205 research outputs found
Applied Plasma Research
Contains reports on two research projects.National Science Foundation (Grant GK-37979X)U. S. Army - Research Office - Durham (Contract DAHC04-72-C-0044
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DEGAS 2 neutral transport modeling of high density, low temperature plasmas
Neutral transport in the high density, low temperature plasma regime is examined using the DEGAS 2 Monte Carlo neutral transport code. DEGAS 2 is shown to agree with an analytic fluid neutral model valid in this regime as long as the grid cell spacing is less than twice the neutral mean-free path. Using new atomic physics data provided by the collisional radiative code CRAMD, DEGAS 2 is applied to a detached Alcator C-Mod discharge. A model plasma with electron temperature {approximately}1 eV along detached flux tubes, between the target and the ionization front, is used to demonstrate that recombination is essential to matching the experimental data. With the CRAMD data, {approximately}20% of the total recombination is due to molecular activated recombination
Chaos in Spin Clusters: Correlation Functions and Spectral Properties
We investigate dynamic correlation functions for a pair of exchange‐coupled classical spins with biaxial exchange and/or single‐site anisotropy. This represents a Hamiltonian system with two degrees of freedom for which we have previously established the integrability criteria. We discuss the impact of (non‐)integrability on the autocorrelation functions and their spectral properties. We point out the role of long‐time anomalies caused by low‐flux cantori, which dominate the convergence properties of time averages and determine the long‐time asymptotic behavior of autocorrelation functions in nonintegrable cases
Dark matter in the solar system III: The distribution function of WIMPs at the Earth from gravitational capture
In this last paper in a series of three on weakly interacting massive
particle (WIMP) dark matter in the solar system, we focus on WIMPs bound to the
system by gravitationally scattering off of planets. We present simulations of
WIMP orbits in a toy solar system consisting of only the Sun and Jupiter. As
previous work suggested, we find that the density of gravitationally captured
WIMPs at the Earth is small and largely insensitive to the details of elastic
scattering in the Sun. However, we find that the density of gravitationally
captured WIMPs may be affected by external Galactic gravitational fields. If
such fields are unimportant, the density of gravitationally captured WIMPs at
the Earth should be similar to the maximum density of WIMPs captured in the
solar system by elastic scattering in the Sun. Using standard assumptions about
the halo WIMP distribution function, we find that the gravitationally captured
WIMPs contribute negligibly to direct detection event rates. While these WIMPs
do dominate the annihilation rate of WIMPs in the Earth, the resulting event
rate in neutrino telescopes is too low to be observed in next-generation
neutrino telescopes.Comment: 24 pages, 11 figures, to be submitte
Nonlinear Radiation Pressure and Stochasticity in Ultraintense Laser Fields
The radiation force on a single electron in an ultraintense plane wave () is calculated and shown to be proportional to in the
high- limit for arbitrary waveform and polarization. The cyclotron motion of
an electron in a constant magnetic field and an ultraintense plane wave is
numerically found to be quasiperiodic even in the high- limit if the
magnetic field is not too strong, as suggested by previous analytical work. A
strong magnetic field causes highly chaotic electron motion and the boundary of
the highly chaotic region of parameter space is determined numerically.
Applications to experiments and astrophysics are briefly discussed.Comment: 5 pages, 4 figures; uses RevTex, epsf macros. Corrected, expanded
versio
Dissipative chaotic scattering
We show that weak dissipation, typical in realistic situations, can have a
metamorphic consequence on nonhyperbolic chaotic scattering in the sense that
the physically important particle-decay law is altered, no matter how small the
amount of dissipation. As a result, the previous conclusion about the unity of
the fractal dimension of the set of singularities in scattering functions, a
major claim about nonhyperbolic chaotic scattering, may not be observable.Comment: 4 pages, 2 figures, revte
Universal diffusion near the golden chaos border
We study local diffusion rate in Chirikov standard map near the critical
golden curve. Numerical simulations confirm the predicted exponent
for the power law decay of as approaching the golden curve via principal
resonances with period (). The universal
self-similar structure of diffusion between principal resonances is
demonstrated and it is shown that resonances of other type play also an
important role.Comment: 4 pages Latex, revtex, 3 uuencoded postscript figure
Quantum Poincar\'e Recurrences
We show that quantum effects modify the decay rate of Poincar\'e recurrences
P(t) in classical chaotic systems with hierarchical structure of phase space.
The exponent p of the algebraic decay P(t) ~ 1/t^p is shown to have the
universal value p=1 due to tunneling and localization effects. Experimental
evidence of such decay should be observable in mesoscopic systems and cold
atoms.Comment: revtex, 4 pages, 4 figure
Environment-independent decoherence rate in classically chaotic systems
We study the decoherence of a one-particle system, whose classical
correpondent is chaotic, when it evolves coupled to a weak quenched
environment. This is done by analytical evaluation of the Loschmidt Echo, (i.e.
the revival of a localized density excitation upon reversal of its time
evolution), in presence of the perturbation. We predict an exponential decay
for the Loschmidt Echo with a (decoherence) rate which is asymptotically given
by the mean Lyapunov exponent of the classical system, and therefore
independent of the perturbation strength, within a given range of strengths.
Our results are consistent with recent experiments of Polarization Echoes in
nuclear magnetic resonance and preliminary numerical simulations.Comment: No figures. Typos corrected and minor modifications to the text and
references. Published versio
Fractal Conductance Fluctuations in a Soft Wall Stadium and a Sinai Billiard
Conductance fluctuations have been studied in a soft wall stadium and a Sinai
billiard defined by electrostatic gates on a high mobility semiconductor
heterojunction. These reproducible magnetoconductance fluctuations are found to
be fractal confirming recent theoretical predictions of quantum signatures in
classically mixed (regular and chaotic) systems. The fractal character of the
fluctuations provides direct evidence for a hierarchical phase space structure
at the boundary between regular and chaotic motion.Comment: 4 pages, 4 figures, data on Sinai geometry added to Fig.1, minor
change
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