132 research outputs found

### Simulations of Ion Acceleration at Non-relativistic Shocks. I. Acceleration Efficiency

We use 2D and 3D hybrid (kinetic ions - fluid electrons) simulations to
investigate particle acceleration and magnetic field amplification at
non-relativistic astrophysical shocks. We show that diffusive shock
acceleration operates for quasi-parallel configurations (i.e., when the
background magnetic field is almost aligned with the shock normal) and, for
large sonic and Alfv\'enic Mach numbers, produces universal power-law spectra
proportional to p^(-4), where p is the particle momentum. The maximum energy of
accelerated ions increases with time, and it is only limited by finite box size
and run time. Acceleration is mainly efficient for parallel and quasi-parallel
strong shocks, where 10-20% of the bulk kinetic energy can be converted to
energetic particles, and becomes ineffective for quasi-perpendicular shocks.
Also, the generation of magnetic turbulence correlates with efficient ion
acceleration, and vanishes for quasi-perpendicular configurations. At very
oblique shocks, ions can be accelerated via shock drift acceleration, but they
only gain a factor of a few in momentum, and their maximum energy does not
increase with time. These findings are consistent with the degree of
polarization and the morphology of the radio and X-ray synchrotron emission
observed, for instance, in the remnant of SN 1006. We also discuss the
transition from thermal to non-thermal particles in the ion spectrum
(supra-thermal region), and we identify two dynamical signatures peculiar of
efficient particle acceleration, namely the formation of an upstream precursor
and the alteration of standard shock jump conditions.Comment: 21 pages, 14 figures, Minor changes reflecting the version accepted
to Ap

### Ab-initio pulsar magnetosphere: three-dimensional particle-in-cell simulations of axisymmetric pulsars

We perform first-principles relativistic particle-in-cell simulations of
aligned pulsar magnetosphere. We allow free escape of particles from the
surface of a neutron star and continuously populate the magnetosphere with
neutral pair plasma to imitate pair production. As pair plasma supply
increases, we observe the transition from a charge-separated electrosphere
solution with trapped plasma and no spin-down to a solution close to the ideal
force-free magnetosphere with electromagnetically-dominated pulsar wind. We
calculate the magnetospheric structure, current distribution and spin-down
power of the neutron star. We also discuss particle acceleration in the
equatorial current sheet.Comment: 6 pages, 5 figures, published in ApJ Letter

### Laser Shaping and Optimization of the Laser-Plasma Interaction

The physics of energy transfer between the laser and the plasma in laser
wakefield accelerators is studied. We find that wake excitation by arbitrary
laser shapes can be parameterized using the total pulse energy and pulse
depletion length. A technique for determining laser profiles that produce the
required plasma excitation is developed. We show that by properly shaping the
longitudinal profile of the driving laser pulse, it is possible to maximize
both the transformer ratio and the wake amplitude, achieving optimal
laser-plasma coupling. The corresponding family of laser pulse shapes is
derived in the nonlinear regime of laser-plasma interaction. Such shapes
provide theoretical upper limit on the magnitude of the wakefield and
efficiency of the accelerating stage by allowing for uniform photon
deceleration inside the laser pulse. We also construct realistic optimal pulse
shapes that can be produced in finite-bandwidth laser systems and propose a
two-pulse wake amplification scheme using the optimal solution.Comment: 12 pages, 5 figures, contributed to the Advanced Accelerator Concepts
2000 worksho

### Relativistic Reconnection: an Efficient Source of Non-Thermal Particles

In magnetized astrophysical outflows, the dissipation of field energy into
particle energy via magnetic reconnection is often invoked to explain the
observed non-thermal signatures. By means of two- and three-dimensional
particle-in-cell simulations, we investigate anti-parallel reconnection in
magnetically-dominated electron-positron plasmas. Our simulations extend to
unprecedentedly long temporal and spatial scales, so we can capture the
asymptotic state of the system beyond the initial transients, and without any
artificial limitation by the boundary conditions. At late times, the
reconnection layer is organized into a chain of large magnetic islands
connected by thin X-lines. The plasmoid instability further fragments each
X-line into a series of smaller islands, separated by X-points. At the
X-points, the particles become unmagnetized and they get accelerated along the
reconnection electric field. We provide definitive evidence that the late-time
particle spectrum integrated over the whole reconnection region is a power-law,
whose slope is harder than -2 for magnetizations sigma>10. Efficient particle
acceleration to non-thermal energies is a generic by-product of the long-term
evolution of relativistic reconnection in both two and three dimensions. In
three dimensions, the drift-kink mode corrugates the reconnection layer at
early times, but the long-term evolution is controlled by the plasmoid
instability, that facilitates efficient particle acceleration, in analogy to
the two-dimensional physics. Our findings have important implications for the
generation of hard photon spectra in pulsar winds and relativistic
astrophysical jets.Comment: 6 pages, 5 figures, ApJL accepted, movies available at
https://www.cfa.harvard.edu/~lsironi/Site/sigma10.no.guide.field

### Simulations of Ion Acceleration at Non-relativistic Shocks. III. Particle Diffusion

We use large hybrid (kinetic protons-fluid electrons) simulations to
investigate the transport of energetic particles in self-consistent
electromagnetic configurations of collisionless shocks. In previous papers of
this series, we showed that ion acceleration may be very efficient (up to
$10-20\%$ in energy), and outlined how the streaming of energetic particles
amplifies the upstream magnetic field. Here, we measure particle diffusion
around shocks with different strengths, finding that the mean free path for
pitch-angle scattering of energetic ions is comparable with their gyroradii
calculated in the self-generated turbulence. For moderately-strong shocks,
magnetic field amplification proceeds in the quasi-linear regime, and particles
diffuse according to the self-generated diffusion coefficient, i.e., the
scattering rate depends only on the amount of energy in modes with wavelengths
comparable with the particle gyroradius. For very strong shocks, instead, the
magnetic field is amplified up to non-linear levels, with most of the energy in
modes with wavelengths comparable to the gyroradii of highest-energy ions, and
energetic particles experience Bohm-like diffusion in the amplified field. We
also show how enhanced diffusion facilitates the return of energetic particles
to the shock, thereby determining the maximum energy that can be achieved in a
given time via diffusive shock acceleration. The parametrization of the
diffusion coefficient that we derive can be used to introduce self-consistent
microphysics into large-scale models of cosmic ray acceleration in
astrophysical sources, such as supernova remnants and clusters of galaxies.Comment: 8 pages, 7 figures, Minor changes reflecting the version accepted to
Ap

### Simulations of relativistic collisionless shocks: shock structure and particle acceleration

We discuss 3D simulations of relativistic collisionless shocks in
electron-positron pair plasmas using the particle-in-cell (PIC) method. The
shock structure is mainly controlled by the shock's magnetization ("sigma"
parameter). We demonstrate how the structure of the shock varies as a function
of sigma for perpendicular shocks. At low magnetizations the shock is mediated
mainly by the Weibel instability which generates transient magnetic fields that
can exceed the initial field. At larger magnetizations the shock is dominated
by magnetic reflections. We demonstrate where the transition occurs and argue
that it is impossible to have very low magnetization collisionless shocks in
nature (in more than one spatial dimension). We further discuss the
acceleration properties of these shocks, and show that higher magnetization
perpendicular shocks do not efficiently accelerate nonthermal particles in 3D.
Among other astrophysical applications, this may pose a restriction on the
structure and composition of gamma-ray bursts and pulsar wind outflows.Comment: 6 pages, invited talk at "Astrophysical Sources of High Energy
Particles and Radiation," Torun, June 20 - 24, 200

- âŠ