638 research outputs found
Coherence properties of the radiation from FLASH
FLASH is the first free electron laser user facility operating in the vacuum
ultraviolet and soft x-ray wavelength range. Many user experiments require
knowledge of the spatial and temporal coherence properties of the radiation. In
this paper we present an analysis of the coherence properties of the radiation
for the fundamental and for the higher odd frequency harmonics. We show that
temporal and spatial coherence reach maximum close to the FEL saturation but
may degrade significantly in the post-saturation regime. We also find that the
pointing stability of short FEL pulses is limited due to the fact that
non-azimuthal FEL eigenmodes are not sufficiently suppressed. We discuss
possible ways for improving the degree of transverse coherence and the pointing
stability.Comment: 17 pages, 19 figure
Statistical properties of the radiation from SASE FEL operating in a post-saturation regime with and without undulator tapering
We describe statistical and coherence properties of the radiation from x-ray
free electron lasers (XFEL) operating in the post-saturation regime. We
consider practical case of the SASE3 FEL at the European XFEL. We perform
comparison of the main characteristics of the X-ray FEL operating in the
post-saturation regime with and without undulator tapering: efficiency,
coherence time and degree of transverse coherence.Comment: 6 pages, 5 figure
A possible upgrade of FLASH for harmonic lasing down to 1.3 nm
We propose the 3rd harmonic lasing in a new FLASH undulator as a way to
produce intense, narrow-band, and stable SASE radiation down to 1.3 nm with the
present accelerator energy of 1.25 GeV. To provide optimal conditions for
harmonic lasing, we suggest to suppress the fundamental with the help of a
special set of phase shifters. We rely on the standard technology of
gap-tunable planar hybrid undulators, and choose the period of 2.3 cm and the
minimum gap of 0.9 cm; total length of the undulator system is 34.5 m. With the
help of numerical simulations we demonstrate that the 3rd harmonic lasing at
1.3 nm provides peak power at a gigawatt level and the narrow intrinsic
bandwidth, 0.1% (FWHM). Pulse duration can be controlled in the range of a few
tens of femtoseconds, and the peak brilliance reaches the value of 10^31
photons/(s mrad^2 mm^2 0.1%BW). With the given undulator design, a standard
option of lasing at the fundamental wavelength to saturation is possible
through the entire water window and at longer wavelengths. In this paper we
briefly consider additional options such as polarization control, bandwidth
reduction, self-seeding, X-ray pulse compression, and two-color operation. We
also discuss possible technical issues and backup solutions.Comment: arXiv admin note: text overlap with arXiv:1205.250
Prospects for CW and LP operation of the European XFEL in hard X-ray regime
The European XFEL will operate nominally at 17.5 GeV in SP (short pulse) mode
with 0.65 ms long bunch train and 10 Hz repetition rate. A possible upgrade of
the linac to CW (continuous wave) or LP (long pulse) modes with a corresponding
reduction of electron beam energy is under discussion since many years. Recent
successes in the dedicated R&D program allow to forecast a technical
feasibility of such an upgrade in the foreseeable future. One of the challenges
is to provide sub-Angstrom FEL operation in CW and LP modes. In this paper we
perform a preliminary analysis of a possible operation of the European XFEL in
the hard X-ray regime in CW and LP modes with the energies of 7 GeV and 10 GeV,
respectively. We consider lasing in the baseline XFEL undulator as well as in a
new undulator with a reduced period. We show that, with reasonable requirements
on electron beam quality, lasing on the fundamental will be possible in
sub-Angstrom regime. As an option for generation of brilliant photon beams at
short wavelengths we also consider harmonic lasing that has recently attracted
a significant attention
A Method for Ultrashort Electron Pulse Shape-Measurement Using Coherent Synchrotron Radiation
In this paper we discuss a method for nondestructive measurements of the
longitudinal profile of sub-picosecond electron bunches for X-Ray Free Electron
Lasers (XFELs). The method is based on the detection of the Coherent
Synchrotron Radiation (CSR) spectrum produced by a bunch passing a dipole
magnet system. This work also contains a systematic treatment of synchrotron
radiation theory which lies at the basis of CSR. Standard theory of synchrotron
radiation uses several approximations whose applicability limits are often
forgotten: here we present a systematic discussion about these assumptions.
