974 research outputs found
Collisionless energy absorption in the short-pulse intense laser-cluster interaction
In a previous Letter [Phys. Rev. Lett. 96, 123401 (2006)] we have shown by
means of three-dimensional particle-in-cell simulations and a simple
rigid-sphere model that nonlinear resonance absorption is the dominant
collisionless absorption mechanism in the intense, short-pulse laser cluster
interaction. In this paper we present a more detailed account of the matter. In
particular we show that the absorption efficiency is almost independent of the
laser polarization. In the rigid-sphere model, the absorbed energy increases by
many orders of magnitude at a certain threshold laser intensity. The
particle-in-cell results display maximum fractional absorption around the same
intensity. We calculate the threshold intensity and show that it is
underestimated by the common over-barrier ionization estimate.Comment: 12 pages, 13 figures, RevTeX
An alternative to the plasma emission model: Particle-In-Cell, self-consistent electromagnetic wave emission simulations of solar type III radio bursts
1.5D PIC, relativistic, fully electromagnetic (EM) simulations are used to
model EM wave emission generation in the context of solar type III radio
bursts. The model studies generation of EM waves by a super-thermal, hot beam
of electrons injected into a plasma thread that contains uniform longitudinal
magnetic field and a parabolic density gradient. In effect, a single magnetic
line connecting Sun to earth is considered, for which several cases are
studied. (i) We find that the physical system without a beam is stable and only
low amplitude level EM drift waves (noise) are excited. (ii) The beam injection
direction is controlled by setting either longitudinal or oblique electron
initial drift speed, i.e. by setting the beam pitch angle. In the case of zero
pitch angle, the beam excites only electrostatic, standing waves, oscillating
at plasma frequency, in the beam injection spatial location, and only low level
EM drift wave noise is also generated. (iii) In the case of oblique beam pitch
angles, again electrostatic waves with same properties are excited. However,
now the beam also generates EM waves with the properties commensurate to type
III radio bursts. The latter is evidenced by the wavelet analysis of transverse
electric field component, which shows that as the beam moves to the regions of
lower density, frequency of the EM waves drops accordingly. (iv) When the
density gradient is removed, electron beam with an oblique pitch angle still
generates the EM radiation. However, in the latter case no frequency decrease
is seen. Within the limitations of the model, the study presents the first
attempt to produce simulated dynamical spectrum of type III radio bursts in
fully kinetic plasma model. The latter is based on 1.5D non-zero pitch angle
(non-gyrotropic) electron beam, that is an alternative to the plasma emission
classical mechanism.Comment: Physics of Plasmas, in press, May 2011 issue (final accepted version
Investigation of Particle-in-Cell Acceleration Techniques for Plasma Simulations
COLISEUM is an application framework that integrates plasma propagation schemes and arbitrary 3D surface geometries. Using Particle-in-Cell (PIC) schemes to model the plasma propagation high fidelity modeling of the plasma and its interactions with the surfaces is possible. In order to improve the computational performance of the Particle-in-Cell scheme with Direct Simulation Monte Carlo collision modeling (PIC-DSMC) within COLISEUM, AQUILA, acceleration techniques have been developed that significantly decrease the amount of CPU time needed to obtain a steady-state solution. These techniques have been demonstrated to decrease the CPU time from 3 to 24 times with little appreciable differences in the global particle properties and number densities. This work investigates the differences in the local plasma properties that result from the application of the different acceleration techniques. Results show that the subcycling acceleration scheme does accurately capture the macroscopic flow properties (such as particle counts and species number densities) and the velocity distributions in the lower density regions of the flow field. However, the higher density regions of the flow field (such as in the main beam of the plasma source) show significant differences that are believed to be associated with the simplifying assumptions used in the original collision modeling scheme within the PIC-DSMC module AQUILA
New combined PIC-MCC approach for fast simulation of a radio frequency discharge at low gas pressure
A new combined PIC-MCC approach is developed for accurate and fast simulation
of a radio frequency discharge at low gas pressure and high density of plasma.
Test calculations of transition between different modes of electron heating in
a ccrf discharge in helium and argon show a good agreement with experimental
data.
