82 research outputs found
Simulations ofthe 100kW TJNAF FEL using a step-tapered undulator
The Thomas Jefferson National Accelerator Facility (TJNAF) free electron laser (FEL) can be upgraded to operate at 100kW average power in the near future using a configuration that recirculates the electron beam to recover energy. It is important to extract the maximum energy from the electron beam in a pass through the undulator while inducing
the minimum amount of exhaust energy spread. A larger energy extraction reduces the requirement for a large recirculating current, while a smaller exhaust energy spread allows the intense electron beam to be recirculated without
damaging components. To improve FEL performance, we explore the use of the step-tapered undulator, which alters the resonance condition halfway through the undulator. Short pulses complicate the desired interaction. Comparisons
are made to the conventional periodic and linearly-tapered undulators.The authors are grateful for the support of the Office of Naval Research, Thomas Jefferson National Accelerator Facility, and contributions of Dave Douglas of TJNAF
A new approach to improving the efficiency of fel oscillator simulations
During the last year we have been benchmarking FEL
oscillator simulation codes against the measured
performance of the three Jefferson Lab oscillator FELs.
While one might think that a full 4D simulation is de
facto the best predictor of performance, the simulations
are computationally intensive, even when analytical
approximations to the electron bunch longitudinal
distribution are used. In this presentation we compare the
predictions of the 4D FEL interaction codes Genesis and
Medusa, in combination with the optical code OPC, with
those using a combination of the 2D & 3D versions of
these codes, which can be run quickly on a single CPU
core desktop computer
'Theory for the enhanced induced magnetization in coupled magnetic trilayers in the presence of spin fluctuations'
Motivated by recent experiments, the effect of the interlayer exchange
interaction on the magnetic properties of coupled Co/Cu/Ni
trilayers is studied theoretically. Here the Ni film has a lower Curie
temperature than the Co film in case of decoupled layers. We
show that by taking into account magnetic fluctuations the interlayer coupling
induces a strong magnetization for T\gtsim T_{C,\rm Ni} in the Ni film. For
an increasing the resonance-like peak of the longitudinal Ni
susceptibility is shifted to larger temperatures, whereas its maximum value
decreases strongly. A decreasing Ni film thickness enhances the induced Ni
magnetization for T\gtsim T_{C,\rm Ni}. The measurements cannot be explained
properly by a mean field estimate, which yields a ten times smaller effect.
Thus, the observed magnetic properties indicate the strong effect of 2D
magnetic fluctuations in these layered magnetic systems. The calculations are
performed with the help of a Heisenberg Hamiltonian and a Green's function
approach.Comment: 4 pages, 3 figure
Simulations of the TJNAF FEL with tapered and inversely tapered undulators
Experiments using the TJNAF FEL have explored the operation with both tapered and inversely tapered undulators.
We present here numerical simulations using the TJNAF experimental parameters, including the effects of taper. Singlemode
simulations show the effect of taper on gain. Multimode simulations describe the evolution of short optical pulses
in the far infrared, and show how taper affects single-pass gain and steady-state power as a function of desynchronism.
