28,233 research outputs found
Laser ignition of an optically sensitised secondary explosive by a diode laser
As a green technology, laser ignition of a relatively insensitive secondary
explosive has been experimentally investigated. The explosive, hexanitrostilbene
(HNS), was doped with one of two optical sensitizers, carbon black or a laser
absorbing dye, and a continuous-wave (CW) infrared diode laser was used as the
igniting source. The ignition sensitivities of HNS with each of the two optical
sensitizers were analysed and compared in terms of: optical power threshold for
ignition, ignition delay and full burn delay at various laser powers. The results
have shown that both the chemical dye and carbon black optically sensitize
the explosive with similar efficiencies. In contrast to the carbon black, the dye
provides wavelength specificity and selectivity in the laser ignition process and its
solubility in some specific solvents improves the coating of the explosive material.
It was therefore concluded that the laser absorbing dye is a better candidate for
optical sensitization in laser ignition than the commonly used carbon black. The
combination of laser ignition sensitivity with wavelength selectivity potentially
offers higher reliability and safety at a low optical power for future ignitors of
secondary explosives
Coronal rain in magnetic arcades: Rebound shocks, Limit cycles, and Shear flows
We extend our earlier multidimensional, magnetohydrodynamic simulations of
coronal rain occurring in magnetic arcades with higher resolution,
grid-adaptive computations covering a much longer ( hour) timespan. We
quantify how in-situ forming blob-like condensations grow along and across
field lines and show that rain showers can occur in limit cycles, here
demonstrated for the first time in 2.5D setups. We discuss dynamical,
multi-dimensional aspects of the rebound shocks generated by the siphon inflows
and quantify the thermodynamics of a prominence-corona-transition-region like
structure surrounding the blobs. We point out the correlation between
condensation rates and the cross-sectional size of loop systems where
catastrophic cooling takes place. We also study the variations of the typical
number density, kinetic energy and temperature while blobs descend, impact and
sink into the transition region. In addition, we explain the mechanisms leading
to concurrent upflows while the blobs descend. As a result, there are plenty of
shear flows generated with relative velocity difference around 80 km s
in our simulations. These shear flows are siphon flows set up by multiple blob
dynamics and they in turn affect the deformation of the falling blobs. In
particular, we show how shear flows can break apart blobs into smaller
fragments, within minutes
Simulating coronal condensation dynamics in 3D
We present numerical simulations in 3D settings where coronal rain phenomena
take place in a magnetic configuration of a quadrupolar arcade system. Our
simulation is a magnetohydrodynamic simulation including anisotropic thermal
conduction, optically thin radiative losses, and parametrised heating as main
thermodynamical features to construct a realistic arcade configuration from
chromospheric to coronal heights. The plasma evaporation from chromospheric and
transition region heights eventually causes localised runaway condensation
events and we witness the formation of plasma blobs due to thermal instability,
that evolve dynamically in the heated arcade part and move gradually downwards
due to interchange type dynamics. Unlike earlier 2.5D simulations, in this case
there is no large scale prominence formation observed, but a continuous coronal
rain develops which shows clear indications of Rayleigh-Taylor or interchange
instability, that causes the denser plasma located above the transition region
to fall down, as the system moves towards a more stable state. Linear stability
analysis is used in the non-linear regime for gaining insight and giving a
prediction of the system's evolution. After the plasma blobs descend through
interchange, they follow the magnetic field topology more closely in the lower
coronal regions, where they are guided by the magnetic dips.Comment: 47 pages, 59 figure
Spectroscopic Observations of Planetary Nebulae in the Northern Spur of M31
We present spectroscopy of three planetary nebulae (PNe) in the Northern Spur
of the Andromeda Galaxy (M31) obtained with the Double Spectrograph on the 5.1
m Hale Telescope at the Palomar Observatory. The samples are selected from the
observations of Merrett et al. Our purpose is to investigate formation of the
substructures of M31 using PNe as a tracer of chemical abundances. The [O III]
4363 auroral line is detected in the spectra of two objects, enabling
temperature determinations. Ionic abundances are derived from the observed
collisionally excited lines, and elemental abundances of nitrogen, oxygen, and
