known
first-principle computer code. This fact shows that under the usually accepted
approximations, the description of the wiggler brightness turns out to be
inconsistent even qualitatively. Therefore, there is a need for a well-defined
procedure for computing the brightness from a wiggler source. We accomplish
this task based on the use of a Wigner function formalism. In the geometrical
optics limit computations can be performed analytically. Within this limit, we
restrict ourselves to the case of the beam size-dominated regime, which is
typical for synchrotron radiation facilities in the X-ray wavelength range. We
give a direct demonstration of the fact that the apparent horizontal source
size is broadened in proportion to the beamline opening angle and to the length
of the wiggler. While this effect is well-understood, a direct proof appears
not to have been given elsewhere. We consider the problem of the calculation of
the wiggler source size by means of numerical simulations alone, which play the
same role of an experiment. We report a significant numerical disagreement
between exact calculations and approximations currently used in literature.Comment: arXiv admin note: substantial text overlap with arXiv:1407.459
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
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