225 research outputs found
Electromagnetic field and short-range wake function in a beam pipe of elliptical cross section
Within the ultrarelativistic limit, analytical expressions are found for the high-frequency resistive-wall coupling impedance of an elliptical cross-section vacuum chamber. Subsequently, the corresponding wake functions are derived by performing inverse Fourier transformations numerically. The electromagnetic fields have been developed working out two systems of solutions, namely for the vacuum and for the resistive wall. The constants involved in these systems have been determined by matching boundary conditions at the interface vacuum wall. Several study cases have been considered concerning the aspect ratio of the elliptical cross section and the transverse position of the leading charge in order to exemplify the behavior of the longitudinal and transverse wake functions
Strained tetragonal states and Bain paths in metals
Paths of tetragonal states between two phases of a material, such as bcc and
fcc, are called Bain paths. Two simple Bain paths can be defined in terms of
special imposed stresses, one of which applies directly to strained epitaxial
films. Each path goes far into the range of nonlinear elasticity and reaches a
range of structural parameters in which the structure is inherently unstable.
In this paper we identify and analyze the general properties of these paths by
density functional theory. Special examples include vanadium, cobalt and
copper, and the epitaxial path is used to identify an epitaxial film as related
uniquely to a bulk phase.Comment: RevTeX, 4 pages, 4 figures, submitted to Phys. Rev. Let
Scaling and Crossover in the Large-N Model for Growth Kinetics
The dependence of the scaling properties of the structure factor on space
dimensionality, range of interaction, initial and final conditions, presence or
absence of a conservation law is analysed in the framework of the large-N model
for growth kinetics. The variety of asymptotic behaviours is quite rich,
including standard scaling, multiscaling and a mixture of the two. The
different scaling properties obtained as the parameters are varied are
controlled by a structure of fixed points with their domains of attraction.
Crossovers arising from the competition between distinct fixed points are
explicitely obtained. Temperature fluctuations below the critical temperature
are not found to be irrelevant when the order parameter is conserved. The model
is solved by integration of the equation of motion for the structure factor and
by a renormalization group approach.Comment: 48 pages with 6 figures available upon request, plain LaTe
The Effect of Lattice Vibrations on Substitutional Alloy Thermodynamics
A longstanding limitation of first-principles calculations of substitutional
alloy phase diagrams is the difficulty to account for lattice vibrations. A
survey of the theoretical and experimental literature seeking to quantify the
impact of lattice vibrations on phase stability indicates that this effect can
be substantial. Typical vibrational entropy differences between phases are of
the order of 0.1 to 0.2 k_B/atom, which is comparable to the typical values of
configurational entropy differences in binary alloys (at most 0.693 k_B/atom).
This paper describes the basic formalism underlying ab initio phase diagram
calculations, along with the generalization required to account for lattice
vibrations. We overview the various techniques allowing the theoretical
calculation and the experimental determination of phonon dispersion curves and
related thermodynamic quantities, such as vibrational entropy or free energy. A
clear picture of the origin of vibrational entropy differences between phases
in an alloy system is presented that goes beyond the traditional bond counting
and volume change arguments. Vibrational entropy change can be attributed to
the changes in chemical bond stiffness associated with the changes in bond
length that take place during a phase transformation. This so-called ``bond
stiffness vs. bond length'' interpretation both summarizes the key phenomenon
driving vibrational entropy changes and provides a practical tool to model
them.Comment: Submitted to Reviews of Modern Physics 44 pages, 6 figure
Effects of anharmonic strain on phase stability of epitaxial films and superlattices: applications to noble metals
Epitaxial strain energies of epitaxial films and bulk superlattices are
studied via first-principles total energy calculations using the local-density
approximation. Anharmonic effects due to large lattice mismatch, beyond the
reach of the harmonic elasticity theory, are found to be very important in
Cu/Au (lattice mismatch 12%), Cu/Ag (12%) and Ni/Au (15%). We find that
is the elastically soft direction for biaxial expansion of Cu and Ni, but it is
for large biaxial compression of Cu, Ag, and Au. The stability of
superlattices is discussed in terms of the coherency strain and interfacial
energies. We find that in phase-separating systems such as Cu-Ag the
