576 research outputs found
Vlasov simulation in multiple spatial dimensions
A long-standing challenge encountered in modeling plasma dynamics is
achieving practical Vlasov equation simulation in multiple spatial dimensions
over large length and time scales. While direct multi-dimension Vlasov
simulation methods using adaptive mesh methods [J. W. Banks et al., Physics of
Plasmas 18, no. 5 (2011): 052102; B. I. Cohen et al., November 10, 2010,
http://meetings.aps.org/link/BAPS.2010.DPP.NP9.142] have recently shown
promising results, in this paper we present an alternative, the Vlasov Multi
Dimensional (VMD) model, that is specifically designed to take advantage of
solution properties in regimes when plasma waves are confined to a narrow cone,
as may be the case for stimulated Raman scatter in large optic f# laser beams.
Perpendicular grid spacing large compared to a Debye length is then possible
without instability, enabling an order 10 decrease in required computational
resources compared to standard particle in cell (PIC) methods in 2D, with
another reduction of that order in 3D. Further advantage compared to PIC
methods accrues in regimes where particle noise is an issue. VMD and PIC
results in a 2D model of localized Langmuir waves are in qualitative agreement
Deformation of Silica Aerogel During Fluid Adsorption
Aerogels are very compliant materials - even small stresses can lead to large
deformations. In this paper we present measurements of the linear deformation
of high porosity aerogels during adsorption of low surface tension fluids,
performed using a Linear Variable Differential Transformer (LVDT). We show that
the degree of deformation of the aerogel during capillary condensation scales
with the surface tension, and extract the bulk modulus of the gel from the
data. Furthermore we suggest limits on safe temperatures for filling and
emptying low density aerogels with helium.Comment: 8 pages, 5 figures, submitted to PR
Influence of the Lower Hybrid Drift Instability on the onset of Magnetic Reconnection
Two-dimensional and three-dimensional kinetic simulation results reveal the
importance of the Lower-Hybrid Drift Instability LHDI to the onset of magnetic
reconnection. Both explicit and implicit kinetic simulations show that the LHDI
heats electrons anisotropically and increases the peak current density. Linear
theory predicts these modifications can increase the growth rate of the tearing
instability by almost two orders of magnitude and shift the fastest growing
modes to significantly shorter wavelengths. These predictions are confirmed by
nonlinear kinetic simulations in which the growth and coalescence of small
scale magnetic islands leads to a rapid onset of large scale reconnection
Vortex core size in interacting cylindrical nanodot arrays
The effect of dipolar interactions among cylindrical nanodots, with a
vortex-core magnetic configuration, is analyzed by means of analytical
calculations. The cylinders are placed in a N x N square array in two
configurations - core oriented parallel to each other and with antiparallel
alignment between nearest neighbors. Results comprise the variation in the core
radius with the number of interacting dots, the distance between them and dot
height. The dipolar interdot coupling leads to a decrease (increase) of the
core radius for parallel (antiparallel) arrays
A Spin-Statistics Theorem for Certain Topological Geons
We review the mechanism in quantum gravity whereby topological geons,
particles made from non-trivial spatial topology, are endowed with nontrivial
spin and statistics. In a theory without topology change there is no
obstruction to ``anomalous'' spin-statistics pairings for geons. However, in a
sum-over-histories formulation including topology change, we show that
non-chiral abelian geons do satisfy a spin-statistics correlation if they are
described by a wave function which is given by a functional integral over
metrics on a particular four-manifold. This manifold describes a topology
changing process which creates a pair of geons from .Comment: 21 pages, Plain TeX with harvmac, 3 figures included via eps
Standard Model with Cosmologically Broken Quantum Scale Invariance
We argue that scale invariance is not anomalous in quantum field theory,
provided it is broken cosmologically. We consider a locally scale invariant
extension of the Standard Model of particle physics and argue that it fits both
the particle and cosmological observations. The model is scale invariant both
classically and quantum mechanically. The scale invariance is broken
cosmologically producing all the dimensionful parameters. The cosmological
constant or dark energy is a prediction of the theory and can be calculated
systematically order by order in perturbation theory. It is expected to be
finite at all orders. The model does not suffer from the hierarchy problem due
to absence of scalar particles, including the Higgs, from the physical
spectrum.Comment: 13 pages, no figures significant revisions, no change in results or
conclusion
Large Fluctuations in the Horizon Area and what they can tell us about Entropy and Quantum Gravity
We evoke situations where large fluctuations in the entropy are induced, our
main example being a spacetime containing a potential black hole whose
formation depends on the outcome of a quantum mechanical event. We argue that
the teleological character of the event horizon implies that the consequent
entropy fluctuations must be taken seriously in any interpretation of the
quantal formalism. We then indicate how the entropy can be well defined despite
the teleological character of the horizon, and we argue that this is possible
only in the context of a spacetime or ``histories'' formulation of quantum
gravity, as opposed to a canonical one, concluding that only a spacetime
formulation has the potential to compute --- from first principles and in the
general case --- the entropy of a black hole. From the entropy fluctuations in
a related example, we also derive a condition governing the form taken by the
entropy, when it is expressed as a function of the quantal density-operator.Comment: 35 pages, plain Tex, needs mathmacros.tex and msmacros.te
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