10,668 research outputs found
Baryon loading and the Weibel instability in gamma-ray bursts
The dynamics of two counter-streaming electron-positron-ion unmagnetized
plasma shells with zero net charge is analyzed in the context of magnetic field
generation in GRB internal shocks due to the Weibel instability. The effects of
large thermal motion of plasma particles, arbitrary mixture of plasma species
and space charge effects are taken into account. We show that, although thermal
effects slow down the instability, baryon loading leads to a non-negligible
growth rate even for large temperatures and different shell velocities, thus
guaranteeing the robustness and the occurrence of the Weibel instability for a
wide range of scenarios.Comment: 6 pages, 4 figures. Accepted for publication in MNRA
Quantum Group Covariance and the Braided Structure of Deformed Oscillators
The connection between braided Hopf algebra structure and the quantum group
covariance of deformed oscillators is constructed explicitly. In this context
we provide deformations of the Hopf algebra of functions on SU(1,1). Quantum
subgroups and their representations are also discussed.Comment: 12 pages, to be published in JM
A Strategy for Efficiently Collecting Aerosol Condensate Using Silica Fibers: Application to Carbonyl Emissions from E-Cigarettes
Analyzing harmful constituents in e-cigarette aerosols typically involves adopting a methodology used for analyzing tobacco smoke. Cambridge filter pads (CFP) are the basis of numerous protocols for analyzing the various classes of compounds representing 93 harmful and potentially harmful constituents identified in tobacco smoke by the FDA. This paper describes a simplified method for trapping the low volatility components of e-cigarette aerosols using a single trapping procedure followed by physical extraction. The trap is a plug of amorphous silica fibers (0.75 g of 4 μm diameter) within a 10 mL syringe inserted between the e-cigarette mouthpiece and the pump of the vaping machine. The method is evaluated for emissions from three generations of e-cigarette device (Kangertech CE4, EVOD, and Subox Mini-C). On average, the silica wool traps about 94% of the vaporized liquid mass in the three devices and higher levels of condensate is retained before reaching saturation compared with CFP. The condensate is then physically extracted from the silica wool plug using a centrifuge. Condensate is then available for use directly in multiple analytical procedures or toxicological experiments. The method is tested by comparison with published analyses of carbonyls, among the most potent toxicants and carcinogens in e-cigarette emissions. Ranges for HPLC-DAD analyses of carbonyl-DNPH derivatives in a laboratory formulation of e-liquid are formaldehyde (0.182 ± 0.023 to 9.896 ± 0.709 μg puff-1), acetaldehyde (0.059 ± 0.005 to 0.791 ± 0.073 μg puff-1), and propionaldehyde (0.008 ± 0.0001 to 0.033 ± 0.023 μg puff-1); other carbonyls are identified and quantified. Carbonyl concentrations are also consistent with published experiments showing marked increases with variable power settings (10W to 50W). Compared with CFPs, e-cigarette aerosol collection by silica wool requires only one vaping session for multiple analyte groups, traps more condensate per puff, and collects more condensate before saturation
Electromechanical tuning of vertically-coupled photonic crystal nanobeams
We present the design, the fabrication and the characterization of a tunable
one-dimensional (1D) photonic crystal cavity (PCC) etched on two
vertically-coupled GaAs nanobeams. A novel fabrication method which prevents
their adhesion under capillary forces is introduced. We discuss a design to
increase the flexibility of the structure and we demonstrate a large reversible
and controllable electromechanical wavelength tuning (> 15 nm) of the cavity
modes.Comment: 11 pages, 4 figure
A global simulation for laser driven MeV electrons in -diameter fast ignition targets
The results from 2.5-dimensional Particle-in-Cell simulations for the
interaction of a picosecond-long ignition laser pulse with a plasma pellet of
50- diameter and 40 critical density are presented. The high density
pellet is surrounded by an underdense corona and is isolated by a vacuum region
from the simulation box boundary. The laser pulse is shown to filament and
create density channels on the laser-plasma interface. The density channels
increase the laser absorption efficiency and help generate an energetic
electron distribution with a large angular spread. The combined distribution of
the forward-going energetic electrons and the induced return electrons is
marginally unstable to the current filament instability. The ions play an
important role in neutralizing the space charges induced by the the temperature
disparity between different electron groups. No global coalescing of the
current filaments resulted from the instability is observed, consistent with
