102 research outputs found
Quasi-perpendicular fast magnetosonic shock with wave precursor in collisionless plasma
A one-dimensional particle-in-cell (PIC) simulation tracks a fast
magnetosonic shock over time scales comparable to an inverse ion gyrofrequency.
The magnetic pressure is comparable to the thermal pressure upstream. The shock
propagates across a uniform background magnetic field with a pressure that
equals the thermal pressure upstream at the angle 85 at a speed that is
1.5 times the fast magnetosonic speed in the electromagnetic limit.
Electrostatic contributions to the wave dispersion increase its phase speed at
large wave numbers, which leads to a convex dispersion curve. A fast
magnetosonic precursor forms ahead of the shock with a phase speed that exceeds
the fast magnetosonic speed by about . The wave is slower than the
shock and hence it is damped.Comment: 4 pages, 3 figure
An Euler Solver Based on Locally Adaptive Discrete Velocities
A new discrete-velocity model is presented to solve the three-dimensional
Euler equations. The velocities in the model are of an adaptive nature---both
the origin of the discrete-velocity space and the magnitudes of the
discrete-velocities are dependent on the local flow--- and are used in a finite
volume context. The numerical implementation of the model follows the
near-equilibrium flow method of Nadiga and Pullin [1] and results in a scheme
which is second order in space (in the smooth regions and between first and
second order at discontinuities) and second order in time. (The
three-dimensional code is included.) For one choice of the scaling between the
magnitude of the discrete-velocities and the local internal energy of the flow,
the method reduces to a flux-splitting scheme based on characteristics. As a
preliminary exercise, the result of the Sod shock-tube simulation is compared
to the exact solution.Comment: 17 pages including 2 figures and CMFortran code listing. All in one
postscript file (adv.ps) compressed and uuencoded (adv.uu). Name mail file
`adv.uu'. Edit so that `#!/bin/csh -f' is the first line of adv.uu On a unix
machine say `csh adv.uu'. On a non-unix machine: uudecode adv.uu; uncompress
adv.tar.Z; tar -xvf adv.ta
Investigating particle acceleration dynamics in interpenetrating magnetized collisionless super-critical shocks
Colliding collisionless shocks appear in a great variety of astrophysical
phenomena and are thought to be possible sources of particle acceleration in
the Universe. We have previously investigated particle acceleration induced by
single super-critical shocks (whose magnetosonic Mach number is higher than the
critical value of 2.7) (Yao et al. 2021, 2022), as well as the collision of two
sub-critical shocks (Fazzini et al. 2022). Here, we propose to make
measurements of accelerated particles from interpenetrating super-critical
shocks to observe the ''phase-locking effect'' (Fazzini et al. 2022) from such
an event. This effect is predicted to significantly boost the energy spectrum
of the energized ions compared to a single supercritical collisionless shock.
We thus anticipate that the results obtained in the proposed experiment could
have a significant impact on our understanding of one type of primary source
(acceleration of thermal ions as opposed to secondary acceleration mechanisms
of already energetic ions) of ion energization of particles in the Universe
Computer simulations of domain growth and phase separation in two-dimensional binary immiscible fluids using dissipative particle dynamics
We investigate the dynamical behavior of binary fluid systems in two
dimensions using dissipative particle dynamics. We find that following a
symmetric quench the domain size R(t) grows with time t according to two
distinct algebraic laws R(t) = t^n: at early times n = 1/2, while for later
times n = 2/3. Following an asymmetric quench we observe only n = 1/2, and if
momentum conservation is violated we see n = 1/3 at early times. Bubble
simulations confirm the existence of a finite surface tension and the validity
of Laplace's law. Our results are compared with similar simulations which have
been performed previously using molecular dynamics, lattice-gas and
lattice-Boltzmann automata, and Langevin dynamics. We conclude that dissipative
particle dynamics is a promising method for simulating fluid properties in such
systems.Comment: RevTeX; 22 pages, 5 low-resolution figures. For full-resolution
figures, connect to http://www.tcm.phy.cam.ac.uk/~ken21/tension/tension.htm
Laser acceleration of high-energy protons in variable density plasmas
The acceleration of protons, induced by electrons generated by a short-pulse laser, is experimentally investigated when varying the density of the plasma target the laser is interacting with. The experimental results are compared with particle-in-cell (PIC) simulations for which the target conditions are inferred from hydrodynamic simulations. High-energy protons are observed only for the two extreme configurations, namely solid-density foils and near- critical-density plasmas having large gradients. Cold solid foils, however, yield the highest energy protons and best proton beam profiles. As suggested by simulations, near-critical-density plasmas could be optimized to further increase the proton energy
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