84 research outputs found
Dynamics of a Dust Crystal with Two Different Size Dust Species
A self-consistent three-dimensional model for a complex (dusty) plasma is
used to study the effects of multiple-sized dust grains in a dust crystal. In
addition to the interparticle forces, which interact through a Yukawa
potential, the model includes the effects of gravity, the variation of the
sheath potential above the powered electrode, and a radial confining potential.
Simulations studied various ratios of a mix of 6.5- and 8.9-micron monodisperse
particles and compared their correlation functions, electric potential energy
of the crystal formations, and the dispersion relations for in-plane and
out-of-plane dust lattice wave (DLW) modes for two different sheath
thicknesses. In the 7 mm sheath, the particles formed two layers in the
vertical direction by size, and acted as a two-layer crystal with weak
correlation between the layers. In the 3 mm sheath, the particles formed an
essentially monolayer crystal; however the crystal dynamics showed some
characteristics of a bilayer crystal
Digital Imaging and Analysis of Dusty Plasmas
Dust particles immersed within a plasma environment, such as those found in
planetary rings or cometary environments, will acquire an electric charge. If
the ratio of interparticle potential energy to average kinetic energy is high
enough the particles will form either a 'liquid' structure with short-range
ordering or a crystalline structure with long-range ordering. Since their
discovery in laboratory environments in 1994, such crystals have been the
subject of a variety of experimental, theoretical, and numerical
investigations. Laboratory experiments analyzing the behavior of dust grains in
a plasma rely on optical diagnostics to provide data about the system in a
non-perturbative manner. In the past, capturing, imaging, and analyzing
crystalline structure in dusty plasmas has been a non-trivial problem.
Utilizing digital imaging and analysis systems, data capture, image formatting,
and analysis can be done quickly. Following data capture, image analysis is
conducted using modified Particle Image Velocimetry (PIV) and Particle Tracking
Velocimetry (PTV) algorithms. The data extracted is then used to construct
Voronoi diagrams, calculate particle density, inter-particle spacing, pair
correlation functions, and thermal energy. From this data other dust plasma
parameters can be inferred such as inter-particle forces and grain charges.Comment: 6 pages, Presented at COSPAR '0
Charged Grains In Saturn's F-Ring: Interaction With Saturn's Magnetic Field
Saturn's dynamic F-Ring still presents a challenge for understanding and
explaining the kinematic processes that lead to the changing structure visible
in our observations of this ring. This study examines the effect of Saturn's
magnetic field on the dynamics of micron-sized grains that may become
electrically charged due to interaction with plasma in Saturn's rigidly
corotating magnetosphere. The numerical model calculates the dynamics of
charged dust grains and includes forces due to Saturn's gravitational field,
the plasma polarization electric field, a third order harmonic expansion of
Saturn's magnetic field, and the F Ring's Shepherding moons, Prometheus and
Pandora.Comment: 6 pages, PDF, to be published in Advances in Space Researc
Precision Measurement of the Proton and Deuteron Spin Structure Functions g2 and Asymmetries A2
We have measured the spin structure functions g2p and g2d and the virtual
photon asymmetries A2p and A2d over the kinematic range 0.02 < x < 0.8 and 0.7
< Q^2 < 20 GeV^2 by scattering 29.1 and 32.3 GeV longitudinally polarized
electrons from transversely polarized NH3 and 6LiD targets. Our measured g2
approximately follows the twist-2 Wandzura-Wilczek calculation. The twist-3
reduced matrix elements d2p and d2n are less than two standard deviations from
zero. The data are inconsistent with the Burkhardt-Cottingham sum rule if there
is no pathological behavior as x->0. The Efremov-Leader-Teryaev integral is
consistent with zero within our measured kinematic range. The absolute value of
A2 is significantly smaller than the sqrt[R(1+A1)/2] limit.Comment: 12 pages, 4 figures, 2 table
Measurement of the Proton and Deuteron Spin Structure Functions g2 and Asymmetry A2
We have measured the spin structure functions g2p and g2d and the virtual
photon asymmetries A2p and A2d over the kinematic range 0.02 < x < 0.8 and 1.0
< Q^2 < 30(GeV/c)^2 by scattering 38.8 GeV longitudinally polarized electrons
from transversely polarized NH3 and 6LiD targets.The absolute value of A2 is
significantly smaller than the sqrt{R} positivity limit over the measured
range, while g2 is consistent with the twist-2 Wandzura-Wilczek calculation. We
obtain results for the twist-3 reduced matrix elements d2p, d2d and d2n. The
Burkhardt-Cottingham sum rule integral - int(g2(x)dx) is reported for the range
0.02 < x < 0.8.Comment: 12 pages, 4 figures, 1 tabl
Quasielastic 12C(e,e'p) Reaction at High Momentum Transfer
We measured the 12C(e,e'p) cross section as a function of missing energy in
parallel kinematics for (q,w) = (970 MeV/c, 330 MeV) and (990 MeV/c, 475 MeV).