Properties of coherent synchrotron radiation from an electron moving along an
arc of a circle are then derived and discussed. We describe also an effective
and practical diagnostic technique based on the utilization of an
electromagnetic undulator to record the energy of the coherent radiation pulse
into the central cone. This measurement must be repeated many times with
different undulator resonant frequencies in order to reconstruct the modulus of
the bunch form-factor. The retrieval of the bunch profile function from these
data is performed by means of deconvolution techniques: for the present work we
take advantage of a constrained deconvolution method. We illustrate with
numerical examples the potential of the proposed method for electron beam
diagnostics at the TESLA Test Facility (TTF) accelerator. Here we choose, for
emphasis, experiments aimed at the measure of the strongly non-Gaussian
electron bunch profile in the TTF femtosecond-mode operation. We demonstrate
that a tandem combination of a picosecond streak camera and a CSR spectrometer
can be used to extract shape information from electron bunches with a narrow
leading peak and a long tail.Comment: 60 pages, 39 figure
Flexoelectric effect in finite samples
Static flexoelectric effect in a finite sample of a solid is addressed in
terms of phenomenological theory for the case of a thin plate subjected to
bending. It has been shown that despite an explicit asymmetry inherent to the
bulk constitutive electromechanical equations which take into account the
flexoelectric coupling, the electromechanical response for a finite sample is
"symmetric". "Symmetric" means that if a sensor and an actuator are made of a
flexoelectric element, performance of such devices can be characterized by the
same effective piezoelectric coefficient. This behavior is consistent with the
thermodynamic arguments offered earlier, being in conflict with the current
point of view on the matter in literature. This result was obtained using
standard mechanical boundary conditions valid for the case where the
polarization vanishes at the surface. It was shown that, for the case where
there is the polarization is nonzero at the surface, the aforementioned
symmetry of electromechanical response may be violated if standard mechanical
boundary conditions are used, leading to a conflict with the thermodynamic
arguments. It was argued that this conflict may be resolved when using modified
mechanical boundary conditions. It was also shown that the contribution of
surface piezoelectricity to the flexoelectric response of a finite sample is
expected to be comparable to that of the static bulk contribution (including
the material with high values of the dielectric constant) and to scale as the
bulk value of the dielectric constant (similar to the bulk contribution). This
finding implies that if the experimentally measured flexoelectric coefficient
scales as the dielectric constant of the material, this does not imply that the
measured flexoelectric response is controlled by the static bulk contribution
to the flexoelectric effect
Discovery of an unusual bright eclipsing binary with the longest known period: TYC 2505-672-1 / MASTER OT J095310.04+335352.8
We report on the MASTER Global Robotic Net discovery of an eclipsing binary,
MASTER OT J095310.04+335352.8, previously known as unremarkable star TYC
2505-672-1, which displays extreme orbital parameters. The orbital period
P=69.1 yr is more than 2.5 times longer than that of epsilon-Aurigae, which is
the previous record holder. The light curve is characterized by an extremely
deep total eclipse with a depth of more than 4.5 mag, which is symmetrically
shaped and has a total duration of 3.5 yrs. The eclipse is essentially gray.
The spectra acquired with the Russian 6 m BTA telescope both at minimum and
maximum light mainly correspond to an M0-1III--type red giant, but the spectra
taken at the bottom of eclipse show small traces of a sufficiently hot source.
The observed properties of this system can be better explained as the red giant
eclipsed by a large cloud (the disk) of small particles surrounding the
invisible secondary companion.Comment: 8 figures, 9 pages, Astronomy and astrophysics in prin
Photometric observations of the supernova 2009nr
We present the results of our UBVRI CCD photometry for the second brightest
supernova of 2009, SN 2009nr, discovered during a sky survey with the
telescopes of the MASTER robotic network. Its light and color curves and
bolometric light curves have been constructed. The light-curve parameters and
the maximum luminosity have been determined. SN 2009nr is shown to be similar
in light-curve shape and maximum luminosity to SN 1991T, which is the prototype
of the class of supernovae Ia with an enhanced luminosity. SN 2009nr exploded
far from the center of the spiral galaxy UGC 8255 and most likely belongs to
its old halo population. We hypothesize that this explosion is a consequence of
the merger of white dwarfs
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