22,567 research outputs found
Energy dynamics in a simulation of LAPD turbulence
Energy dynamics calculations in a 3D fluid simulation of drift wave
turbulence in the linear Large Plasma Device (LAPD) [W. Gekelman et al., Rev.
Sci. Inst. 62, 2875 (1991)] illuminate processes that drive and dissipate the
turbulence. These calculations reveal that a nonlinear instability dominates
the injection of energy into the turbulence by overtaking the linear drift wave
instability that dominates when fluctuations about the equilibrium are small.
The nonlinear instability drives flute-like () density
fluctuations using free energy from the background density gradient. Through
nonlinear axial wavenumber transfer to fluctuations, the
nonlinear instability accesses the adiabatic response, which provides the
requisite energy transfer channel from density to potential fluctuations as
well as the phase shift that causes instability. The turbulence characteristics
in the simulations agree remarkably well with experiment. When the nonlinear
instability is artificially removed from the system through suppressing
modes, the turbulence develops a coherent frequency spectrum
which is inconsistent with experimental data
Quasi-Adiabatic Continuation in Gapped Spin and Fermion Systems: Goldstone's Theorem and Flux Periodicity
We apply the technique of quasi-adiabatic continuation to study systems with
continuous symmetries. We first derive a general form of Goldstone's theorem
applicable to gapped nonrelativistic systems with continuous symmetries. We
then show that for a fermionic system with a spin gap, it is possible to insert
-flux into a cylinder with only exponentially small change in the energy
of the system, a scenario which covers several physically interesting cases
such as an s-wave superconductor or a resonating valence bond state.Comment: 19 pages, 2 figures, final version in press at JSTA
Topological entropy of realistic quantum Hall wave functions
The entanglement entropy of the incompressible states of a realistic quantum
Hall system are studied by direct diagonalization. The subdominant term to the
area law, the topological entanglement entropy, which is believed to carry
information about topologic order in the ground state, was extracted for
filling factors 1/3, 1/5 and 5/2. The results for 1/3 and 1/5 are consistent
with the topological entanglement entropy for the Laughlin wave function. The
5/2 state exhibits a topological entanglement entropy consistent with the
Moore-Read wave function.Comment: 6 pages, 6 figures; improved computations and graphics; added
reference
Structure and Response in the World Trade Network
We examine how the structure of the world trade network has been shaped by
globalization and recessions over the last 40 years. We show that by treating
the world trade network as an evolving system, theory predicts the trade
network is more sensitive to evolutionary shocks and recovers more slowly from
them now than it did 40 years ago, due to structural changes in the world trade
network induced by globalization. We also show that recession-induced change to
the world trade network leads to an \emph{increased} hierarchical structure of
the global trade network for a few years after the recession.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Soil Moisture Workshop
The Soil Moisture Workshop was held at the United States Department of Agriculture National Agricultural Library in Beltsville, Maryland on January 17-19, 1978. The objectives of the Workshop were to evaluate the state of the art of remote sensing of soil moisture; examine the needs of potential users; and make recommendations concerning the future of soil moisture research and development. To accomplish these objectives, small working groups were organized in advance of the Workshop to prepare position papers. These papers served as the basis for this report
Polaron Excitations in Doped C60: Effects of Disorders
Effects on C by thermal fluctuations of phonons, misalignment of
C molecules in a crystal, and other intercalated impurities (remaining
C, oxygens, and so on) are simulated by disorder potentials. The
Su-Schrieffer-Heeger--type electron-phonon model for doped C is solved
with gaussian bond disorders and also with site disorders. Sample average is
performed over sufficient number of disorder configurations. The distributions
of bond lengths and electron densities are shown as functions of the disorder
strength and the additional electron number. Stability of polaron excitations
as well as dimerization patterns is studied. Polarons and dimerizations in
lightly doped cases (C) are relatively stable against disorders,
indicated by peak structures in distribution functions. In more heavily doped
cases, the several peaks merge into a single peak, showing the breakdown of
polaron structures as well as the decrease of the dimerization strength.
Possibility of the observation of polaronic lattice distortions and electron
structures in doped C is discussed.Comment: Note: This manusript was accepted for publication in Physical Review
B. Figures will be sent to you via snail (conventional) mai
Analysis of plasma instabilities and verification of the BOUT code for the Large Plasma Device
The properties of linear instabilities in the Large Plasma Device [W.
Gekelman et al., Rev. Sci. Inst., 62, 2875 (1991)] are studied both through
analytic calculations and solving numerically a system of linearized
collisional plasma fluid equations using the 3D fluid code BOUT [M. Umansky et
al., Contrib. Plasma Phys. 180, 887 (2009)], which has been successfully
modified to treat cylindrical geometry. Instability drive from plasma pressure
gradients and flows is considered, focusing on resistive drift waves, the
Kelvin-Helmholtz and rotational interchange instabilities. A general linear
dispersion relation for partially ionized collisional plasmas including these
modes is derived and analyzed. For LAPD relevant profiles including strongly
driven flows it is found that all three modes can have comparable growth rates
and frequencies. Detailed comparison with solutions of the analytic dispersion
relation demonstrates that BOUT accurately reproduces all characteristics of
linear modes in this system.Comment: Published in Physics of Plasmas, 17, 102107 (2010
Quantifying Model Complexity via Functional Decomposition for Better Post-Hoc Interpretability
Post-hoc model-agnostic interpretation methods such as partial dependence
plots can be employed to interpret complex machine learning models. While these
interpretation methods can be applied regardless of model complexity, they can
produce misleading and verbose results if the model is too complex, especially
w.r.t. feature interactions. To quantify the complexity of arbitrary machine
learning models, we propose model-agnostic complexity measures based on
functional decomposition: number of features used, interaction strength and
main effect complexity. We show that post-hoc interpretation of models that
minimize the three measures is more reliable and compact. Furthermore, we
demonstrate the application of these measures in a multi-objective optimization
approach which simultaneously minimizes loss and complexity
Modification of turbulent transport with continuous variation of flow shear in the Large Plasma Device
Continuous control over azimuthal flow and shear in the edge of the Large
Plasma Device (LAPD) has been achieved using a biasable limiter which has
allowed a careful study of the effect of flow shear on pressure-gradient-driven
turbulence and transport in LAPD. LAPD rotates spontaneously in the ion
diamagnetic direction (IDD); positive limiter bias first reduces, then
minimizes (producing a near-zero shear state), and finally reverses the flow
into the electron diamagnetic direction (EDD). Degradation of particle
confinement is observed in the minimum shearing state and reduction in
turbulent particle flux is observed with increasing shearing in both flow
directions. Near-complete suppression of turbulent particle flux is observed
for shearing rates comparable to the turbulent autocorrelation rate measured in
the minimum shear state. Turbulent flux suppression is dominated by amplitude
reduction in low-frequency (kHz) density fluctuations. An increase in
fluctuations for the highest shearing states is observed with the emergence of
a coherent mode which does not lead to net particle transport. The variations
of density fluctuations are fit well with power-laws and compare favorably to
simple models of shear suppression of transport.Comment: 10 pages, 5 figures; Submitted to Phys. Rev. Let
- …