the observed large angular spread of the energetic electrons.Comment: 9 pages, 6 figures, to appear in Physics of Plasmas (May 2006
Using dark modes for high-fidelity optomechanical quantum state transfer
In a recent publication [Y.D. Wang and A.A. Clerk, Phys. Rev. Lett. 108,
153603 (2012)], we demonstrated that one can use interference to significantly
increase the fidelity of state transfer between two electromagnetic cavities
coupled to a common mechanical resonator over a naive sequential-transfer
scheme based on two swap operations. This involved making use of a delocalized
electromagnetic mode which is decoupled from the mechanical resonator, a
so-called "mechanically-dark" mode. Here, we demonstrate the existence of a new
"hybrid" state transfer scheme which incorporates the best elements of the
dark-mode scheme (protection against mechanical dissipation) and the
double-swap scheme (fast operation time). Importantly, this new scheme also
does not require the mechanical resonator to be prepared initially in its
ground state. We also provide additional details on the previously-described
interference-enhanced transfer schemes, and provide an enhanced discussion of
how the interference physics here is intimately related to the optomechanical
analogue of electromagnetically-induced transparency (EIT). We also compare the
various transfer schemes over a wide range of relevant experimental parameters,
producing a "phase diagram" showing the the optimal transfer scheme for
different points in parameter space.Comment: 39 pages, 11 figures NJP 14 (Focus issue on Optomechanics
Agent-Based Modeling of Intracellular Transport
We develop an agent-based model of the motion and pattern formation of
vesicles. These intracellular particles can be found in four different modes of
(undirected and directed) motion and can fuse with other vesicles. While the
size of vesicles follows a log-normal distribution that changes over time due
to fusion processes, their spatial distribution gives rise to distinct
patterns. Their occurrence depends on the concentration of proteins which are
synthesized based on the transcriptional activities of some genes. Hence,
differences in these spatio-temporal vesicle patterns allow indirect
conclusions about the (unknown) impact of these genes.
By means of agent-based computer simulations we are able to reproduce such
patterns on real temporal and spatial scales. Our modeling approach is based on
Brownian agents with an internal degree of freedom, , that represents
the different modes of motion. Conditions inside the cell are modeled by an
effective potential that differs for agents dependent on their value .
Agent's motion in this effective potential is modeled by an overdampted
Langevin equation, changes of are modeled as stochastic transitions
with values obtained from experiments, and fusion events are modeled as
space-dependent stochastic transitions. Our results for the spatio-temporal
vesicle patterns can be used for a statistical comparison with experiments. We
also derive hypotheses of how the silencing of some genes may affect the
intracellular transport, and point to generalizations of the model
Ionized Absorbers in AGN: the Role of Collisional Ionization and Time-Evolving Photoionization
In this paper we explore collisional ionization and time-evolving
photoionization in the, X-ray discovered, ionized absorbers in Seyfert
galaxies. These absorbers show temporal changes inconsistent with simple
equilibrium models. We develop a simple code to follow the temporal evolution
of non-equilibrium photoionized gas. As a result several effects appear that
are easily observable; and which, in fact, may explain otherwise paradoxical
behavior.
Specifically we find that: 1) In many important astrophysical conditions pure
collisional and photoionization equilibria can be distinguished with moderate
spectral resolution observations, due to a strong absorption structure between
1 and 3 keV. 2) In time-evolving non-equilibrium photoionization models the
response of the ionization state of the gas to sudden changes of the ionizing
continuum is smoothed and delayed at low gas densities, even when the
luminosity increases. 3) If the changes of the ionizing luminosity are not
instantaneous, and the electron density is low enough (the limit depends on the
average ionization state of the gas), the ionization state of the gas can
continue to increase while the source luminosity decreases, so a maximum in the
ionization state of a given element may occur during a minimum of the ionizing
intensity (the opposite of the prediction of equilibrium models). 4) Different
ions of different elements reach their equilibrium configuration on different
time-scales.
These properties are similar to those seen in several ionized absorbers in
AGN, properties which had hitherto been puzzling. We applied these models to a
high S/N ROSAT PSPC observation of the Seyfert 1 galaxy NGC 4051.Comment: 36 pages, 10 figures, accepted for publication on Apj, in pres
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