25 research outputs found
Multidimensional simulations of magnetic field amplification and electron acceleration to near-energy equipartition with ions by a mildly relativistic quasi-parallel plasma collision
The energetic electromagnetic eruptions observed during the prompt phase of
gamma-ray bursts are attributed to synchrotron emissions. The internal shocks
moving through the ultrarelativistic jet, which is ejected by an imploding
supermassive star, are the likely source of this radiation. Synchrotron
emissions at the observed strength require the simultaneous presence of
powerful magnetic fields and highly relativistic electrons. We explore with one
and three-dimensional relativistic particle-in-cell simulations the transition
layer of a shock, that evolves out of the collision of two plasma clouds at a
speed 0.9c and in the presence of a quasi-parallel magnetic field. The cloud
densities vary by a factor of 10. The number densities of ions and electrons in
each cloud, which have the mass ratio 250, are equal. The peak Lorentz factor
of the electrons is determined in the 1D simulation, as well as the orientation
and the strength of the magnetic field at the boundary of the two colliding
clouds. The relativistic masses of the electrons and ions close to the shock
transition layer are comparable as in previous work. The 3D simulation shows
rapid and strong plasma filamentation behind the transient precursor. The
magnetic field component orthogonal to the initial field direction is amplified
in both simulations to values that exceed those expected from the shock
compression by over an order of magnitude. The forming shock is
quasi-perpendicular due to this amplification. The simultaneous presence of
highly relativistic electrons and strong magnetic fields will give rise to
significant synchrotron emissions.Comment: 8 pages, 5 figures. This work was presented at 21st International
Conference on Numerical Simulation of Plasmas (ICNSP'09). Accepted for
publication IEEE Trans. on Plasma Scienc
Magnetic field amplification and electron acceleration to near-energy equipartition with ions by a mildly relativistic quasi-parallel plasma protoshock
The prompt emissions of gamma-ray bursts are seeded by radiating
ultrarelativistic electrons. Internal shocks propagating through a jet launched
by a stellar implosion, are expected to amplify the magnetic field & accelerate
electrons. We explore the effects of density asymmetry & a quasi-parallel
magnetic field on the collision of plasma clouds. A 2D relativistic PIC
simulation models the collision of two plasma clouds, in the presence of a
quasi-parallel magnetic field. The cloud density ratio is 10. The densities of
ions & electrons & the temperature of 131 keV are equal in each cloud. The mass
ratio is 250. The peak Lorentz factor of the electrons is determined, along
with the orientation & strength of the magnetic field at the cloud collision
boundary. The magnetic field component orthogonal to the initial plasma flow
direction is amplified to values that exceed those expected from shock
compression by over an order of magnitude. The forming shock is
quasi-perpendicular due to this amplification, caused by a current sheet which
develops in response to the differing deflection of the incoming upstream
electrons & ions. The electron deflection implies a charge separation of the
upstream electrons & ions; the resulting electric field drags the electrons
through the magnetic field, whereupon they acquire a relativistic mass
comparable to the ions. We demonstrate how a magnetic field structure
resembling the cross section of a flux tube grows in the current sheet of the
shock transition layer. Plasma filamentation develops, as well as signatures of
orthogonal magnetic field striping. Localized magnetic bubbles form. Energy
equipartition between the ion, electron & magnetic energy is obtained at the
shock transition layer. The electronic radiation can provide a seed photon
population that can be energized by secondary processes (e.g. inverse Compton).Comment: 12 pages, 15 Figures, accepted to A&
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
Theory and modelling of fast electron transport in laser-plasma interactions
The interaction of a high-intensity laser beam with a solid target generates a large number
of fast electrons with long mean free paths. The study of these fast electrons is still
the subject of active research, given their relevance to Tabak's [2] proposed fast-ignition
approach to inertial confinement fusion. Conventional methods for simulating this system
fall into two categories: kinetic and hybrid codes. Kinetic codes (Vlasov Fokker-Planck
(VFP) and Particle in Cell (PIC) codes) provide an almost complete description of the
system, but are often computationally expensive. Conventional hybrid codes simulate
the fast-electrons well using a PIC code, but simplify the simulation of the background
by using a rudimentary fluid model.
