69 research outputs found
A fast current-driven instability in relativistic collisionless shocks
We report here on a fast current-driven instability at relativistic
collisionless shocks, triggered by the perpendicular current carried by the
supra-thermal particles as they gyrate around the background magnetic field in
the shock precursor. We show that this instability grows faster than any other
instability studied so far in this context, and we argue that it is likely to
shape the physics of the shock and of particle acceleration in a broad
parameter range.Comment: 6 pages, 5 figures -- version to appear in EP
Origins of plateau formation in ion energy spectra under target normal sheath acceleration
Target normal sheath acceleration (TNSA) is a method employed in
laser--matter interaction experiments to accelerate light ions (usually
protons). Laser setups with durations of a few 10 fs and relatively low
intensity contrasts observe plateau regions in their ion energy spectra when
shooting on thin foil targets with thicknesses of order 10 m. In
this paper we identify a mechanism which explains this phenomenon using one
dimensional particle-in-cell simulations. Fast electrons generated from the
laser interaction recirculate back and forth through the target, giving rise to
time-oscillating charge and current densities at the target backside. Periodic
decreases in the electron density lead to transient disruptions of the TNSA
sheath field: peaks in the ion spectra form as a result, which are then spread
in energy from a modified potential driven by further electron recirculation.
The ratio between the laser pulse duration and the recirculation period
(dependent on the target thickness, including the portion of the pre-plasma
which is denser than the critical density) determines if a plateau forms in the
energy spectra.Comment: 11 pages, 12 figure
Particle acceleration at magnetized, relativistic turbulent shock fronts
The efficiency of particle acceleration at shock waves in relativistic,
magnetized astrophysical outflows is a debated topic with far-reaching
implications. Here, for the first time, we study the impact of turbulence in
the pre-shock plasma. Our simulations demonstrate that, for a mildly
relativistic, magnetized pair shock (Lorentz factor , magnetization level ), strong turbulence can revive
particle acceleration in a superluminal configuration that otherwise prohibits
it. Depending on the initial plasma temperature and magnetization,
stochastic-shock-drift or diffusive-type acceleration governs particle
energization, producing powerlaw spectra with . At larger magnetization levels, stochastic
acceleration within the pre-shock turbulence becomes competitive and can even
take over shock acceleration
Origin of intense electron heating in relativistic blast waves
The modeling of gamma-ray burst afterglow emission bears witness to strong
electron heating in the precursor of Weibel-mediated, relativistic
collisionless shock waves propagating in unmagnetized electron-ion plasmas. In
this Letter, we propose a theoretical model, which describes electron heating
via a Joule-like process caused by pitch-angle scattering in the decelerating,
self-induced microturbulence and the coherent charge-separation field induced
by the difference in inertia between electrons and ions. The emergence of this
electric field across the precursor of electron-ion shocks is confirmed by
large-scale particle-in-cell (PIC) simulations. Integrating the model using a
Monte Carlo-Poisson method, we compare the main observables to the PIC
simulations to conclude that the above mechanism can indeed account for the
bulk of electron heating.Comment: 9 pages, 8 figures; to be published in Astrophysical Journal Letter
The various manifestations of collisionless dissipation in wave propagation
The propagation of an electrostatic wave packet inside a collisionless and
initially Maxwellian plasma is always dissipative because of the irreversible
acceleration of the electrons by the wave. Then, in the linear regime, the wave
packet is Landau damped, so that in the reference frame moving at the group
velocity, the wave amplitude decays exponentially with time. In the nonlinear
regime, once phase mixing has occurred and when the electron motion is nearly
adiabatic, the damping rate is strongly reduced compared to the Landau one, so
that the wave amplitude remains nearly constant along the characteristics. Yet,
we show here that the electrons are still globally accelerated by the wave
packet, and, in one dimension, this leads to a non local amplitude dependence
of the group velocity. As a result, a freely propagating wave packet would
shrink, and, therefore, so would its total energy. In more than one dimension,
not only does the magnitude of the group velocity nonlinearly vary, but also
its direction. In the weakly nonlinear regime, when the collisionless damping
rate is still significant compared to its linear value, this leads to an
effective defocussing effect which we quantify, and which we compare to the
self-focussing induced by wave front bowing.Comment: 23 pages, 6 figure
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