At w=475 MeV, at the maximum of the quasielastic peak, there is a large
continuum (E_m > 50 MeV) cross section extending out to the deepest missing
energy measured, amounting to almost 50% of the measured cross section. The
ratio of data to DWIA calculation is 0.4 for both the p- and s-shells. At w=330
MeV, well below the maximum of the quasielastic peak, the continuum cross
section is much smaller and the ratio of data to DWIA calculation is 0.85 for
the p-shell and 1.0 for the s-shell. We infer that one or more mechanisms that
increase with transform some of the single-nucleon-knockout into
multinucleon knockout, decreasing the valence knockout cross section and
increasing the continuum cross section.Comment: 14 pages, 7 figures, Revtex (multicol, prc and aps styles), to appear
in Phys Rev
Measurements of the -Dependence of the Proton and Neutron Spin Structure Functions g1p and g1n
The structure functions g1p and g1n have been measured over the range 0.014 <
x < 0.9 and 1 < Q2 < 40 GeV2 using deep-inelastic scattering of 48 GeV
longitudinally polarized electrons from polarized protons and deuterons. We
find that the Q2 dependence of g1p (g1n) at fixed x is very similar to that of
the spin-averaged structure function F1p (F1n). From a NLO QCD fit to all
available data we find at
Q2=5 GeV2, in agreement with the Bjorken sum rule prediction of 0.182 \pm
0.005.Comment: 17 pages, 3 figures. Submitted to Physics Letters
Virtual Compton Scattering and Neutral Pion Electroproduction in the Resonance Region up to the Deep Inelastic Region at Backward Angles
We have made the first measurements of the virtual Compton scattering (VCS)
process via the H exclusive reaction in the nucleon resonance
region, at backward angles. Results are presented for the -dependence at
fixed GeV, and for the -dependence at fixed near 1.5 GeV.
The VCS data show resonant structures in the first and second resonance
regions. The observed -dependence is smooth. The measured ratio of
H to H cross sections emphasizes the different
sensitivity of these two reactions to the various nucleon resonances. Finally,
when compared to Real Compton Scattering (RCS) at high energy and large angles,
our VCS data at the highest (1.8-1.9 GeV) show a striking -
independence, which may suggest a transition to a perturbative scattering
mechanism at the quark level.Comment: 20 pages, 8 figures. To appear in Phys.Rev.
Inclusive hadron photoproduction from longitudinally polarized protons and deuterons.
We report measurements of the asymmetry A_parallel for inclusive hadron production on longitudinally polarized proton and deuteron targets by circularly polarized photons. The photons were produced via internal and external bremsstrahlung from an electron beam of 48.35 GeV. Asymmetries for both positive and negative signed hadrons, and a subset of identified pions, were measured in the momentum range 1
Measurements of the dependence of the proton and neutron spin structure functions and
he structure functions g1p and g1n have been measured over the range 0.014 < x < 0.9 and 1 < Q2 < 40 GeV2 using deep-inelastic scattering of 48 GeV longitudinally polarized electrons from polarized protons and deuterons. We find that the Q2 dependence of g1p (g1n) at fixed x is very similar to that of the spin-averaged structure function F1p (F1n). From a NLO QCD fit to all available data we find at Q2=5 GeV2, in agreement with the Bjorken sum rule prediction of 0.182 \pm 0.005
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