116,376 research outputs found
Magnetic reconnection in plasma under inertial confinement fusion conditions driven by heat flux effects in Ohm's law
In the interaction of high-power laser beams with solid density plasma there
are a number of mechanisms that generate strong magnetic fields. Such fields
subsequently inhibit or redirect electron flows, but can themselves be advected
by heat fluxes, resulting in complex interplay between thermal transport and
magnetic fields.We show that for heating by multiple laser spots reconnection
of magnetic field lines can occur, mediated by these heat fluxes, using a fully
implicit 2D Vlasov-Fokker-Planck code. Under such conditions, the reconnection
rate is dictated by heat flows rather than Alfv\`enic flows. We find that this
mechanism is only relevant in a high plasma. However, the Hall
parameter can be large so that thermal transport is
strongly modified by these magnetic fields, which can impact longer time scale
temperature homogeneity and ion dynamics in the system
Some implications of sampling choices on comparisons between satellite and model aerosol optical depth fields
The comparison of satellite and model aerosol optical depth (AOD) fields provides useful information on the strengths and weaknesses of both. However, the sampling of satellite and models is very different and some subjective decisions about data selection and aggregation must be made in order to perform such comparisons. This work examines some implications of these decisions, using GlobAerosol AOD retrievals at 550 nm from Advanced Along-Track Scanning Radiometer (AATSR) measurements, and aerosol fields from the GEOS-Chem chemistry transport model. It is recommended to sample the model only where the satellite flies over on a particular day; neglecting this can cause regional differences in model AOD of up to 0.1 on monthly and annual timescales. The comparison is observed to depend strongly upon thresholds for sparsity of satellite retrievals in the model grid cells. Requiring at least 25% coverage of the model grid cell by satellite data decreases the observed difference between the two by approximately half over land. The impact over ocean is smaller. In both model and satellite datasets, there is an anticorrelation between the proportion <i>p</i> of a model grid cell covered by satellite retrievals and the AOD. This is attributed to small <i>p</i> typically occuring due to high cloud cover and lower AODs being found in large clear-sky regions. Daily median AATSR AODs were found to be closer to GEOS-Chem AODs than daily means (with the root mean squared difference being approximately 0.05 smaller). This is due to the decreased sensitivity of medians to outliers such as cloud-contaminated retrievals, or aerosol point sources not included in the model
Electromagnetic contribution to charge symmetry violation in parton distributions
We report a calculation of the combined effect of photon radiation and quark
mass differences on charge symmetry violation (CSV) in the parton distribution
functions of the nucleon. Following a recent suggestion of Martin and Ryskin,
the initial photon distribution is calculated in terms of coherent radiation
from the proton as a whole, while the effect of the quark mass difference is
based on a recent lattice QCD simulation. The distributions are then evolved to
a scale at which they can be compared with experiment by including both QCD and
QED radiation. Overall, at a scale of 5 GeV, the total CSV effect on the
phenomenologically important difference between the and -quark
distributions is some 20\% larger than the value based on quark mass
differences alone. In total these sources of CSV account for approximately 40\%
of the NuTeV anomaly.Comment: 5 pages, 2 figure
The effect of non-linear quantum electrodynamics on relativistic transparency and laser absorption in ultra-relativistic plasmas
With the aid of large-scale three-dimensional QED-PIC simulations, we
describe a realistic experimental configuration to measure collective effects
that couple strong field quantum electrodynamics to plasma kinetics. For two
counter propagating lasers interacting with a foil at intensities exceeding
Wcm, a binary result occurs; when quantum effects are
included, a foil that classically would effectively transmit the laser pulse
becomes opaque. This is a dramatic change in plasma behavior, directly as a
consequence of the coupling of radiation reaction and pair production to plasma
dynamics
The Extended Power Law as Intrinsic Signature For a Black Hole
We analyze the exact general relativistic exact integro-differential equation
of radiative transfer describing the interaction of low energy photons with a
Maxwellian distribution of hot electrons in gravitational field of a
Schwarzschild black hole. We prove that due to Comptonization an initial
arbitrary spectrum of low energy photons unavoidably results in spectra
characterized by an extended power-law feature. We examine the spectral index
by using both analytical and numerical methods for a variety of physical
parameters as such the plasma temperature and the mass accretion rate. The
presence of the event horizon as well as the behaviour of the null geodesics in
its vicinity largely determine the dependence of the spectral index on the flow
parameters. We come to the conclusion that the bulk motion of a converging flow
is more efficient in upscattering photons than thermal Comptonization provided
that the electron temperature in the flow is of order of a few keV or less. In
this case, the spectrum observed at infinity consists of a soft component
produced by those input photons that escape after a few scatterings without any
significant energy change and of hard component (described by a power law)
produced by the photons that underwent significant upscattering. The luminosity
of the power-law component is relatively small compared to that of the soft
component. For accretion into black hole the spectral energy index of the
power-law is always higher than one for plasma temperature of order of a few
keV. This result suggests that the bulk motion Comptonization might be
responsible for the power-law spectra seen in the black-hole X-ray sources.Comment: 31 pages, 3 figures; Astrophysical Journal accepte
Suppression of complete fusion due to breakup in the reactions B + Bi
Above-barrier cross sections of -active heavy reaction products, as
well as fission, were measured for the reactions of B with
Bi. Detailed analysis showed that the heavy products include components
from incomplete fusion as well as complete fusion (CF), but fission originates
almost exclusively from CF. Compared with fusion calculations without breakup,
the CF cross sections are suppressed by 15% for B and 7% for B. A
consistent and systematic variation of the suppression of CF for reactions of
the weakly bound nuclei Li, Be, B on targets of
Pb and Bi is found as a function of the breakup threshold
energy
Finite Nuclei in the Quark-Meson Coupling (QMC) Model
We report the first use of the effective QMC energy density functional (EDF),
derived from a quark model of hadron structure, to study a broad range of
ground state properties of even-even nuclei across the periodic table in the
non-relativistic Hartree-Fock+BCS framework. The novelty of the QMC model is
that the nuclear medium effects are treated through modification of the
internal structure of the nucleon. The density dependence is microscopically
derived and the spin-orbit term arises naturally. The QMC EDF depends on a
single set of four adjustable parameters having clear physical basis. When
applied to diverse ground state data the QMC EDF already produces, in its
present simple form, overall agreement with experiment of a quality comparable
to a representative Skyrme EDF. There exist however multiple Skyrme paramater
sets, frequently tailored to describe selected nuclear phenomena. The QMC EDF
set of fewer parameters, as derived in this work, is not open to such
variation, chosen set being applied, without adjustment, to both the properties
of finite nuclei and nuclear matter.Comment: 9 pages, 1 table, 4 figures; in print in Phys. Rev. Letters. A minor
change in the abstract, a few typos corrected and some small technical
adjustments made to comply with the journal regulation
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