2,046 research outputs found
Electric field generation by the electron beam filamentation instability: Filament size effects
The filamentation instability (FI) of counter-propagating beams of electrons
is modelled with a particle-in-cell simulation in one spatial dimension and
with a high statistical plasma representation. The simulation direction is
orthogonal to the beam velocity vector. Both electron beams have initially
equal densities, temperatures and moduli of their nonrelativistic mean
velocities. The FI is electromagnetic in this case. A previous study of a small
filament demonstrated, that the magnetic pressure gradient force (MPGF) results
in a nonlinearly driven electrostatic field. The probably small contribution of
the thermal pressure gradient to the force balance implied, that the
electrostatic field performed undamped oscillations around a background
electric field. Here we consider larger filaments, which reach a stronger
electrostatic potential when they saturate. The electron heating is enhanced
and electrostatic electron phase space holes form. The competition of several
smaller filaments, which grow simultaneously with the large filament, also
perturbs the balance between the electrostatic and magnetic fields. The
oscillations are damped but the final electric field amplitude is still
determined by the MPGF.Comment: 14 pages, 10 plots, accepted for publication in Physica Script
Diffusive radiation in Langmuir turbulence produced by jet shocks
Anisotropic distributions of charged particles including two-stream
distributions give rise to generation of either stochastic electric fields (in
the form of Langmuir waves, Buneman instability) or random quasi-static
magnetic fields (Weibel and filamentation instabilities) or both. These
two-stream instabilities are known to play a key role in collisionless shock
formation, shock-shock interactions, and shock-induced electromagnetic
emission. This paper applies the general non-perturbative stochastic theory of
radiation to study electromagnetic emission produced by relativistic particles,
which random walk in the stochastic electric fields of the Langmuir waves. This
analysis takes into account the cumulative effect of uncorrelated Langmuir
waves on the radiating particle trajectory giving rise to angular diffusion of
the particle, which eventually modifies the corresponding radiation spectra. We
demonstrate that the radiative process considered is probably relevant for
emission produced in various kinds of astrophysical jets, in particular, prompt
gamma-ray burst spectra, including X-ray excesses and prompt optical flashes.Comment: 9 pages, 5 figures, MNRAS, accepte
Shock creation and particle acceleration driven by plasma expansion into a rarefied medium
The expansion of a dense plasma through a more rarefied ionised medium is a
phenomenon of interest in various physics environments ranging from
astrophysics to high energy density laser- matter laboratory experiments. Here
this situation is modeled via a 1D Particle-In-Cell simulation; a jump in the
plasma density of a factor of 100 is introduced in the middle of an otherwise
equally dense electron-proton plasma with an uniform proton and electron
temperature of 10eV and 1keV respectively. The diffusion of the dense plasma,
through the rarified one, triggers the onset of different nonlinear phenomena
such as a strong ion-acoustic shock wave and a rarefaction wave. Secondary
structures are detected, some of which are driven by a drift instability of the
rarefaction wave. Efficient proton acceleration occurs ahead of the shock,
bringing the maximum proton velocity up to 60 times the initial ion thermal
speed
The filamentation instability driven by warm electron beams: Statistics and electric field generation
The filamentation instability of counterpropagating symmetric beams of
electrons is examined with 1D and 2D particle-in-cell (PIC) simulations, which
are oriented orthogonally to the beam velocity vector. The beams are uniform,
warm and their relative speed is mildly relativistic. The dynamics of the
filaments is examined in 2D and it is confirmed that their characteristic size
increases linearly in time. Currents orthogonal to the beam velocity vector are
driven through the magnetic and electric fields in the simulation plane. The
fields are tied to the filament boundaries and the scale size of the
flow-aligned and the perpendicular currents are thus equal. It is confirmed
that the electrostatic and the magnetic forces are equally important, when the
filamentation instability saturates in 1D. Their balance is apparently the
saturation mechanism of the filamentation instability for our initial
conditions. The electric force is relatively weaker but not negligible in the
2D simulation, where the electron temperature is set higher to reduce the
computational cost. The magnetic pressure gradient is the principal source of
the electrostatic field, when and after the instability saturates in the 1D
simulation and in the 2D simulation.Comment: 10 pages, 6 figures, accepted by the Plasma Physics and Controlled
Fusion (Special Issue EPS 2009
Particle-in-cell simulation of a mildly relativistic collision of an electron-ion plasma carrying a quasi-parallel magnetic field: Electron acceleration and magnetic field amplification at supernova shocks
Plasma processes close to SNR shocks result in the amplification of magnetic
fields and in the acceleration of electrons, injecting them into the diffusive
acceleration mechanism. The acceleration of electrons and the B field
amplification by the collision of two plasma clouds, each consisting of
electrons and ions, at a speed of 0.5c is investigated. A quasi-parallel
guiding magnetic field, a cloud density ratio of 10 and a plasma temperature of
25 keV are considered. A quasi-planar shock forms at the front of the dense
plasma cloud. It is mediated by a circularly left-hand polarized
electromagnetic wave with an electric field component along the guiding
magnetic field. Its propagation direction is close to that of the guiding field
and orthogonal to the collision boundary. It has a low frequency and a
wavelength that equals several times the ion inertial length, which would be
indicative of a dispersive Alfven wave close to the ion cyclotron resonance
frequency of the left-handed mode (ion whistler), provided that the frequency
