30,028 research outputs found
Vlasov simulation of laser-driven shock acceleration and ion turbulence
We present a Vlasov, i.e. a kinetic Eulerian simulation study of nonlinear
collisionless ion-acoustic shocks and solitons excited by an intense laser
interacting with an overdense plasma. The use of the Vlasov code avoids
problems with low particle statistics and allows a validation of
particle-in-cell results. A simple original correction to the splitting method
for the numerical integration of the Vlasov equation has been implemented in
order to ensure the charge conservation in the relativistic regime. We show
that the ion distribution is affected by the development of a turbulence driven
by the relativistic "fast" electron bunches generated at the laser-plasma
interaction surface. This leads to the onset of ion reflection at the shock
front in an initially cold plasma where only soliton solutions without ion
reflection are expected to propagate. We give a simple analytic model to
describe the onset of the turbulence as a nonlinear coupling of the ion density
with the fast electron currents, taking the pulsed nature of the relativistic
electron bunches into account
Lessons from Non-Abelian Plasma Instabilities in Two Spatial Dimensions
Plasma instabilities can play a fundamental role in quark-gluon plasma
equilibration in the high energy (weak coupling) limit. Early simulations of
the evolution of plasma instabilities in non-abelian gauge theory, performed in
one spatial dimension, found behavior qualitatively similar to traditional QED
plasmas. Later simulations of the fully three-dimensional theory found
different behavior, unlike traditional QED plasmas. To shed light on the origin
of this difference, we study the intermediate case of two spatial dimensions.
Depending on how the "two-dimensional'' theory is formulated, we can obtain
either behavior.Comment: 15 pages, 10 figure
Studies of dynamic processes related to active experiments in space plasmas
This is the final report for grant NAGw-2055, 'Studies of Dynamic Processes Related to Active Experiments in Space Plasmas', covering research performed at the University of Michigan. The grant was awarded to study: (1) theoretical and data analysis of data from the CHARGE-2 rocket experiment (1keV; 1-46 mA electron beam ejections) and the Spacelab-2 shuttle experiment (1keV; 100 mA); (2) studies of the interaction of an electron beam, emitted from an ionospheric platform, with the ambient neutral atmosphere and plasma by means of a newly developed computer simulation model, relating model predictions with CHARGE-2 observations of return currents observed during electron beam emissions; and (3) development of a self-consistent model for the charge distribution on a moving conducting tether in a magnetized plasma and for the potential structure in the plasma surrounding the tether. Our main results include: (1) the computer code developed for the interaction of electrons beams with the neutral atmosphere and plasma is able to model observed return fluxes to the CHARGE-2 sounding rocket payload; and (2) a 3-D electromagnetic and relativistic particle simulation code was developed
Anomalous diffusion, clustering, and pinch of impurities in plasma edge turbulence
The turbulent transport of impurity particles in plasma edge turbulence is
investigated. The impurities are modeled as a passive fluid advected by the
electric and polarization drifts, while the ambient plasma turbulence is
modeled using the two-dimensional Hasegawa--Wakatani paradigm for resistive
drift-wave turbulence. The features of the turbulent transport of impurities
are investigated by numerical simulations using a novel code that applies
semi-Lagrangian pseudospectral schemes. The diffusive character of the
turbulent transport of ideal impurities is demonstrated by relative-diffusion
analysis of the evolution of impurity puffs. Additional effects appear for
inertial impurities as a consequence of compressibility. First, the density of
inertial impurities is found to correlate with the vorticity of the electric
drift velocity, that is, impurities cluster in vortices of a precise
orientation determined by the charge of the impurity particles. Second, a
radial pinch scaling linearly with the mass--charge ratio of the impurities is
discovered. Theoretical explanation for these observations is obtained by
analysis of the model equations.Comment: This article has been submitted to Physics of Plasmas. After it is
published, it will be found at http://pop.aip.org/pop
Magnetohydrodynamics of the Weakly Ionized Solar Photosphere
We investigate the importance of ambipolar diffusion and Hall currents for
high-resolution comprehensive ('realistic') photospheric simulations. To do so
we extended the radiative magnetohydrodynamics code \emph{MURaM} to use the
generalized Ohm's law under the assumption of local thermodynamic equilibrium.
We present test cases comparing analytical solutions with numerical simulations
for validation of the code. Furthermore, we carried out a number of numerical
experiments to investigate the impact of these neutral-ion effects in the
photosphere. We find that, at the spatial resolutions currently used (5-20 km
per grid point), the Hall currents and ambipolar diffusion begin to become
significant -- with flows of 100 m/s in sunspot light bridges, and changes of a
few percent in the thermodynamic structure of quiet-Sun magnetic features. The
magnitude of the effects is expected to increase rapidly as smaller-scale
variations are resolved by the simulations.Comment: accepted Ap
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