7,187 research outputs found
Electromagnetic field generation in the downstream of electrostatic shocks due to electron trapping
A new magnetic field generation mechanism in electrostatic shocks is found,
which can produce fields with magnetic energy density as high as 0.01 of the
kinetic energy density of the flows on time scales . Electron trapping during the shock formation process
creates a strong temperature anisotropy in the distribution function, giving
rise to the pure Weibel instability. The generated magnetic field is
well-confined to the downstream region of the electrostatic shock. The shock
formation process is not modified and the features of the shock front
responsible for ion acceleration, which are currently probed in laser-plasma
laboratory experiments, are maintained. However, such a strong magnetic field
determines the particle trajectories downstream and has the potential to modify
the signatures of the collisionless shock
DC magnetic field generation in unmagnetized shear flows
The generation of DC magnetic fields in unmagnetized plasmas with velocity
shear is predicted for non relativistic and relativistic scenarios either due
to thermal effects or due to the onset of the Kelvin-Helmholtz instability
(KHI). A kinetic model describes the growth and the saturation of the DC field.
The predictions of the theory are confirmed by multidimensional
particle-in-cell simulations, demonstrating the formation of long lived
magnetic fields () along the full longitudinal
extent of the shear layer, with transverse width on the electron length scale
(), reaching magnitudes
Electron-scale shear instabilities: magnetic field generation and particle acceleration in astrophysical jets
Strong shear flow regions found in astrophysical jets are shown to be
important dissipation regions, where the shear flow kinetic energy is converted
into electric and magnetic field energy via shear instabilities. The emergence
of these self-consistent fields make shear flows significant sites for
radiation emission and particle acceleration. We focus on electron-scale
instabilities, namely the collisionless, unmagnetized Kelvin-Helmholtz
instability (KHI) and a large-scale dc magnetic field generation mechanism on
the electron scales. We show that these processes are important candidates to
generate magnetic fields in the presence of strong velocity shears, which may
naturally originate in energetic matter outburst of active galactic nuclei and
gamma-ray bursters. We show that the KHI is robust to density jumps between
shearing flows, thus operating in various scenarios with different density
contrasts. Multidimensional particle-in-cell (PIC) simulations of the KHI,
performed with OSIRIS, reveal the emergence of a strong and large-scale dc
magnetic field component, which is not captured by the standard linear fluid
theory. This dc component arises from kinetic effects associated with the
thermal expansion of electrons of one flow into the other across the shear
layer, whilst ions remain unperturbed due to their inertia. The electron
expansion forms dc current sheets, which induce a dc magnetic field. Our
results indicate that most of the electromagnetic energy developed in the KHI
is stored in the dc component, reaching values of equipartition on the order of
in the electron time-scale, and persists longer than the proton
time-scale. Particle scattering/acceleration in the self generated fields of
these shear flow instabilities is also analyzed
Transverse electron-scale instability in relativistic shear flows
Electron-scale surface waves are shown to be unstable in the transverse plane
of a shear flow in an initially unmagnetized plasma, unlike in the
(magneto)hydrodynamics case. It is found that these unstable modes have a
higher growth rate than the closely related electron-scale Kelvin-Helmholtz
instability in relativistic shears. Multidimensional particle-in-cell
simulations verify the analytic results and further reveal the emergence of
mushroom-like electron density structures in the nonlinear phase of the
instability, similar to those observed in the Rayleigh Taylor instability
despite the great disparity in scales and different underlying physics.
Macroscopic () fields are shown to be generated by these
microscopic shear instabilities, which are relevant for particle acceleration,
radiation emission and to seed MHD processes at long time-scales
Spatial-temporal evolution of the current filamentation instability
The spatial-temporal evolution of the purely transverse current filamentation
instability is analyzed by deriving a single partial differential equation for
the instability and obtaining the analytical solutions for the spatially and
temporally growing current filament mode. When the beam front always encounters
fresh plasma, our analysis shows that the instability grows spatially from the
beam front to the back up to a certain critical beam length; then the
instability acquires a purely temporal growth. This critical beam length
increases linearly with time and in the non-relativistic regime it is
proportional to the beam velocity. In the relativistic regime the critical
length is inversely proportional to the cube of the beam Lorentz factor
. Thus, in the ultra-relativistic regime the instability
immediately acquires a purely temporal growth all over the beam. The analytical
results are in good agreement with multidimensional particle-in-cell
simulations performed with OSIRIS. Relevance of current study to recent and
future experiments on fireball beams is also addressed
Slow down of a globally neutral relativistic beam shearing the vacuum
The microphysics of relativistic collisionless sheared flows is investigated
in a configuration consisting of a globally neutral, relativistic beam
streaming through a hollow plasma/dielectric channel. We show through
multidimensional PIC simulations that this scenario excites the Mushroom
instability (MI), a transverse shear instability on the electron-scale, when
there is no overlap (no contact) between the beam and the walls of the
hollow plasma channel. The onset of the MI leads to the conversion of the
beam's kinetic energy into magnetic (and electric) field energy, effectively
slowing down a globally neutral body in the absence of contact. The
collisionless shear physics explored in this configuration may operate in
astrophysical environments, particularly in highly relativistic and supersonic
settings where macroscopic shear processes are stable
All-optical trapping and acceleration of heavy particles
A scheme for fast, compact, and controllable acceleration of heavy particles
in vacuum is proposed, in which two counterpropagating lasers with variable
frequencies drive a beat-wave structure with variable phase velocity, thus
allowing for trapping and acceleration of heavy particles, such as ions or
muons. Fine control over the energy distribution and the total charge of the
beam is obtained via tuning of the frequency variation. The acceleration scheme
is described with a one-dimensional theory, providing the general conditions
for trapping and scaling laws for the relevant features of the particle beam.
Two-dimensional, electromagnetic particle-in-cell simulations confirm the
validity and the robustness of the physical mechanism.Comment: 10 pages, 3 figures, to appear in New Journal of Physic
Anisotropic heating and magnetic field generation due to Raman scattering in laser-plasma interactions
We identify a mechanism for magnetic field generation in the interaction of intense electromagnetic waves and
underdense plasmas. We show that Raman scattered plasma waves trap and heat the electrons preferentially in
their propagation direction, resulting in a temperature anisotropy. In the trail of laser pulse, we observe magnetic
field growth that matches the Weibel mechanism due to the temperature anisotropy. We discuss the role of the
initial electron temperature in our results. The predictions are confirmed with multidimensional particle-in-cell
simulations. We show how this configuration is an experimental platform to study the long-time evolution of the
Weibel instabilityinfo:eu-repo/semantics/publishedVersio
Amplification and generation of ultra-intense twisted laser pulses via stimulated Raman scattering
Twisted Laguerre-Gaussian lasers, with orbital angular momentum and
characterised by doughnut shaped intensity profiles, provide a transformative
set of tools and research directions in a growing range of fields and
applications, from super-resolution microcopy and ultra-fast optical
communications to quantum computing and astrophysics. The impact of twisted
light is widening as recent numerical calculations provided solutions to
long-standing challenges in plasma-based acceleration by allowing for high
gradient positron acceleration. The production of ultrahigh intensity twisted
laser pulses could then also have a broad influence on relativistic
laser-matter interactions. Here we show theoretically and with ab-initio
three-dimensional particle-in-cell simulations, that stimulated Raman
backscattering can generate and amplify twisted lasers to Petawatt intensities
in plasmas. This work may open new research directions in non-linear optics and
high energy density science, compact plasma based accelerators and light
sources.Comment: 18 pages, 4 figures, 1 tabl
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