3,594 research outputs found
Vlasov simulations of Kinetic Alfv\'en Waves at proton kinetic scales
Kinetic Alfv\'en waves represent an important subject in space plasma
physics, since they are thought to play a crucial role in the development of
the turbulent energy cascade in the solar wind plasma at short wavelengths (of
the order of the proton inertial length and beyond). A full understanding
of the physical mechanisms which govern the kinetic plasma dynamics at these
scales can provide important clues on the problem of the turbulent dissipation
and heating in collisionless systems. In this paper, hybrid Vlasov-Maxwell
simulations are employed to analyze in detail the features of the kinetic
Alfv\'en waves at proton kinetic scales, in typical conditions of the solar
wind environment. In particular, linear and nonlinear regimes of propagation of
these fluctuations have been investigated in a single-wave situation, focusing
on the physical processes of collisionless Landau damping and wave-particle
resonant interaction. Interestingly, since for wavelengths close to and
proton plasma beta of order unity the kinetic Alfv\'en waves have small
phase speed compared to the proton thermal velocity, wave-particle interaction
processes produce significant deformations in the core of the particle velocity
distribution, appearing as phase space vortices and resulting in flat-top
velocity profiles. Moreover, as the Eulerian hybrid Vlasov-Maxwell algorithm
allows for a clean almost noise-free description of the velocity space,
three-dimensional plots of the proton velocity distribution help to emphasize
how the plasma departs from the Maxwellian configuration of thermodynamic
equilibrium due to nonlinear kinetic effects
Design and optimization of a laser-PIXE beamline for material science applications
Multi-MeV proton beams can be generated by irradiating thin solid foils with ultra-intense (>10^18 W/cm^2) short laser pulses. Several of their characteristics, such as high bunch charge and short pulse duration, make them a complementary alternative to conventional radio frequency-based accelerators. A potential material science application is the chemical analysis of cultural heritage (CH) artifacts. The complete chemistry of the bulk material (ceramics, metals) can be retrieved through sophisticated nuclear techniques such as particle-induced X-ray emission (PIXE). Recently, the use of laser-generated proton beams was introduced as diagnostics in material science (laser-PIXE or laser-driven PIXE): Coupling laser-generated proton sources to conventional beam steering devices successfully enhances the capture and transport of the laser-accelerated beam. This leads to a reduction of the high divergence and broad energy spread at the source. The design of our hybrid beamline is composed of an energy selector, followed by permanent quadrupole magnets aiming for better control and manipulation of the final proton beam parameters. This allows tailoring both, mean proton energy and spot sizes, yet keeping the system compact. We performed a theoretical study optimizing a beamline for laser-PIXE applications. Our design enables monochromatizing the beam and shaping its final spot size. We obtain spot sizes ranging between a fraction of mm up to cm scale at a fraction of nC proton charge per shot. These results pave the way for a versatile and tunable laserPIXE at a multi-Hz repetition rate using modern commercially available laser systems
Theoretical models for MHD turbulence in the solar wind
The in situ measurements of velocity, magnetic field, density and temperature fluctuations performed in the solar wind have greatly improved our knowledge of MHD turbulence not only from the point of view of space physics but also
from the more general point of view of plasma physics. These fluctuations which extend over a wide range of frequencies (about 5 decades), a fact which seems to be the
signature of turbulent nonlinear energy cascade, display, mainly in the trailing edge of high-speed streams, a number of features characteristic of a self-organized situation:
i) a high degree of correlation between magnetic and velocity field fluctuations, ii) a very low level of fluctuations in mass density and magnetic-field intensity, iii) a considerable anisotropy revealed by minimum variance analysis of the magnetic-field correlation tensor. Many fundamental processes in plasma physics, which were largely
unknown or not understood before their observations in the solar wind, have been explained, by building up analytical models or performing numerical simulations. We discuss the most recent analytical theories and numerical simulations and outline the limits implicit in any analysis which consider the low-frequency solar-wind fluctuations as a superposition of linear modes. The characterization of
low-frequency fluctuations during Alfv´enic periods, which results from the models discussed, is finally presented
Plasma transport in the interplanetary space: Percolation and anomalous diffusion of magnetic-field lines
The magnetic fluctuations due to, e.g., magnetohydrodynamic turbulence cause a magnetic-field line random walk that influences many cosmic plasma phenomena. The results of a three-dimensional numerical simulation of a turbulent magnetic field in plane geometry are presented here. Magnetic percolation, L´evy flights, and non-Gaussian random walk of the magnetic-field lines are found for moderate perturbation levels. In such a case plasma transport can be anomalous, i.e., either superdiffusive
or subdiffusive. Increasing the perturbation level a Gaussian diffusion regime is attained. The implications on the structure of the electron foreshock and of planetary
magnetopauses are discussed
Laser-Generated Proton Beams for High-Precision Ultra-Fast Crystal Synthesis
We present a method for the synthesis of micro-crystals and micro-structured surfaces using laseraccelerated
protons. In this method, a solid surface material having a low melting temperature is
irradiated with very-short laser-generated protons, provoking in the ablation process thermodynamic
conditions that are between the boiling and the critical point. The intense and very quick proton energy
deposition (in the ns range) induces an explosive boiling and produces microcrystals that nucleate in a
plasma plume composed by ions and atoms detached from the laser-irradiated surface. The synthesized
particles in the plasma plume are then deposited onto a cold neighboring, non-irradiated, solid
secondary surface. We experimentally verify the synthesizing methods by depositing low-meltingmaterial
microcrystals - such as gold - onto nearby silver surfaces and modeling the proton/matter
interaction via a Monte Carlo code, confrming that we are in the above described thermodynamic
conditions. Morphological and crystallinity measurements indicate the formation of gold octahedral
crystals with dimensions around 1.2 μm, uniformly distributed onto a silver surface with dimensions
in the tens of mm2. This laser-accelerated particle based synthesis method paves the way for the
development of new material synthesis using ultrashort laser-accelerated particle beams
A Monte Carlo simulation of magnetic field line tracing in the solar wind
International audienceIt is well known that the structure of magnetic field lines in solar wind can be influenced by the presence of the magnetohydrodynamic turbulence. We have developed a Monte Carlo simulation which traces the magnetic field lines in the heliosphere, including the effects of magnetic turbulence. These effects are modelled by random operators which are proportional to the square root of the magnetic field line diffusion coefficient. The modelling of the random terms is explained, in detail, in the case of numerical integration by discrete steps. Furthermore, a proper evaluation of the diffusion coefficient is obtained by a numerical simulation of transport in anisotropic magnetic turbulence. The scaling of the fluctuation level and of the correlation lengths with the distance from the Sun are also taken into account. As a consequence, plasma transport across the average magnetic field direction is obtained. An application to the propagation of energetic particles from corotating interacting regions to high heliographic latitudes is considered
Response to Comment on `Undamped electrostatic plasma waves' [Phys. Plasmas 19, 092103 (2012)]
Numerical and experimental evidence is given for the occurrence of the
plateau states and concomitant corner modes proposed in \cite{valentini12}. It
is argued that these states provide a better description of reality for small
amplitude off-dispersion disturbances than the conventional
Bernstein-Greene-Kruskal or cnoidal states such as those proposed in
\cite{comment
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