2,000 research outputs found
Temporal and Spatial Turbulent Spectra of MHD Plasma and an Observation of Variance Anisotropy
The nature of MHD turbulence is analyzed through both temporal and spatial
magnetic fluctuation spectra. A magnetically turbulent plasma is produced in
the MHD wind-tunnel configuration of the Swarthmore Spheromak Experiment (SSX).
The power of magnetic fluctuations is projected into directions perpendicular
and parallel to a local mean field; the ratio of these quantities shows the
presence of variance anisotropy which varies as a function of frequency.
Comparison amongst magnetic, velocity, and density spectra are also made,
demonstrating that the energy of the turbulence observed is primarily seeded by
magnetic fields created during plasma production. Direct spatial spectra are
constructed using multi-channel diagnostics and are used to compare to
frequency spectra converted to spatial scales using the Taylor Hypothesis.
Evidence for the observation of dissipation due to ion inertial length scale
physics is also discussed as well as the role laboratory experiment can play in
understanding turbulence typically studied in space settings such as the solar
wind. Finally, all turbulence results are shown to compare fairly well to a
Hall-MHD simulation of the experiment.Comment: 17 pages, 17 figures, Submitted to Astrophysical Journa
Temporal And Spatial Turbulent Spectra Of MHD Plasma And An Observation Of Variance Anisotropy
The nature of magnetohydrodynamic (MHD) turbulence is analyzed through both temporal and spatial magnetic fluctuation spectra. A magnetically turbulent plasma is produced in the MHD wind tunnel configuration of the Swarthmore Spheromak Experiment. The power of magnetic fluctuations is projected into directions perpendicular and parallel to a local mean field; the ratio of these quantities shows the presence of variance anisotropy which varies as a function of frequency. Comparisons among magnetic, velocity, and density spectra are also made, demonstrating that the energy of the turbulence observed is primarily seeded by magnetic fields created during plasma production. Direct spatial spectra are constructed using multi-channel diagnostics and are used to compare to frequency spectra converted to spatial scales using the Taylor hypothesis. Evidence for the observation of dissipation due to ion inertial length scale physics is also discussed, as well as the role laboratory experiments can play in understanding turbulence typically studied in space settings such as the solar wind. Finally, all turbulence results are shown to compare fairly well to a Hall-MHD simulation of the experiment
Turbulence Analysis Of An Experimental Flux-Rope Plasma
We have previously generated elongated Taylor double-helix flux-rope plasmas in the SSX MHD wind tunnel. These plasmas are remarkable in their rapid relaxation (about one Alfven time) and their description by simple analytical Taylor force-free theory despite their high plasma beta and high internal flow speeds. We report on the turbulent features observed in these plasmas including frequency spectra, autocorrelation function, and probability distribution functions of increments. We discuss here the possibility that the turbulence facilitating access to the final state supports coherent structures and intermittency revealed by non-Gaussian signatures in the statistics. Comparisons to a Hall-MHD simulation of the SSX MHD wind tunnel show similarity in several statistical measures
Parametric Self-Oscillation via Resonantly Enhanced Multiwave Mixing
We demonstrate an efficient nonlinear process in which Stokes and anti-Stokes
components are generated spontaneously in a Raman-like, near resonant media
driven by low power counter-propagating fields. Oscillation of this kind does
not require optical cavity and can be viewed as a spontaneous formation of
atomic coherence grating
Coherent processing of a light pulse stored in a medium of four-level atoms
It is demonstrated that the properties of light stored in a four-level atomic
system can be modified by an additional control interaction present during the
storage stage. By choosing the pulse area of this interaction one can in
particular continuously switch between two channels into which light is
released.Comment: text+4 figure
Fault-tolerant Quantum Communication with Minimal Physical Requirements
We describe a novel protocol for a quantum repeater which enables long
distance quantum communication through realistic, lossy photonic channels.
Contrary to previous proposals, our protocol incorporates active purification
of arbitrary errors at each step of the protocol using only two qubits at each
repeater station. Because of these minimal physical requirements, the present
protocol can be realized in simple physical systems such as solid-state single
photon emitters. As an example, we show how nitrogen vacancy color centers in
diamond can be used to implement the protocol, using the nuclear and electronic
spin to form the two qubits.Comment: 4 pages, 3 figures. V2: Minor modifications. V3: Major changes in the
presentation and new titl
Decoherence Dynamics in Low-Dimensional Cold Atom Interferometers
We report on a study of the dynamics of decoherence of a matter-wave
interferometer, consisting of a pair of low-dimensional cold atom condensates
at finite temperature. We identify two distinct regimes in the time dependence
of the coherence factor of the interferometer: quantum and classical. Explicit
analytical results are obtained in both regimes. In particular, in the
two-dimensional (2D) case in the classical (long time) regime, we find that the
dynamics of decoherence is universal, exhibiting a power-law decay with an
exponent, proportional to the ratio of the temperature to the
Kosterlitz-Thouless temperature of a single 2D condensate. In the
one-dimensional (1D) case in the classical regime we find a universal
nonanalytic time dependence of decoherence, which is a consequence of the
nonhydrodynamic nature of damping in 1D liquids.Comment: 4 pages, published versio
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