306 research outputs found
Coherent instabilities of intense high-energy "white" charged-particle beams in the presence of nonlocal effects within the context of the Madelung fluid description
A hydrodynamical description of coherent instabilities that take place in the
longitudinal dynamics of a charged-particle coasting beam in a high-energy
accelerating machine is presented. This is done in the framework of the
Madelung fluid picture provided by the Thermal Wave Model. The well known
coherent instability charts in the complex plane of the longitudinal coupling
impedance for monochromatic beams are recovered. The results are also
interpreted in terms of the deterministic approach to modulational instability
analysis usually given for monochromatic large amplitude wave train propagation
governed by the nonlinear Schr\"odinger equation. The instability analysis is
then extended to a non-monochromatic coasting beam with a given thermal
equilibrium distribution, thought as a statistical ensemble of monochromatic
incoherent coasting beams ("white" beam). In this hydrodynamical framework, the
phenomenon of Landau damping is predicted without using any kinetic equation
governing the phase space evolution of the system.Comment: 14 pages, 1 figur
Transverse modulational instability of partially incoherent soliton stripes
Based on the Wigner distribution approach, an analysis of the effect of
partial incoherence on the transverse instability of soliton structures in
nonlinear Kerr media is presented. It is explicitly shown, that for a
Lorentzian incoherence spectrum the partial incoherence gives rise to a damping
which counteracts, and tends to suppress, the transverse instability growth.
However, the general picture is more complicated and it is shown that the
effect of the partial incoherence depends crucially on the form of the
incoherence spectrum. In fact, for spectra with finite rms-width, the partial
incoherence may even increase both the growth rate and the range of unstable,
transverse wave numbers.Comment: 5 pages, submitted to Phys. Rev.
Multi--hump soliton--like structures in interactions of lasers and Bose--Einstein condensates
An investigation is made of multi-hump and periodic solutions of the
semi-classical coupled equations describing laser radiation copropagating with
a Bose-Einstein condensate. Solutions reminiscent of optical vector solitons
have been found and have been used to gain understanding of the dynamics
observed in the numerical simulations, in particular to shed light on the
phenomenon of jet emission from a condensate interacting with a laser.Comment: 6 pages, 4 figures; submitted to European Physics Letter
Landau damping of partially incoherent Langmuir waves
It is shown that partial incoherence, in the form of stochastic phase noise,
of a Langmuir wave in an unmagnetized plasma gives rise to a Landau-type
damping. Starting from the Zakharov equations, which describe the nonlinear
interaction between Langmuir and ion-acoustic waves, a kinetic equation is
derived for the plasmons by introducing the Wigner-Moyal transform of the
complex Langmuir wave field. This equation is then used to analyze the
stability properties of small perturbations on a stationary solution consisting
of a constant amplitude wave with stochastic phase noise. The concomitant
dispersion relation exhibits the phenomenon of Landau-like damping. However,
this damping differs from the classical Landau damping in which a Langmuir
wave, interacting with the plasma electrons, loses energy. In the present
process, the damping is non-dissipative and is caused by the resonant
interaction between an instantaneously-produced disturbance, due to the
parametric interactions, and a partially incoherent Langmuir wave, which can be
considered as a quasi-particle composed of an ensemble of partially incoherent
plasmons.Comment: 12 page
Fermat's principle and variational analysis of an optical model for light propagation exhibiting a critical radius
Fermat's principle and variational analysis is used to analyze the
trajectories of light propagating in a radially inhomogeneous medium with a
singularity in the center. It is found that the light trajectories are similar
to those around a black hole, in the sense that there exists a critical radius
within which the light cannot escape, but spirals into the singularity.Comment: 5 pages from tex file + 3 figures (eps files
Nonlocal effects in high energy charged particle beams
Within the framework of the thermal wave model, an investigation is made of
the longitudinal dynamics of high energy charged particle beams. The model
includes the self-consistent interaction between the beam and its surroundings
in terms of a nonlinear coupling impedance, and when resistive as well as
reactive parts are included, the evolution equation becomes a generalised
nonlinear Schroedinger equation including a nonlocal nonlinear term. The
consequences of the resistive part on the propagation of particle bunches are
examined using analytical as well as numerical methods.Comment: 6 pages, 6 figures, uses RevTeX
Strontium optical lattice clocks for practical realization of the metre and secondary representation of the second
We present a system of two independent strontium optical lattice standards
probed with a single shared ultra-narrow laser. The absolute frequency of the
clocks can be verified by the use of Er:fiber optical frequency comb with the
GPS-disciplined Rb frequency standard. We report hertz-level spectroscopy of
the clock line and measurements of frequency stability of the two strontium
optical lattice clocks.Comment: This is an author-created, un-copyedited version of an article
accepted for publication in Meas. Sci. Technol. The publisher is not
responsible for any errors or omissions in this version of the manuscript or
any version derived from it. The Version of Record is available online at
doi:10.1088/0957-0233/26/7/07520
LOCALIZED FAST MAGNETOACOUSTIC EIGENMODES IN TOKAMAK PLASMAS
ABSTRACT. An equation is derived for fast magnetoacoustic eigenmodes (FMEs) in a tokamak fusion plasma with an elongated cross-section. It is shown that edge localized fast magnetoacoustic waves (FMWs) with both positive and negative poloidal wavenumbers are possible, provided that the productna 2 exceeds a certain critical magnitude Ncr, wheren is the plasma density at a point near the plasma edge and a is the plasma radius. Fulfilment of this condition in various devices is analysed, which is important in explaining the fine structure of the spectrum of suprathermal ICE from tokamak plasmas. It is found that a small population of high energy ions with a non-equilibrium distribution function can significantly affect the FMEs
Fibre-optic delivery of time and frequency to VLBI station
The quality of Very Long Baseline Interferometry (VLBI) radio observations
predominantly relies on precise and ultra-stable time and frequency (T&F)
standards, usually hydrogen masers (HM), maintained locally at each VLBI
station. Here, we present an operational solution in which the VLBI
observations are routinely carried out without use of a local HM, but using
remote synchronization via a stabilized, long-distance fibre-optic link. The
T&F reference signals, traceable to international atomic timescale (TAI), are
delivered to the VLBI station from a dedicated timekeeping laboratory.
Moreover, we describe a proof-of-concept experiment where the VLBI station is
synchronized to a remote strontium optical lattice clock during the
observation.Comment: 8 pages, 8 figures, matches the version published in A&A, section
Astronomical instrumentatio
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