693 research outputs found
Impact of surface phenomena on direct bulk flexoelectric effect in finite samples
In the framework of a continuum theory, it is shown that the direct
flexoelectric response of a finite sample essentially depends on the surface
polarization energy, even in the thermodynamic limit where the body size tends
to infinity. It is found that a modification of the surface energy can lead to
a change of the polarization response by a factor of two. The origin of the
effect is an electric field produced by surface dipoles induced by the strain
gradient. The unexpected sensitivity of the polarization response to the
surface energy in the thermodynamic limit is conditioned by the fact that the
moments of the surface dipoles may scale as the body size
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
- …