16,882 research outputs found
DC magnetic field generation in unmagnetized shear flows
The generation of DC magnetic fields in unmagnetized plasmas with velocity
shear is predicted for non relativistic and relativistic scenarios either due
to thermal effects or due to the onset of the Kelvin-Helmholtz instability
(KHI). A kinetic model describes the growth and the saturation of the DC field.
The predictions of the theory are confirmed by multidimensional
particle-in-cell simulations, demonstrating the formation of long lived
magnetic fields () along the full longitudinal
extent of the shear layer, with transverse width on the electron length scale
(), reaching magnitudes
Electron-scale shear instabilities: magnetic field generation and particle acceleration in astrophysical jets
Strong shear flow regions found in astrophysical jets are shown to be
important dissipation regions, where the shear flow kinetic energy is converted
into electric and magnetic field energy via shear instabilities. The emergence
of these self-consistent fields make shear flows significant sites for
radiation emission and particle acceleration. We focus on electron-scale
instabilities, namely the collisionless, unmagnetized Kelvin-Helmholtz
instability (KHI) and a large-scale dc magnetic field generation mechanism on
the electron scales. We show that these processes are important candidates to
generate magnetic fields in the presence of strong velocity shears, which may
naturally originate in energetic matter outburst of active galactic nuclei and
gamma-ray bursters. We show that the KHI is robust to density jumps between
shearing flows, thus operating in various scenarios with different density
contrasts. Multidimensional particle-in-cell (PIC) simulations of the KHI,
performed with OSIRIS, reveal the emergence of a strong and large-scale dc
magnetic field component, which is not captured by the standard linear fluid
theory. This dc component arises from kinetic effects associated with the
thermal expansion of electrons of one flow into the other across the shear
layer, whilst ions remain unperturbed due to their inertia. The electron
expansion forms dc current sheets, which induce a dc magnetic field. Our
results indicate that most of the electromagnetic energy developed in the KHI
is stored in the dc component, reaching values of equipartition on the order of
in the electron time-scale, and persists longer than the proton
time-scale. Particle scattering/acceleration in the self generated fields of
these shear flow instabilities is also analyzed
Transverse electron-scale instability in relativistic shear flows
Electron-scale surface waves are shown to be unstable in the transverse plane
of a shear flow in an initially unmagnetized plasma, unlike in the
(magneto)hydrodynamics case. It is found that these unstable modes have a
higher growth rate than the closely related electron-scale Kelvin-Helmholtz
instability in relativistic shears. Multidimensional particle-in-cell
simulations verify the analytic results and further reveal the emergence of
mushroom-like electron density structures in the nonlinear phase of the
instability, similar to those observed in the Rayleigh Taylor instability
despite the great disparity in scales and different underlying physics.
Macroscopic () fields are shown to be generated by these
microscopic shear instabilities, which are relevant for particle acceleration,
radiation emission and to seed MHD processes at long time-scales
Slow down of a globally neutral relativistic beam shearing the vacuum
The microphysics of relativistic collisionless sheared flows is investigated
in a configuration consisting of a globally neutral, relativistic beam
streaming through a hollow plasma/dielectric channel. We show through
multidimensional PIC simulations that this scenario excites the Mushroom
instability (MI), a transverse shear instability on the electron-scale, when
there is no overlap (no contact) between the beam and the walls of the
hollow plasma channel. The onset of the MI leads to the conversion of the
beam's kinetic energy into magnetic (and electric) field energy, effectively
slowing down a globally neutral body in the absence of contact. The
collisionless shear physics explored in this configuration may operate in
astrophysical environments, particularly in highly relativistic and supersonic
settings where macroscopic shear processes are stable
Benchmark calculation of p-3H and n-3He scattering
p-3H and n-3He scattering in the energy range above the n-3He but below the
d-d thresholds is studied by solving the 4-nucleon problem with a realistic
nucleon-nucleon interaction. Three different methods -- Alt, Grassberger and
Sandhas, Hyperspherical Harmonics, and Faddeev-Yakubovsky -- have been employed
and their results for both elastic and charge-exchange processes are compared.
We observe a good agreement between the three different methods, thus the
obtained results may serve as a benchmark. A comparison with the available
experimental data is also reported and discussed.Comment: 13 pages, 6 figures. arXiv admin note: text overlap with
arXiv:1109.362
Multiple scattering effects in quasi free scattering from halo nuclei: a test to Distorted Wave Impulse Approximation
Full Faddeev-type calculations are performed for Be breakup on proton
target at 38.4, 100, and 200 MeV/u incident energies. The convergence of the
multiple scattering expansion is investigated. The results are compared with
those of other frameworks like Distorted Wave Impulse Approximation that are
based on an incomplete and truncated multiple scattering expansion.Comment: 7 pages, 16 figures, to be published in Phys. Rev.
Effects of live-bait shrimp trawling on seagrass beds and fish bycatch in Tampa Bay, Florida
The use of live shrimp for bait in
recreational fishing has resulted in
a controversial fishery for shrimp in
Florida. In this fishery, night collections
are conducted over seagrass
beds with roller beam trawls to capture
live shrimp, primarily pink
shrimp, Penaeus duorarum. These
shrimp are culled from the catch on
sorting tables and placed in onboard
aerated “live” wells. Beds of
turtlegrass, Thalassia testudinum,
a species that has highest growth
rates and biomass during summer
and lowest during the winter (Fonseca
et al., 1996) are predominant
areas for live-bait shrimp trawling
(Tabb and Kenny, 1969).
Our study objectives were 1) to
determine effects of a roller beam
trawl on turtlegrass biomass and
morphometrics during intensive
(up to 18 trawls over a turtlegrass
bed), short-term (3-hour duration)
use and 2) to examine the mortality
of bycatch finfish following capture
by a trawl
Fuzzy short-run control charts
Statistical control charts are useful tools in monitoring the state of a manufacturing process. Control charts are used to plot process data and compare it to the limits set for the process. Points plotting outside these limits indicate an out-of-control condition. Standard control charting procedures, however, are limited in that they cannot take into account the case when data is of a fuzzy nature. Another limitation of standard charting methods is when the data produced by the process is short-run data. Often, the situation where the data is short-run occurs in conjunction with data that is considered fuzzy. This paper dicusses the development of a fuzzy control chartting technique, called short-Run α-cut p Control Chart, to account for fuzzy data in a short-run situation. The developed chart parameters accounted for the fuzzy nature of the data in a short-run situation. The parameters were validated by comparing the false alarm rates for various combinations of subgroup numbers (m) and subgroup sizes (n). It was shown that for every combination of m and n, the Short-Run α-cut p Control Chart limits produced a lower false alarm rate than that of the standard fuzzy α-cut control chart.Peer Reviewe
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