13,216 research outputs found
Photon emission by ultra-relativistic positrons in crystalline undulators: the high-energy regime
This paper discusses the undulator radiation emitted by high-energy positrons
during planar channeling in periodically bent crystals. We demonstrate that the
construction of the undulator for positrons with energies of 10 GeV and above
is only possible if one takes into account the radiative energy losses. The
frequency of the undulator radiation depends on the energy of the particle.
Thus the decrease of the particle's energy during the passage of the crystal
should result in the destruction of the undulator radiation regime. However, we
demonstrate that it is possible to avoid the destructive influence of the
radiative losses on the frequency of the undulator radiation by the appropriate
variation of the shape of the crystal channels. We also discuss a method by
which, to our mind, it would be possible to prepare the crystal with the
desired properties of its channels.Comment: submitted for the proceedings of the International Workshop on
``Electron-Photon Interaction in Dense Media'' in Nor-Hamberd, Armenia, 2001;
10 pages, 9 figures, LaTe
Spontaneous and stimulated undulator radiation by an ultra-relativistic positron channeling in a periodically bent crystal
We discuss the radiation generated by positrons channeling in a crystalline
undulator. The undulator is produced by periodically bending a single crystal
with an amplitude much larger than the interplanar spacing. Different
approaches for bending the crystal are described and the restrictions on the
parameters of the bending are discussed. We also present numeric calculations
of the spontaneous emitted radiation and estimate the conditions for stimulated
emission. Our investigations show that the proposed mechanism could be an
interesting source for high energy photons and is worth to be studied
experimentally.Comment: long version of our contribution to the 22nd International Free
Electron Laser Conference, Durham, NC, USA, 13-18 August 2000, Reprinted from
Nuclear Instruments and Methods A, Volume 474, 1--3, in press, with
permission from Elsevier Science. http://www.elsevier.com/locate/nim
Channeling of Charged Particles Through Periodically Bent Crystals: on the Possibility of a Gamma Laser
We discuss radiation generated by positrons channeling in a crystalline
undulator. The undulator is produced by periodically bending a single crystal
with an amplitude much larger than the interplanar spacing. Different
approaches for bending the crystal are described and the restrictions on the
parameters of the bending are established. We present the results of numeric
calculations of the spectral distributions of the spontaneous emitted radiation
and estimate the conditions for stimulated emission. Our investigations show
that the proposed mechanism provides an efficient source for high energy
photons, which is worth to be studied experimentally.Comment: contributed to the conference ``Fundamental and Applied Aspects of
Modern Physics'' in Luederitz, Namibia, 200
An engineering feasibility study of an orbiting scanning radiometer
Engineering feasibility study of lunar orbiting optical scanning radiometer
ACREAGE RESPONSES TO EXPECTED REVENUES AND PRICE RISK FOR MINOR OILSEEDS AND PROGRAM CROPS IN THE NORTHERN PLAINS
Wheat, barely, flaxseed, and oilseed sunflower acreage respond to different economic variables. Wheat and barely acreage must be divided among program-complying, program-planted, and nonprogram-planted acreage because these categories respond to different variables and respond to own expected-revenue and price-risk variables in opposite ways. Flaxseed, sunflower, and nonprogram-planted acreage of wheat and barley have highly significant, positive responses to their own expected revenue and negative responses to their own-price risk. Flaxseed and sunflower acreage have been more responsive to their lagged values than to expected revenues for wheat.Crop Production/Industries,
Safe Exploration for Optimization with Gaussian Processes
We consider sequential decision problems under uncertainty, where we seek to optimize an unknown function from noisy samples. This requires balancing exploration (learning about the objective) and exploitation (localizing the maximum), a problem well-studied in the multi-armed bandit literature. In many applications, however, we require that the sampled function values exceed some prespecified "safety" threshold, a requirement that existing algorithms fail to meet. Examples include medical applications where patient comfort must be guaranteed, recommender systems aiming to avoid user dissatisfaction, and robotic control, where one seeks to avoid controls causing physical harm to the platform. We tackle this novel, yet rich, set of problems under the assumption that the unknown function satisfies regularity conditions expressed via a Gaussian process prior. We develop an efficient algorithm called SafeOpt, and theoretically guarantee its convergence to a natural notion of optimum reachable under safety constraints. We evaluate SafeOpt on synthetic data, as well as two real applications: movie recommendation, and therapeutic spinal cord stimulation
Numerical modelling of the lobes of radio galaxies in cluster environments -- IV. Remnant radio galaxies
© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.We examine the remnant phase of radio galaxies using three-dimensional hydrodynamical simulations of relativistic jets propagating through cluster environments. By switching the jets off once the lobes have reached a certain length we can study how the energy distribution between the lobes and shocked intra-cluster medium compares to that of an active source, as well as calculate synchrotron emission properties of the remnant sources. We see that as a result of disturbed cluster gas beginning to settle back into the initial cluster potential, streams of dense gas are pushed along the jet axis behind the remnant lobes, causing them to rise out of the cluster faster than they would due to buoyancy. This leads to increased adiabatic losses and a rapid dimming. The rapid decay of total flux density and surface brightness may explain the small number of remnant sources found in samples with a high flux density limit and may cause analytic models to overestimate the remnant fraction expected in sensitive surveys such as those now being carried out with LOFAR.Peer reviewedFinal Accepted Versio
Substrate effects on surface magetetism of Fe/W(110) from first principles
Surface magnetic properties of the pseudomorphic Fe(110) monolayer on a
W(110) substrate are investigated from first principles as a function of the
substrate thickness (up to eight layers). Analyzing the magnetocrystalline
anisotropy energies, we find stable (with respect to the number of substrate
layers) in-plane easy and hard axes of magnetization along the [1[overline 1]0]
and [001] directions, respectively, reaching a value in good agreement with
experiment for thick substrates. Additionally, the changes to the magnetic spin
moments and the density of the Fe d states are analyzed with respect to the
number of substrate layers as well as with respect to the direction of
magnetization. With respect to the number of W(110) substrate layers beneath
the Fe(110) surface, we find that the first four substrate layers have a large
influence on the electronic and magnetic properties of the surface. Beyond the
fourth layer, the substrate has only marginal influence on the surface
properties.Comment: 8 Pages, 3 Figures, 3 Table
Excretion of organic acids associated with biotin deficiency in chronic anticonvulsant therapy
Electrodynamic Structure of an Outer Gap Accelerator: Location of the Gap and the Gamma-ray Emission from the Crab Pulsar
We investigate a stationary pair production cascade in the outer
magnetosphere of a spinning neutron star. The charge depletion due to global
flows of charged particles, causes a large electric field along the magnetic
field lines. Migratory electrons and/or positrons are accelerated by this field
to radiate curvature gamma-rays, some of which collide with the X-rays to
materialize as pairs in the gap. The replenished charges partially screen the
electric field, which is self-consistently solved together with the
distribution functions of particles and gamma-rays. If no current is injected
at neither of the boundaries of the accelerator, the gap is located around the
conventional null surface, where the local Goldreich-Julian charge density
vanishes. However, we first find that the gap position shifts outwards (or
inwards) when particles are injected at the inner (or outer) boundary. Applying
the theory to the Crab pulsar, we demonstrate that the pulsed TeV flux does not
exceed the observational upper limit for moderate infrared photon density and
that the gap should be located near to or outside of the conventional null
surface so that the observed spectrum of pulsed GeV fluxes may be emitted via a
curvature process. Some implications of the existence of a solution for a super
Goldreich-Julian current are discussed.Comment: 17 pages, 12 figures, submitted to Ap
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