superlattice formation energies decrease with superlattice period, and the
interfacial energy is positive. Superlattices are formed easiest on (001) and
hardest on (111) substrates. For ordering systems, such as Cu-Au and Ag-Au, the
formation energy of superlattices increases with period, and interfacial
energies are negative. These superlattices are formed easiest on (001) or (110)
and hardest on (111) substrates. For Ni-Au we find a hybrid behavior:
superlattices along and like in phase-separating systems, while for
they behave like in ordering systems. Finally, recent experimental
results on epitaxial stabilization of disordered Ni-Au and Cu-Ag alloys,
immiscible in the bulk form, are explained in terms of destabilization of the
phase separated state due to lattice mismatch between the substrate and
constituents.Comment: RevTeX galley format, 16 pages, includes 9 EPS figures, to appear in
Physical Review
Instrumentation for Longitudinal Beam Gymnastics in FEL's and at the CLIC Test Facility 3
Built at CERN by an international collaboration, the CLIC Test Facility 3 (CTF3) aims at demonstrating the feasibility of a high luminosity 3 TeV e+-e- collider by the year 2010. One of the main issues to be demonstrated is the generation of a high average current (30 A) high frequency (12 GHz) bunched beam by means of RF manipulation. At the same time, Free Electron Lasers (FEL) are developed in several places all over the world with the aim of providing high brilliance photon sources. These machines rely on the production of high peak current electron bunches. The required performances put high demands on the diagnostic equipment and innovative longitudinal monitors have been developed during the past years. This paper gives an overview of the longitudinal instrumentation developed at ELETTRA and CTF3, where a special effort was made in order to implement at the same time non-intercepting devices for online monitoring, and destructive diagnostics which have the advantage of providing more detailed information
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FERMI&Elettra Accelerator Technical Optimization FinalReport
This report describes the accelerator physics aspects, theengineering considerations and the choice of parameters that led to theaccelerator design of the FERMI Free-Electron-Laser. The accelerator(also called the "electron beam delivery system") covers the region fromthe exit of the injector to the entrance of the first FEL undulator. Theconsiderations that led to the proposed configuration were made on thebasis of a study that explored various options and performance limits.This work follows previous studies of x-ray FEL facilities (SLAC LCLS[1], DESY XFEL [2], PAL XFEL [3], MIT [4], BESSY FEL[5], LBNL LUX [6],Daresbury 4GLS [7]) and integrates many of the ideas that were developedthere. Several issues specific to harmonic cascade FELs, and that had notyet been comprehensively studied, were also encountered and tackled. Aparticularly difficult issue was the need to meet the requirement forhigh peak current and small slice energy spread, as the specification forthe ratio of these two parameters (that defines the peak brightness ofthe electron beam) is almost a factor of two higher than that of theLCLS's SASE FEL. Another challenging aspect was the demand to produce anelectron beam with as uniform as possible peak current and energydistributions along the bunch, a condition that was met by introducingnovel beam dynamics techniques. Part of the challenge was due to the factthat there were no readily available computational tools to carry outreliable calculations, and these had to be developed. Most of theinformation reported in this study is available in the form of scientificpublications, and is partly reproduced here for the convenience of thereader
Tunability experiments at the FERMI@Elettra free-electron laser
FERMI@Elettra is a free electron-laser (FEL)-based user facility that, after two years of commissioning, started preliminary users' dedicated runs in 2011. At variance with other FEL user facilities, FERMI@Elettra has been designed to deliver improved spectral stability and longitudinal coherence. The adopted scheme, which uses an external laser to initiate the FEL process, has been demonstrated to be capable of generating FEL pulses close to the Fourier transform limit. We report on the first instance of FEL wavelength tuning, both in a narrow and in a large spectral range (fine- and coarse-tuning). We also report on two different experiments that have been performed exploiting such FEL tuning. We used fine-tuning to scan across the 1s–4p resonance in He atoms, at ≈23.74 eV (52.2 nm), detecting both UV–visible fluorescence (4p–2s, 400 nm) and EUV fluorescence (4p–1s, 52.2 nm). We used coarse-tuning to scan the M4,5 absorption edge of Ge (∼29.5 eV) in the wavelength region 30–60 nm, measured in transmission geometry with a thermopile positioned on the rear side of a Ge thin foil
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