9,138 research outputs found
Evidence cross-validation and Bayesian inference of MAST plasma equilibria
In this paper, current profiles for plasma discharges on the Mega-Ampere
Spherical Tokamak (MAST) are directly calculated from pickup coil, flux loop
and Motional-Stark Effect (MSE) observations via methods based in the
statistical theory of Bayesian analysis. By representing toroidal plasma
current as a series of axisymmetric current beams with rectangular
cross-section and inferring the current for each one of these beams,
flux-surface geometry and q-profiles are subsequently calculated by elementary
application of Biot-Savart's law. The use of this plasma model in the context
of Bayesian analysis was pioneered by Svensson and Werner on the Joint-European
Tokamak (JET) [J. Svensson and A. Werner. Current tomography for axisymmetric
plasmas. Plasma Physics and Controlled Fusion, 50(8):085002, 2008]. In
this framework, linear forward models are used to generate diagnostic
predictions, and the probability distribution for the currents in the
collection of plasma beams was subsequently calculated directly via application
of Bayes' formula. In this work, we introduce a new diagnostic technique to
identify and remove outlier observations associated with diagnostics falling
out of calibration or suffering from an unidentified malfunction. These
modifications enable good agreement between Bayesian inference of the last
closed flux-surface (LCFS) with other corroborating data, such as such as that
from force balance considerations using EFIT++ [L. Appel et al., Proc. 33rd EPS
Conf., Rome, Italy, 2006]. In addition, this analysis also yields errors on the
plasma current profile and flux-surface geometry, as well as directly
predicting the Shafranov shift of the plasma core.This work was jointly funded by the Australian Government
through International Science Linkages Grant No.
CG130047, the Australian National University, the United
Kingdom Engineering and Physical Sciences Research
Council under Grant No. EP/G003955, and by the European
Communities under the contract of Association between EURATOM and CCFE
Radiation Front Sweeping the Ambient Medium of Gamma-Ray Bursts
Gamma-ray bursts (GRBs) are emitted by relativistic ejecta from powerful
cosmic explosions. Their light curves suggest that the gamma-ray emission
occurs at early stages of the ejecta expansion, well before it decelerates in
the ambient medium. If so, the launched gamma-ray front must overtake the
ejecta and sweep the ambient medium outward. As a result a gap is opened
between the ejecta and the medium that surfs the radiation front ahead.
Effectively, the ejecta moves in a cavity until it reaches a radius
R_{gap}=10^{16}E_{54}^{1/2} cm where E is the isotropic energy of the GRB. At
R=R_{gap} the gap is closed, a blast wave forms and collects the medium swept
by radiation. Further development of the blast wave is strongly affected by the
leading radiation front: the front plays the role of a precursor where the
medium is loaded with e+- pairs and preaccelerated just ahead of the blast. It
impacts the emission from the blast at R < R_{load}=5R_{gap} (the early
afterglow). A spectacular observational effect results: GRB afterglows should
start in optical/UV and evolve fast (< min) to a normal X-ray afterglow. The
early optical emission observed in GRB 990123 may be explained in this way. The
impact of the front is especially strong if the ambient medium is a wind from a
massive progenitor of the GRB. In this case three phenomena are predicted: (1)
The ejecta decelerates at R<R_{load} producing a lot of soft radiation. (2) The
light curve of soft emission peaks at
t_{peak}=40(1+z)E_{54}^{1/2}(Gamma_{ej}/100)^{-2} s where Gamma_{ej} is the
Lorentz factor of the ejecta. Given measured redshift z and t_{peak}, one finds
Gamma_{ej}. (3) The GRB acquires a spectral break at 5 - 50 MeV because harder
photons are absorbed by radiation scattered in the wind.Comment: 20 pages, accepted to Ap
Quasi-thermal Comptonization and gamma-ray bursts
Quasi-thermal Comptonization in internal shocks formed between relativistic
shells can account for the high energy emission of gamma-ray bursts. This is in
fact the dominant cooling mechanism if the typical energy of the emitting
particles is achieved either through the balance between heating and cooling or
as a result of electron-positron pair production. Both processes yield sub or
mildly relativistic energies. In this case the synchrotron spectrum is
self-absorbed, providing the seed soft photons for the Comptonization process,
whose spectrum is flat [F(v) ~ const], ending either in an exponential cutoff
or a Wien peak, depending on the scattering optical depth of the emitting
particles. Self-consistent particle energy and optical depth are estimated and
found in agreement with the observed spectra.Comment: 10 pages, ApJ Letters, accepted for publicatio
Allergic Rhinoconjunctivitis: the Role of Histamine
Allergic rhinoconjunctivitis is the most common atopic condition
encountered in clinical practice. Analysis of the pathogenesis of
this condition permits identification of optimal therapeutic
targets. The increased knowledge of the underlying pathophysiology
suggests that multiple inflammatory mediators are involved in the
pathogenesis of the allergic reaction in the ocular and nasal
mucosa. However, despite the presence of a wide range of different
mediators, it would appear that histamine plays a key role.