We demonstrate high efficiency of the combined PIC-MCC algorithm, especially
for the collisionless regime of electron heating.Comment: 6 paged, 8 figure
Electron beam induced radio emission from ultracool dwarfs
We present the numerical simulations for an electron-beam-driven and
loss-cone-driven electron-cyclotron maser (ECM) with different plasma
parameters and different magnetic field strengths for a relatively small region
and short time-scale in an attempt to interpret the recent discovered intense
radio emission from ultracool dwarfs. We find that a large amount of
electromagnetic field energy can be effectively released from the beam-driven
ECM, which rapidly heats the surrounding plasma. A rapidly developed
high-energy tail of electrons in velocity space (resulting from the heating
process of the ECM) may produce the radio continuum depending on the initial
strength of the external magnetic field and the electron beam current. Both
significant linear polarization and circular polarization of electromagnetic
waves can be obtained from the simulations. The spectral energy distributions
of the simulated radio waves show that harmonics may appear from 10 to
70 ( is the electron plasma frequency) in the
non-relativistic case and from 10 to 600 in the relativistic
case, which makes it difficult to find the fundamental cyclotron frequency in
the observed radio frequencies. A wide frequency band should therefore be
covered by future radio observations.Comment: 10 pages, 19 figures, accepted for publication in the Astrophysical
Journa
Heterogeneity of cell membrane structure studied by single molecule tracking
Heterogeneity in cell membrane structure, typified by microdomains with different biophysical and biochemical properties, is thought to impact on a variety of cell functions. Integral membrane proteins act as nanometre-sized probes of the lipid environment and their thermally-driven movements can be used to report local variations in membrane properties. In the current study, we have used total internal reflection fluorescence microscopy (TIRFM) combined with super-resolution tracking of multiple individual molecules, in order to create high-resolution maps of local membrane viscosity. We used a quadrat sampling method and show how statistical tests for membrane heterogeneity can be conducted by analysing the paths of many molecules that pass through the same unit area of membrane. We describe experiments performed on cultured primary cells, stable cell lines and ex vivo tissue slices using a variety of membrane proteins, under different imaging conditions. In some cell types, we find no evidence for heterogeneity in mobility across the plasma membrane, but in others we find statistically significant differences with some regions of membrane showing significantly higher viscosity than others
Statistical kinetic treatment of relativistic binary collisions
In particle-based algorithms, the effect of binary collisions is commonly
described in a statistical way, using Monte Carlo techniques. It is shown that,
in the relativistic regime, stringent constraints should be considered on the
sampling of particle pairs for collision, which are critical to ensure
physically meaningful results, and that nonrelativistic sampling criteria
(e.g., uniform random pairing) yield qualitatively wrong results, including
equilibrium distributions that differ from the theoretical J\"uttner
distribution. A general procedure for relativistically consistent algorithms is
provided, and verified with three-dimensional Monte Carlo simulations, thus
opening the way to the numerical exploration of the statistical properties of
collisional relativistic systems.Comment: Accepted for publication as a Rapid Communication in Phys. Rev.
A numerical investigation of the stability of steady states and critical phenomena for the spherically symmetric Einstein-Vlasov system
The stability features of steady states of the spherically symmetric
Einstein-Vlasov system are investigated numerically. We find support for the
conjecture by Zeldovich and Novikov that the binding energy maximum along a
steady state sequence signals the onset of instability, a conjecture which we
extend to and confirm for non-isotropic states. The sign of the binding energy
of a solution turns out to be relevant for its time evolution in general. We
relate the stability properties to the question of universality in critical
collapse and find that for Vlasov matter universality does not seem to hold.Comment: 29 pages, 10 figure
Critical collapse of collisionless matter - a numerical investigation
In recent years the threshold of black hole formation in spherically
symmetric gravitational collapse has been studied for a variety of matter
models. In this paper the corresponding issue is investigated for a matter
model significantly different from those considered so far in this context. We
study the transition from dispersion to black hole formation in the collapse of
collisionless matter when the initial data is scaled. This is done by means of
a numerical code similar to those commonly used in plasma physics. The result
is that for the initial data for which the solutions were computed, most of the
matter falls into the black hole whenever a black hole is formed. This results
in a discontinuity in the mass of the black hole at the onset of black hole
formation.Comment: 22 pages, LaTeX, 7 figures (ps-files, automatically included using
psfig
Reaching Inward Not Outward: Marketing via the Internet at the UK 2010 General Election
The Internet has been to date used as a space for simple promotion by political parties; websites present an opportunity for the delivery of non-mediated communication directly to the online audience and nothing more. However, new patterns in usage during campaigns, particularly that of Barack Obama, aided by the technological innovations that fall under the umbrella of Web 2.0, offer new models of online political communication. Through an analysis of the websites and linked online presences of six parties that stood across the UK at the 2010 General Election, we find a dual strategy for Internet campaigning emerging. The persuasive traditions of electioneering remain a feature; however, the key emergent function is one of internal marketing to party supporters and activists. Large sections of party websites are being dedicated to harnessing supporters and converting them to being donators, promoters, and campaigners both online and offline. This suggests that the Internet is increasingly embedded within election communication and online communication strategies are becoming a feature of most of the parties' marketing communication mix. © 2013 Copyright Taylor and Francis Group, LLC
- …