is appropriate. However, it moves with the super-alfvenic plasma collision
speed, suggesting that it is an Alfven precursor or a nonlinear MHD wave such
as a Short Large-Amplitude Magnetic Structure (SLAMS). The growth of the
magnetic amplitude of this wave to values well in excess of those of the
quasi-parallel guiding field and of the filamentation modes results in a
quasi-perpendicular shock. We present evidence for the instability of this mode
to a four wave interaction. The waves developing upstream of the dense cloud
give rise to electron acceleration ahead of the collision boundary. Energy
equipartition between the ions and the electrons is established at the shock
and the electrons are accelerated to relativistic speeds.Comment: 16 pages, 18 figures, Accepted for publication by Astron & Astrophy
Systemic-risk Dilemmas Emerging from Reactive Investments
The stability and prosperity of human societies depend on cooperative exchanges at many levels. While the emergence and stability of such exchanges has been studied extensively, the majority of these investigations have relied on the prisoner’s dilemma game. However, cooperative exchanges are typically more complex and involve decisions about continuous investments, instead of simple choices between cooperation and non-cooperation. Our aim here is to understand the factors promoting the emergence and stability of cooperative exchanges based on continuous investments, performed by individuals with reactive strategies. Through such strategies, agents continuously re-evaluate and adjust their investments according to the gains obtained from an exchange. Such reactivity provides a natural safeguard against exploitation, as it allows agents to fade out unprofitable investments. Here we show that these benefits of reactivity, which are so crucial at the level of agents, exact a high price at the level of the society, by exacerbating systemic risk. In particular, the spread of exuberant investors causes the emergence of boom-bust cycles, characterized by an increase in investment levels followed by a decline. We demonstrate that an optimal level of reactivity can stabilize cooperation while offering safeguards against both exploitation and exuberance. We also study three other fundamental factors: the pace of innovation enhancing strategy diversity, the modularity arising from dividing the collective of agents into smaller groups with sparse interactions between them, and the heterogeneity of such groups structuring social exchanges. We demonstrate how intermediate levels of these additional factors are optimal in terms of stabilizing cooperation while minimizing systemic risk. Our study identifies generally applicable countermeasures against ubiquitous threats to the stability of cooperative exchanges, with a view towards facilitating the design of corresponding future policies
PIC Simulations of the Temperature Anisotropy-Driven Weibel Instability: Analyzing the perpendicular mode
An instability driven by the thermal anisotropy of a single electron species
is investigated in a 2D particle-in-cell (PIC) simulation. This instability is
the one considered by Weibel and it differs from the beam driven filamentation
instability. A comparison of the simulation results with analytic theory
provides similar exponential growth rates of the magnetic field during the
linear growth phase of the instability. We observe in accordance with previous
works the growth of electric fields during the saturation phase of the
instability. Some components of this electric field are not accounted for by
the linearized theory. A single-fluid-based theory is used to determine the
source of this nonlinear electric field. It is demonstrated that the magnetic
stress tensor, which vanishes in a 1D geometry, is more important in this
2-dimensional model used here. The electric field grows to an amplitude, which
yields a force on the electrons that is comparable to the magnetic one. The
peak energy density of each magnetic field component in the simulation plane
agrees with previous estimates. Eddy currents develop, which let the amplitude
of the third magnetic field component grow, which is not observed in a 1D
simulation.Comment: accepted by Plasma Physics and Controlled Fusio
How large can the electron to proton mass ratio be in Particle-In-Cell simulations of unstable systems?
Particle-in-cell (PIC) simulations are widely used as a tool to investigate
instabilities that develop between a collisionless plasma and beams of charged
particles. However, even on contemporary supercomputers, it is not always
possible to resolve the ion dynamics in more than one spatial dimension with
such simulations. The ion mass is thus reduced below 1836 electron masses,
which can affect the plasma dynamics during the initial exponential growth
phase of the instability and during the subsequent nonlinear saturation. The
goal of this article is to assess how far the electron to ion mass ratio can be
increased, without changing qualitatively the physics. It is first demonstrated
that there can be no exact similarity law, which balances a change of the mass
ratio with that of another plasma parameter, leaving the physics unchanged.
Restricting then the analysis to the linear phase, a criterion allowing to
define a maximum ratio is explicated in terms of the hierarchy of the linear
unstable modes. The criterion is applied to the case of a relativistic electron
beam crossing an unmagnetized electron-ion plasma.Comment: To appear in Physics of Plasma
The origin of green icebergs in Antarctica
A comparison of samples from a translucent green iceberg with a core from the Ronne Ice Shelf revealed an excellent agreement in isotopic composition, crystal structure, and incorporated sediment particles. Marine shelf ice which constitutes the basal portion of some ice shelves is considered to be the source of green icebergs. It most likely results from "ice pump" processes which produce large amounts of ice platelets in the water column beneath ice shelves. These subsequently accumulate and become compacted into bubble-free, desalinated ice. Iceberg and drift-buoy trajectories indicate that green icebergs observed in the Weddell Sea originate from the Amery Ice Shelf rather than from the Ronne Ice Shelf, although the latter ice shelf is also a potential source
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