21,012 research outputs found
Physics of collisionless shocks - theory and simulation
Collisionless shocks occur in various fields of physics. In the context of
space and astrophysics they have been investigated for many decades. However, a
thorough understanding of shock formation and particle acceleration is still
missing. Collisionless shocks can be distinguished into electromagnetic and
electrostatic shocks. Electromagnetic shocks are of importance mainly in
astrophysical environments and they are mediated by the Weibel or filamentation
instability. In such shocks, charged particles gain energy by diffusive shock
acceleration. Electrostatic shocks are characterized by a strong electrostatic
field, which leads to electron trapping. Ions are accelerated by reflection
from the electrostatic potential. Shock formation and particle acceleration
will be discussed in theory and simulations
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
The impact of kinetic effects on the properties of relativistic electron-positron shocks
We assess the impact of non-thermally shock-accelerated particles on the
magnetohydrodynamic (MHD) jump conditions of relativistic shocks. The adiabatic
constant is calculated directly from first principle particle-in-cell
simulation data, enabling a semi-kinetic approach to improve the standard fluid
model and allowing for an identification of the key parameters that define the
shock structure. We find that the evolving upstream parameters have a stronger
impact than the corrections due to non-thermal particles. We find that the
decrease of the upstream bulk speed yields deviations from the standard MHD
model up to 10%. Furthermore, we obtain a quantitative definition of the shock
transition region from our analysis. For Weibel-mediated shocks the inclusion
of a magnetic field in the MHD conservation equations is addressed for the
first time
Exploring the nature of collisionless shocks under laboratory conditions
Collisionless shocks are pervasive in astrophysics and they are critical to
understand cosmic ray acceleration. Laboratory experiments with intense lasers
are now opening the way to explore and characterise the underlying
microphysics, which determine the acceleration process of collisionless shocks.
We determine the shock character - electrostatic or electromagnetic - based on
the stability of electrostatic shocks to transverse electromagnetic
fluctuations as a function of the electron temperature and flow velocity of the
plasma components, and we compare the analytical model with particle-in-cell
simulations. By making the connection with the laser parameters driving the
plasma flows, we demonstrate that shocks with different and distinct underlying
microphysics can be explored in the laboratory with state-of-the-art laser
systems
Cosmic Sculpture: A new way to visualise the Cosmic Microwave Background
3D printing presents an attractive alternative to visual representation of
physical datasets such as astronomical images that can be used for research,
outreach or teaching purposes, and is especially relevant to people with a
visual disability. We here report the use of 3D printing technology to produce
a representation of the all-sky Cosmic Microwave Background (CMB) intensity
anisotropy maps produced by the Planck mission. The success of this work in
representing key features of the CMB is discussed as is the potential of this
approach for representing other astrophysical data sets. 3D printing such
datasets represents a highly complementary approach to the usual 2D projections
used in teaching and outreach work, and can also form the basis of
undergraduate projects. The CAD files used to produce the models discussed in
this paper are made available.Comment: Accepted for publication in the European Journal of Physic
Four-dimensional gravity on supersymmetric dilatonic domain walls
We investigate the localization of four-dimensional metastable gravity in
supersymmetric dilatonic domain walls through massive modes by considering
several scenarios in the model. We compute corrections to the Newtonian
potential for small and long distances compared with a crossover scale given in
terms of the dilatonic coupling. 4D gravity behavior is developed on the brane
for distance very much below the crossover scale, while for distance much
larger, the 5D gravity is recovered. Whereas in the former regime gravity is
always attractive, in the latter regime due to non-normalizable unstable
massive graviton modes present on the spectrum, in some special cases, gravity
appears to be repulsive and signalizes a gravitational confining phase which is
able to produce an inflationary phase of the Universe.Comment: 11 pages, 4 figures, Latex. Version to appear in PL
Benchmark calculation of p-3H and n-3He scattering
p-3H and n-3He scattering in the energy range above the n-3He but below the
d-d thresholds is studied by solving the 4-nucleon problem with a realistic
nucleon-nucleon interaction. Three different methods -- Alt, Grassberger and
Sandhas, Hyperspherical Harmonics, and Faddeev-Yakubovsky -- have been employed
and their results for both elastic and charge-exchange processes are compared.
We observe a good agreement between the three different methods, thus the
obtained results may serve as a benchmark. A comparison with the available
experimental data is also reported and discussed.Comment: 13 pages, 6 figures. arXiv admin note: text overlap with
arXiv:1109.362
Three-body Faddeev-Alt-Grassberger-Sandhas approach to direct nuclear reactions
Momentum space three-body Faddeev-like equations are used to calculate
elastic, transfer and charge exchange reactions resulting from the scattering
of deuterons on 12C and 16O or protons on 13C and 17O; 12C and 16O are treated
as inert cores. All possible reactions are calculated in the framework of the
same model space. Comparison with previous calculations based on approximate
methods used in nuclear reaction theory is discussed.Comment: 10 pages, 13 figures, to be published in Phys. Rev.
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