53,839 research outputs found
RF power generation
This paper reviews the main types of r.f. power amplifiers which are, or may
be, used for particle accelerators. It covers solid-state devices, tetrodes,
inductive output tubes, klystrons, magnetrons, and gyrotrons with power outputs
greater than 10 kW c.w. or 100 kW pulsed at frequencies from 50 MHz to 30 GHz.
Factors affecting the satisfactory operation of amplifiers include cooling,
matching and protection circuits are discussed. The paper concludes with a
summary of the state of the art for the different technologies.Comment: 35 pages, contribution to the CAS - CERN Accelerator School:
Specialised Course on RF for Accelerators; 8 - 17 Jun 2010, Ebeltoft, Denmar
Inter-ethnic Issues in Lorraine Hansberry\u27s The Sign in Sidney Brustein\u27s Window by Steven R. Carter
When Lorraine Hansberry\u27s second produced play, The Sign in Sidney Brustein\u27s Window, opened on Broadway in 1964, some white critics expressed surprise that her protagonist was Jewish rather than black and some blacks were disappointed or even outraged by this, feeling that she was deserting the cause and trying too hard to win acclaim as a universal writer.[1] Others from both groups, of course, warmly defended her
Recent developments in Vorton Theory
This article provides a concise overview of recent theoretical results
concerning the theory of vortons, which are defined to be (centrifugally
supported) equilibrium configurations of (current carrying) cosmic string
loops. Following a presentation of the results of work on the dynamical
evolution of small circular string loops, whose minimum energy states are the
simplest examples of vortons, recent order of magnitude estimates of the
cosmological density of vortons produced in various kinds of theoretical
scenario are briefly summarised.Comment: 6 pages Latex. Contribution to 1996 Cosmology Meeting, Peyresq,
Franc
Kink oscillations in magnetic tubes with twisted annulus
Aims.We study kink waves in a magnetic flux tube modelled as a straight core surrounded by a magnetically twisted annulus, both embedded in a straight ambient external field, and derive the dispersion relation for this configuration.
Methods.The existence and behaviour of the kink modes are examined with specific attention to the effect that the addition of magnetic twist has on phase speeds and periods. Analytic expansions to the short and long wavelength approximations are also considered.
Results.The magnetic twist is found to introduce of an infinite set of body modes into solutions of the dispersion relation not present in the untwisted case. Moreover, for the kink modes, the width of interval of this infinite set, generally found to occupy phase speeds around the annulus' longitudinal Alfvén speed, increases for longer wavelengths. Two surface modes are also present in the solution, one at each surface: the internal and the external edges of the annulus. The magnetic twist is found to increase or decrease the phase speeds of these surface modes that are depending on the ratio of internal and external Alfvén speeds in the flux tube.
Conclusions.The magnetic twist of the annulus region of a flux tube is found to have a marked effect on the phase speeds of occurring modes. A straight annulus layer increased (or decreased) the periods of the surface modes for a tube modelled as a density (magnetic) enhancement. The addition of twist reduces the periods of the modes in both cases
Renormalisation of gravitational self interaction for wiggly strings
It is shown that for any elastic string model with energy density and
tension , the divergent contribution from gravitational self interaction can
be allowed for by an action renormalisation proportional to . This
formula is applied to the important special case of a bare model of the
transonic type (characterised by a constant value of the product ) that
represents the macroscopically averaged effect of shortwavelength wiggles on an
underlying microscopic model of the Nambu-Goto type (characterised by ).Comment: 11 pages, Latex; original 8 page version extended to include
estimates of relevant orders of magnitude. To be published in Physical
Review,
Symplectic structure for elastic and chiral conducting cosmic string models
This article is based on the covariant canonical formalism and corresponding
symplectic structure on phase space developed by Witten, Zuckerman and others
in the context of field theory. After recalling the basic principles of this
procedure, we construct the conserved bilinear symplectic current for generic
elastic string models. These models describe current carrying cosmic strings
evolving in an arbitrary curved background spacetime. Particular attention is
paid to the special case of the chiral string for which the worldsheet current
is null. Different formulations of the chiral string action are discussed in
detail, and as a result the integrability property of the chiral string is
clarified.Comment: 18 page
Cosmic Vortons and Particle Physics Constraints
We investigate the cosmological consequences of particle physics theories
that admit stable loops of superconducting cosmic string - {\it vortons}.
General symmetry breaking schemes are considered, in which strings are formed
at one energy scale and subsequently become superconducting in a secondary
phase transition at what may be a considerably lower energy scale. We estimate
the abundances of the ensuing vortons, and thereby derive constraints on the
relevant particle physics models from cosmological observations. These
constraints significantly restrict the category of admissible Grand Unified
theories, but are quite compatible with recently proposed effects whereby
superconducting strings may have been formed close to the electroweak phase
transition.Comment: 33 pages, 2 figures, RevTe
Poly-essential and general Hyperelastic World (brane) models
This article provides a unified treatment of an extensive category of
non-linear classical field models whereby the universe is represented (perhaps
as a brane in a higher dimensional background) in terms of a structure of a
mathematically convenient type describable as hyperelastic, for which a
complete set of equations of motion is provided just by the energy-momentum
conservation law. Particular cases include those of a perfect fluid in
quintessential backgrounds of various kinds, as well as models of the elastic
solid kind that has been proposed to account for cosmic acceleration. It is
shown how an appropriately generalised Hadamard operator can be used to
construct a symplectic structure that controles the evolution of small
perturbations, and that provides a characteristic equation governing the
propagation of weak discontinuities of diverse (extrinsic and extrinsic) kinds.
The special case of a poly-essential model - the k-essential analogue of an
ordinary polytropic fluid - is examined and shown to be well behaved (like the
fluid) only if the pressure to density ratio is positive.Comment: 16 pages Latex, Contrib. to 10th Peyresq Pysics Meeting, June 2005:
Micro and Macro Structures of Spacetim
Convergence analysis of Crank-Nicolson and Rannacher time-marching
This paper presents a convergence analysis of Crank-Nicolson and Rannacher time-marching methods which are often used in finite difference discretisations of the Black-Scholes equations. Particular attention is paid to the important role of Rannacher's startup procedure, in which one or more initial timesteps use Backward Euler timestepping, to achieve second order convergence for approximations of the first and second derivatives. Numerical results confirm the sharpness of the error analysis which is based on asymptotic analysis of the behaviour of the Fourier transform. The relevance to Black-Scholes applications is discussed in detail, with numerical results supporting recommendations on how to maximise the accuracy for a given computational cost
- …