732 research outputs found
Measurement and analysis of SPS kicker magnet heating and outgassing with Different Bunch Spacing
Fast kicker magnets are used to inject beam into and eject beam out of the CERN SPS accelerator ring. These kickers are generally ferrite loaded transmission line type magnets with a rectangular shaped aperture through which the beam passes. Unless special precautions are taken the impedance of the ferrite yoke can provoke significant beam induced heating, over several hours, even above the Curie temperature of the ferrite. At present the nominal bunch spacing in the SPS is 25 ns, however for an early stage of LHC operation it is preferable to have 50 ns bunch spacing. Machine Development (MD) studies have been carried out with an inter-bunch spacing of 25 ns, 50 ns or 75 ns. For some of the SPS kicker magnets the 75 ns bunch spacing resulted in considerable beam induced heating. In addition the MDs showed that 50 ns bunch spacing could result in a very rapid pressure rise in the kicker magnet and thus cause an interlock. This paper discusses the MD observations of the SPS kickers and analyses the available data to provide explanations for the phenomena: possible remedies are also discussed
Quantum-Mechanical Position Operator and Localization in Extended Systems
We introduce a fundamental complex quantity, , which allows us to
discriminate between a conducting and non-conducting thermodynamic phase in
extended quantum systems. Its phase can be related to the expectation value of
the position operator, while its modulus provides an appropriate definition of
a localization length. The expressions are valid for {\it any} fractional
particle filling. As an illustration we use to characterize insulator
to ``superconducting'' and Mott transitions in one-dimensional lattice models
with infinite on-site Coulomb repulsion at quarter filling.Comment: 4 pages, REVTEX, 1 ps figure
Constructive pointfree topology eliminates non-constructive representation theorems from Riesz space theory
In Riesz space theory it is good practice to avoid representation theorems
which depend on the axiom of choice. Here we present a general methodology to
do this using pointfree topology. To illustrate the technique we show that
almost f-algebras are commutative. The proof is obtained relatively
straightforward from the proof by Buskes and van Rooij by using the pointfree
Stone-Yosida representation theorem by Coquand and Spitters
A non-Hermitian critical point and the correlation length of strongly correlated quantum systems
We study a non-Hermitian generalization of quantum systems in which an
imaginary vector potential is added to the momentum operator. In the
tight-binding approximation, we make the hopping energy asymmetric in the
Hermitian Hamiltonian. In a previous article, we conjectured that the
non-Hermitian critical point where the energy gap vanishes is equal to the
inverse correlation length of the Hermitian system and we confirmed the
conjecture for two exactly solvable systems. In this article, we present more
evidence for the conjecture. We also argue the basis of our conjecture by
noting the dispersion relation of the elementary excitation.Comment: 25 pages, 18 figure
Finite Temperature DMRG Investigation of the Spin-Peierls Transition in CuGeO
We present a numerical study of thermodynamical properties of dimerized
frustrated Heisenberg chains down to extremely low temperatures with
applications to CuGeO. A variant of the finite temperature density matrix
renormalization group (DMRG) allows the study of the dimerized phase previously
unaccessible to ab initio calculations. We investigate static dimerized systems
as well as the instability of the quantum chain towards lattice dimerization.
The crossover from a quadratic response in the free energy to the distortion
field at finite temperature to nonanalytic behavior at zero temperature is
studied quantitatively. Various physical quantities are derived and compared
with experimental data for CuGeO such as magnetic dimerization, critical
temperature, susceptibility and entropy.Comment: LaTeX, 5 pages, 5 eps figures include
Disordered Flat Phase and Phase Diagram for Restricted Solid on Solid Models of Fcc(110) Surfaces
We discuss the results of a study of restricted solid-on-solid models for fcc
(110) surfaces. These models are simple modifications of the exactly solvable
BCSOS model, and are able to describe a missing-row reconstructed
surface as well as an unreconstructed surface. They are studied in two
different ways. The first is by mapping the problem onto a quantum spin-1/2
one-dimensional hamiltonian of the Heisenberg type, with competing
couplings. The second is by standard Monte Carlo simulations. We find phase
diagrams with the following features, which we believe to be quite generic: (i)
two flat, ordered phases (unreconstructed and missing-row reconstructed); a
rough, disordered phase; an intermediate disordered flat (DF) phase,
characterized by monoatomic steps, whose physics is shown to be akin to that of
a dimer spin state. (ii) a transition line from the reconstructed
phase to the DF phase showing exponents which appear to be close, within our
numerical accuracy, to the 2D-Ising universality class. (iii) a critical
(preroughening) line with variable exponents, separating the unreconstructed
phase from the DF phase. Possible signatures and order parameters of the DF
phase are investigated.Comment: Revtex (22 pages) + 15 figures (uuencoded file
Status of Schottky Diagnostics in the ANKA Storage Ring
The status of longitudinal and transverse Schottky observation systems for the synchrotron light source ANKA is presented. ANKA regularly operates in a dedicated low alpha mode with short bunches for the generation of coherent THz radiation. The Schottky measurement results are shown and compared with theoretical predictions for the regular as well as the different stages of the low alpha mode of operation. Special care had to be taken to control and mitigate the impact from strong coherent lines of the short bunches on the signal processing chain. The system setup is shown, expected and unexpected observations as well as applications are discussed
Deconfinement Transition and Bound States in Frustrated Heisenberg Chains: Regimes of Forced and Spontaneous Dimerization
We use recently developed strong-coupling expansion methods to study the
two-particle spectra for the frustrated alternating Heisenberg model,
consisting of an alternating nearest neighbor antiferromagnetic exchange and a
uniform second neighbor antiferromagnetic exchange. Starting from the limit of
weakly coupled dimers, we develop high order series expansions for the
effective Hamiltonian in the two-particle subspace. In the limit of a strong
applied dimerization, we calculate accurately various properties of singlet and
triplet bound states and quintet antibound states. We also develop series
expansions for bound state energies in various sectors, which can be
extrapolated using standard methods to cases where the external
bond-alternation goes to zero. We study the properties of singlet and triplet
bound states in the latter limit and suggest a crucial role for the bound
states in the unbinding of triplets and deconfinement of spin-half excitations.Comment: 17 figures, revte
Elementary excitations in the gapped phase of a frustrated S=1/2 spin ladder: from spinons to the Haldane triplet
We use the variational matrix-product ansatz to study elementary excitations
in the S=1/2 ladder with additional diagonal coupling, equivalent to a single
S=1/2 chain with alternating exchange and next-nearest neighbor interaction. In
absence of alternation the elementary excitation consists of two free S=1/2
particles ("spinons") which are solitons in the dimer order. When the
nearest-neighbor exchange alternates, the "spinons" are confined into one S=1
excitation being a soliton in the generalized string order. Variational results
are found to be in a qualitative agreement with the exact diagonalization data
for 24 spins. We argue that such an approach gives a reasonably good
description in a wide range of the model parameters.Comment: RevTeX, 13 pages, 11 embedded figures, uses psfig and multico
Observation of Electron Clouds in the ANKA Undulator by Means of the Microwave Transmission Method
A superconducting undulator is installed in the ANKA electron storage ring. Electron clouds could potentially contribute to the heat load of this device. A microwave transmission type electron cloud diagnostic has been installed for the undulator section of the ANKA machine. We present the system layout with particular emphasis on the electron machine aspects. Hardware transfer function results and e-cloud data for different machine settings are discussed. Special care has been taken for front end filter design both on the microwave injection and pick-up side
- …