In this thesis I present a new approach to modelling relativistic electrons propagating
through a background plasma. This novel approach includes an improved classical transport
description of the background plasma by using the VFP code IMPACT [21]. The
fast electrons are modelled in two ways. Firstly, a 1D crude rigid beam model is used
for the fast electrons. This gives rise to interesting transport effects in the background,
such as transverse heat flow and non-local transport. It is found that the transverse heat flow is sufficient to reverse the `beam hollowing' effect of Davies et al [74] , allowing the
reemergence of a fast electron collimating magnetic field over picosecond timescales. The
second approach is to couple a PIC code into IMPACT to model the dynamic evolution of
the fast electron beam. The scheme is tested against relevant beam-plasma phenomena.
The code is used to model fast electron transport in 2D through a near-solid density
background plasma. The significant result from this 2D investigation is the suppression
of the filamentation instability by the resistively collimating field that surrounds the main
beam
Cosmic Plasmas and Electromagnetic Phenomena
During the past few decades, plasma science has witnessed a great growth in laboratory studies, in simulations, and in space. Plasma is the most common phase of ordinary matter in the universe. It is a state in which ionized matter (even as low as 1%) becomes highly electrically conductive. As such, long-range electric and magnetic fields dominate its behavior. Cosmic plasmas are mostly associated with stars, supernovae, pulsars and neutron stars, quasars and active galaxies at the vicinities of black holes (i.e., their jets and accretion disks). Cosmic plasma phenomena can be studied with different methods, such as laboratory experiments, astrophysical observations, and theoretical/computational approaches (i.e., MHD, particle-in-cell simulations, etc.). They exhibit a multitude of complex magnetohydrodynamic behaviors, acceleration, radiation, turbulence, and various instability phenomena. This Special Issue addresses the growing need of the plasma science principles in astrophysics and presents our current understanding of the physics of astrophysical plasmas, their electromagnetic behaviors and properties (e.g., shocks, waves, turbulence, instabilities, collimation, acceleration and radiation), both microscopically and macroscopically. This Special Issue provides a series of state-of-the-art reviews from international experts in the field of cosmic plasmas and electromagnetic phenomena using theoretical approaches, astrophysical observations, laboratory experiments, and state-of-the-art simulation studies
Magnetohydrodynamic oscillations in the solar corona and Earth's magnetosphere : towards consolidated understanding
Magnetohydrodynamic (MHD) oscillatory processes in different plasma systems, such as the corona of the Sun and the Earth’s magnetosphere, show interesting similarities and differences, which so far received little attention and remain underexploited. The successful commissioning within the past ten years of THEMIS, Hinode, STEREO and SDO spacecraft, in combination with matured analysis of data from earlier spacecraft (Wind, SOHO, ACE, Cluster, TRACE and RHESSI) makes it very timely to survey the breadth of observations giving evidence for MHD oscillatory processes in solar and space plasmas, and state-of-the-art theoretical modelling. The paper reviews several important topics, such as Alfv´enic resonances and mode conversion; MHD waveguides, such as the magnetotail, coronal loops, coronal streamers; mechanisms for periodicities produced in energy releases during substorms and solar flares, possibility of Alfv´enic resonators along open field lines; possible drivers of MHD waves; diagnostics of plasmas with MHD waves; interaction of MHD waves with partlyionised boundaries (ionosphere and chromosphere). The review is mainly oriented to specialists in magnetospheric physics and solar physics, but not familiar with specifics of the adjacent research fields
Magnetohydrodynamic Oscillations in the Solar Corona and Earth's Magnetosphere: Towards Consolidated Understanding
Magnetohydrodynamic (MHD) oscillatory processes in di�erent plasma systems, such as the corona of the Sun and the Earth's magnetosphere show interesting similarities and di�erences, which so far received little attention and remain underexploited. The successful commissioning within the past ten years of SDO, Hinode, STEREO and THEMIS spacecraft, in combination with matured analysis of data from earlier spacecraft (Wind, SOHO, ACE, Cluster, TRACE and RHESSI) makes it very timely to survey the breadth of observations giving evidence for MHD oscillatory processes in solar and space plasmas, and state-of-the-art theoretical modelling. The paper reviews several important topics, such as Alfv�enic resonances and mode conversion; MHD waveguides, such as the magnetotail, coronal loops, coronal streamers; mechanisms
for periodicities produced in energy releases during substorms and solar flares, possibility of Alfv�enic resonators along open �eld lines; possible drivers of MHD waves; diagnostics of plasmas with MHD waves; interaction of MHD waves with partly-ionised boundaries (ionosphere and chromosphere). The review is mainly oriented to specialists
in magnetospheric physics and solar physics, but not familiar with speci�cs of the adjacent research �elds