Experimental allergen challenge studies have demonstrated that
histamine is the only mediator which produces the full spectrum of
clinical manifestations of the acute allergic reaction when applied
to the mucosal surface. While both H1- and H2-receptors are present
in the nasal and ocular mucosa, only H1-receptor antagonists are
capable of inhibiting histamine-induced symptoms of allergic
rhinoconjunctivitis. Furthermore, although the exact role of
histamine in the immediate and prolonged allergic reaction has not
yet been fully elucidated, these findings do not exclude the
possibility that histamine is involved in these processes. The
available evidence therefore supports current clinical practice for
use of H1-receptor antagonist as a first-line therapy in
patients with this atopic condition
Emission Spectra from Internal Shocks in Gamma-Ray-Burst Sources
Unsteady activity of gamma-ray burst sources leads to internal shocks in
their emergent relativistic wind. We study the emission spectra from such
shocks, assuming that they produce a power-law distribution of relativistic
electrons and posses strong magnetic fields. The synchrotron radiation emitted
by the accelerated electrons is Compton up-scattered multiple times by the same
electrons. A substantial component of the scattered photons acquires high
energies and produces e+e- pairs. The pairs transfer back their kinetic energy
to the radiation through Compton scattering. The generic spectral signature
from pair creation and multiple Compton scattering is highly sensitive to the
radius at which the shock dissipation takes place and to the Lorentz factor of
the wind. The entire emission spectrum extends over a wide range of photon
energies, from the optical regime up to TeV energies. For reasonable values of
the wind parameters, the calculated spectrum is found to be in good agreement
with the burst spectra observed by BATSE.Comment: 12 pages, latex, 2 figures, submitted to ApJ
Money in monetary policy design: monetary cross-checking in the New-Keynesian model
In the New-Keynesian model, optimal interest rate policy under uncertainty is formulated without reference to monetary aggregates as long as certain standard assumptions on the distributions of unobservables are satisfied. The model has been criticized for failing to explain common trends in money growth and inflation, and that therefore money should be used as a cross-check in policy formulation (see Lucas (2007)). We show that the New-Keynesian model can explain such trends if one allows for the possibility of persistent central bank misperceptions. Such misperceptions motivate the search for policies that include additional robustness checks. In earlier work, we proposed an interest rate rule that is near-optimal in normal times but includes a cross-check with monetary information. In case of unusual monetary trends, interest rates are adjusted. In this paper, we show in detail how to derive the appropriate magnitude of the interest rate adjustment following a significant cross-check with monetary information, when the New-Keynesian model is the central bank’s preferred model. The cross-check is shown to be effective in offsetting persistent deviations of inflation due to central bank misperceptions. Keywords: Monetary Policy, New-Keynesian Model, Money, Quantity Theory, European Central Bank, Policy Under Uncertaint
Bayesian Road Estimation Using Onboard Sensors
This paper describes an algorithm for estimating the road ahead of a host vehicle based on the measurements from several onboard sensors: a camera, a radar, wheel speed sensors,and an inertial measurement unit.We propose a novel road model that is able to describe the road ahead with higher accuracy than the usual polynomial model. We also develop a Bayesian fusionsystem that uses the following information from the surroundings: lane marking measurements obtained by the camera and leading vehicle and stationary object measurements obtained bya radar–camera fusion system. The performance of our fusion algorithm is evaluated in several drive tests. As expected, the more information we use, the better the performance is.Index Terms—Camera, information fusion, radar, road geometry,unscented Kalman filter (UKF)
Journal Staff
The aluminum–zinc-vacancy (Al Zn −V Zn ) complex is identified as one of the dominant defects in Al-containing n -type ZnO after electron irradiation at room temperature with energies above 0.8 MeV. The complex is energetically favorable over the isolated V Zn , binding more than 90% of the stable V Zn ’s generated by the irradiation. It acts as a deep acceptor with the (0/− ) energy level located at approximately 1 eV above the valence band. Such a complex is concluded to be a defect of crucial and general importance that limits the n -type doping efficiency by complex formation with donors, thereby literally removing the donors, as well as by charge compensation
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