A short optical pulse presents an ever-changing field strength to each section of the electron pulse so that idealized
operation is not possible. Yet, advantages for the recirculation of the electron beam can be explored.The authors are grateful for support by the Naval Postgraduate School
In-plane dipole coupling anisotropy of a square ferromagnetic Heisenberg monolayer
In this study we calculate the dipole-coupling-induced quartic in-plane
anisotropy of a square ferromagnetic Heisenberg monolayer. This anisotropy
increases with an increasing temperature, reaching its maximum value close to
the Curie temperature of the system. At T=0 the system is isotropic, besides a
small remaining anisotropy due to the zero-point motion of quantum mechanical
spins. The reason for the dipole-coupling-induced anisotropy is the disturbance
of the square spin lattice due to thermal fluctuations ('order-by-disorder'
effect). For usual ferromagnets its strength is small as compared to other
anisotropic contributions, and decreases by application of an external magnetic
field. The results are obtained from a Heisenberg Hamiltonian by application of
a mean field approach for a spin cluster, as well as from a many-body Green's
function theory within the Tyablikov-decoupling (RPA).Comment: 6 pages, 2 figures, accepted for publication in RP
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Coherent transition radiation produced by a 1.2 MeV electron beam
We describe a method of generating very high-frequency coherent radiation using an electron beam source with a maximum beam energy of 1.2 MeV. We show that, though the high frequency cutoff for the radiation generated when the beam impacts a target at normal incidence is reduced by transverse beam size effects, it is nevertheless possible to generate much higher frequencies by a judicious choice of the angles of incidence and observation. 5 refs., 4 figs., 1 tab
Resonance lens antenna analysis for MM-wave applications
We report what is to our knowledge the first accurate theoretical investigation of the electromagnetic behavior of 2-D elliptical lenses of finite wavelength-scale size. The role of internal resonances in the focal domain formation is studied. A proposal of a narrow-band receiver based on a hemielliptic lens tuned to a resonance is discussed. Possible features of such a lens-coupled receiver are stability of the resonance field with respect to the angle of arrival of incident wave and several times greater values of the peak field intensity that may potentially lead to higher sensitivity and better scanning performance. In the analysis, we use the Muller boundary integral equation (BIE) technique. This full-wave mathematically rigorous method is combined with trigonometric Galerkin discretization to result in the efficient numerical solution for an arbitrary set of the electrical, geometrical, and material parameters. Numerical results are generated for a quartz elliptical lens (ε= 3.8) with dimensions typical to mm-wave radar applications. Near field analysis, lens-focusing properties and lens frequency-dependent performance are presented
Lens or resonator? Electromagnetic behavior of an extended hemielliptic lens for a sub-millimeter-wave receiver
The behavior of a 2D model of an extended hemielliptic silicon lens of a size typical for THz applications is accurately studied for the case of a plane E-wave illumination. The full-wave analysis of the scattering problem is based on the Mutter's boundary integral-equations (MB1E) that are uniquely solvable. A Calerkin discretization scheme with a trigonometric basis leads tu a very efficient numerical algorithm. The numerical results related to the focusability of the lens versus its rear-side extension and the angle of the plane-wave incidence, as well as near-field profiles, demonstrate strong resonances. Such effects can change the principles of optimal design of lens-based receivers. © 2004 Wiley Periodicals, Inc
An entangled two photon source using biexciton emission of an asymmetric quantum dot in a cavity
A semiconductor based scheme has been proposed for generating entangled
photon pairs from the radiative decay of an electrically-pumped biexciton in a
quantum dot. Symmetric dots produce polarisation entanglement, but
experimentally-realised asymmetric dots produce photons entangled in both
polarisation and frequency. In this work, we investigate the possibility of
erasing the `which-path' information contained in the frequencies of the
photons produced by asymmetric quantum dots to recover polarisation-entangled
photons. We consider a biexciton with non-degenerate intermediate excitonic
states in a leaky optical cavity with pairs of degenerate cavity modes close to
the non-degenerate exciton transition frequencies. An open quantum system
approach is used to compute the polarisation entanglement of the two-photon
state after it escapes from the cavity, measured by the visibility of
two-photon interference fringes. We explicitly relate the two-photon visibility
to the degree of Bell-inequality violation, deriving a threshold at which
Bell-inequality violations will be observed. Our results show that an ideal
cavity will produce maximally polarisation-entangled photon pairs, and even a
non-ideal cavity will produce partially entangled photon pairs capable of
violating a Bell-inequality.Comment: 16 pages, 10 figures, submitted to PR
Polarization-Correlated Photon Pairs from a Single Quantum Dot
Polarization correlation in a linear basis, but not entanglement, is observed
between the biexciton and single-exciton photons emitted by a single InAs
quantum dot in a two-photon cascade. The results are well described
quantitatively by a probabilistic model that includes two decay paths for a
biexciton through a non-degenerate pair of one-exciton states, with the
polarization of the emitted photons depending on the decay path. The results
show that spin non-degeneracy due to quantum-dot asymmetry is a significant
obstacle to the realization of an entangled-photon generation device.Comment: 4 pages, 4 figures, revised discussio
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