neon as well as sulphur and argon are estimated. Correlations between oxygen
and the alpha-element abundance ratios are studied, using our sample and the
M31 disk and bulge PNe from the literature. In one of the three PNe, we
observed relatively higher oxygen abundance compared to the disk sample in M31
at similar galactocentric distances. The results of at least one of the three
Northern Spur PNe might be in line with the proposed possible origin of the
Northern Spur substructure of M31, i.e. the Northern Spur is connected to the
Southern Stream and both substructures comprise the tidal debris of the
satellite galaxies of M31.Comment: 5 tables, 17 figures; accepted for publication in Ap
The Serpens filament: at the onset of slightly supercritical collapse
The Serpens filament, as one of the nearest infrared dark clouds, is regarded
as a pristine filament at a very early evolutionary stage of star formation. In
order to study its molecular content and dynamical state, we mapped this
filament in seven species. Among them, HCO, HNC, HCN, and CS show
self-absorption, while CO is most sensitive to the filamentary
structure. A kinematic analysis demonstrates that this filament forms a
velocity-coherent (trans-)sonic structure, a large part of which is one of the
most quiescent regions in the Serpens cloud. Widespread CO depletion is
found throughout the Serpens filament. Based on the Herschel dust-derived
H column density map, the line mass of the filament is
36--41~M~pc, and its full width at half maximum is
0.170.01~pc, while its length is ~1.6~pc. The inner radial column density
profile of this filament can be well fitted with a Plummer profile with an
exponent of 2.20.1, a scale radius of pc, and a central
density of ~cm. The Serpens filament appears
to be slightly supercritical. The widespread blue-skewed HNC and CS line
profiles and HCN hyperfine line anomalies across this filament indicate radial
infall in parts of the Serpens filament. CO velocity gradients also
indicate accretion flows along the filament. The velocity and density
structures suggest that such accretion flows are likely due to a longitudinal
collapse parallel to the filament's long axis. Both the radial infall rate and
the longitudinal accretion rate along the Serpens filament are lower than all
previously reported values in other filaments. This indicates that the Serpens
filament lies at an early evolutionary stage when collapse has just begun, or
that thermal and non-thermal support are effective in providing support against
gravity.Comment: 22 pages, 14 figures, 4 tables, accepted for publication in A&A; for
the draft showing figures with full resolution, see
http://gongyan2444.github.io/pdf/absfil.pd
Formation of Enhanced Uniform Chiral Fields in Symmetric Dimer Nanostructures
Chiral fields with large optical chirality are very important in chiral molecules analysis, sensing and other measurements. Plasmonic nanostructures have been proposed to realize such super chiral fields for enhancing weak chiral signals. However, most of them cannot provide uniform chiral near-fields close to the structures, which makes these nanostructures not so efficient for applications. Plasmonic helical nanostructures and blocked squares have been proved to provide uniform chiral near-fields, but structure fabrication is a challenge. In this paper, we show that very simple plasmonic dimer structures can provide uniform chiral fields in the gaps with large enhancement of both near electric fields and chiral fields under linearly polarized light illumination with polarization off the dimer axis at dipole resonance. An analytical dipole model is utilized to explain this behavior theoretically. 30 times of volume averaged chiral field enhancement is gotten in the whole gap. Chiral fields with opposite handedness can be obtained simply by changing the polarization to the other side of the dimer axis. It is especially useful in Raman optical activity measurement and chiral sensing of small quantity of chiral molecule
Approximate broadcasting of quantum correlations
© 2017 American Physical Society. Broadcasting quantum and classical information is a basic task in quantum information processing, and is also a useful model in the study of quantum correlations including quantum discord. We establish a full operational characterization of two-sided quantum discord in terms of bilocal broadcasting of quantum correlations. Moreover, we show that both the optimal fidelity of unilocal broadcasting of the correlations in an arbitrary bipartite quantum state and that of broadcasting an arbitrary set of quantum states can be formulized as semidefinite programs (SDPs), which are efficiently computable. We also analyze some properties of these SDPs and evaluate the broadcasting fidelities for some cases of interest
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