28,206 research outputs found
Spectroscopic measurements of temperature and plasma impurity concentration during magnetic reconnection at the Swarthmore Spheromak Experiment
Electron temperature measurements during counterhelicity spheromak merging studies at the Swarthmore Spheromak Experiment (SSX) [M. R. Brown, Phys. Plasmas 6, 1717 (1999)] are presented. VUV monochromator measurements of impurity emission lines are compared with model spectra produced by the non-LTE excitation kinematics code PRISMSPECT [J. J. MacFarlane et al., in Proceedings of the Third Conference on Inertial Fusion Science and Applications (2004)] to yield the electron temperature in the plasma with 1 µs time resolution. Average T_e is seen to increase from 12 to 19 eV during spheromak merging. Average C III ion temperature, measured with a new ion Doppler spectrometer (IDS) [C. D. Cothran et al., Rev. Sci. Instrum. 77, 063504 (2006)], likewise rises during spheromak merging, peaking at ~22 eV, but a similar increase in T_i is seen during single spheromak discharges with no merging. The VUV emission line measurements are also used to constrain the concentrations of various impurities in the SSX plasma, which are dominated by carbon, but include some oxygen and nitrogen. A burst of soft x-ray emission is seen during reconnection with a new four-channel detector (SXR). There is evidence for spectral changes in the soft x-ray emission as reconnection progresses, although our single-temperature equilibrium spectral models are not able to provide adequate fits to all the SXR data
Model Energy Landscapes of Low-Temperature Fluids: Dipolar Hard Spheres
An analytical model of non-Gaussian energy landscape of low-temperature
fluids is developed based on the thermodynamics of the fluid of dipolar hard
spheres. The entire excitation profile of the liquid, from the high
temperatures to the point of ideal-glass transition, has been obtained from the
Monte Carlo simulations. The fluid of dipolar hard spheres loses stability when
reaching the point of ideal-glass transition transforming via a first-order
transition into a columnar liquid phase of dipolar chains locally arranged in a
body-centered tetragonal order.Comment: 4 pages, 3 figure
Paraelectric and ferroelectric order in two-state dipolar fluids
Monte Carlo simulations are used to examine cooperative creation of polar
state in fluids of two-state particles with nonzero dipole in the excited
state. With lowering temperature such systems undergo a second order transition
from nonpolar to polar, paraelectric phase. The transition is accompanied by a
dielectric anomaly of polarization susceptibility increasing by three orders of
magnitude. The paraelectric phase is then followed by formation of a nematic
ferroelectric which further freezes into an fcc ferroelectric crystal by a
first order transition. A mean-field model of phase transitions is discussed.Comment: 5 pages, 4 figure
Fluid-Induced Propulsion of Rigid Particles in Wormlike Micellar Solutions
In the absence of inertia, a reciprocal swimmer achieves no net motion in a
viscous Newtonian fluid. Here, we investigate the ability of a reciprocally
actuated particle to translate through a complex fluid that possesses a network
using tracking methods and birefringence imaging. A geometrically polar
particle, a rod with a bead on one end, is reciprocally rotated using magnetic
fields. The particle is immersed in a wormlike micellar (WLM) solution that is
known to be susceptible to the formation of shear bands and other localized
structures due to shear-induced remodeling of its microstructure. Results show
that the nonlinearities present in this WLM solution break time-reversal
symmetry under certain conditions, and enable propulsion of an artificial
"swimmer." We find three regimes dependent on the Deborah number (De): net
motion towards the bead-end of the particle at low De, net motion towards the
rod-end of the particle at intermediate De, and no appreciable propulsion at
high De. At low De, where the particle time-scale is longer then the fluid
relaxation time, we believe that propulsion is caused by an imbalance in the
fluid first normal stress differences between the two ends of the particle
(bead and rod). At De~1, however, we observe the emergence of a region of
network anisotropy near the rod using birefringence imaging. This anisotropy
suggests alignment of the micellar network, which is "locked in" due to the
shorter time-scale of the particle relative to the fluid
Yukawa Textures From Heterotic Stability Walls
A holomorphic vector bundle on a Calabi-Yau threefold, X, with h^{1,1}(X)>1
can have regions of its Kahler cone where it is slope-stable, that is, where
the four-dimensional theory is N=1 supersymmetric, bounded by "walls of
stability". On these walls the bundle becomes poly-stable, decomposing into a
direct sum, and the low energy gauge group is enhanced by at least one
anomalous U(1) gauge factor. In this paper, we show that these additional
symmetries can strongly constrain the superpotential in the stable region,
leading to non-trivial textures of Yukawa interactions and restrictions on
allowed masses for vector-like pairs of matter multiplets. The Yukawa textures
exhibit a hierarchy; large couplings arise on the stability wall and some
suppressed interactions "grow back" off the wall, where the extended U(1)
symmetries are spontaneously broken. A number of explicit examples are
presented involving both one and two stability walls, with different
decompositions of the bundle structure group. A three family standard-like
model with no vector-like pairs is given as an example of a class of SU(4)
bundles that has a naturally heavy third quark/lepton family. Finally, we
present the complete set of Yukawa textures that can arise for any holomorphic
bundle with one stability wall where the structure group breaks into two
factors.Comment: 53 pages, 4 figures and 13 table
A possible new phase of antagonistic nematogens in a disorienting field
A simple model is proposed for nematogenic molecules that favor perpendicular
orientations as well as parallel ones. (Charged rods, for example, show this
antagonistic tendency.) When a small disorienting field is applied along , a
low density phase of nematic order parameter coexists with a
dense biaxial nematic . (At zero field, becomes isotropic and
uniaxial.) But at stronger fields, a new phase , invariant under
rotations around the field axis, appears in between and .
Prospects for finding the phase experimentally are briefly discussed.Comment: 4 pages, 2 figures. Accepted for publication in PR
A New Method for Finding Vacua in String Phenomenology
One of the central problems of string-phenomenology is to find stable vacua
in the four dimensional effective theories which result from compactification.
We present an algorithmic method to find all of the vacua of any given
string-phenomenological system in a huge class. In particular, this paper
reviews and then extends hep-th/0606122 to include various non-perturbative
effects. These include gaugino condensation and instantonic contributions to
the superpotential.Comment: 27 pages, 5 .eps figures. V2: Minor corrections, reference adde
Coherent population trapping in quantized light field
A full quantum treatment of coherent population trapping (CPT) is given for a
system of resonantly coupled atoms and electromagnetic field. We develop a
regular analytical method of the construction of generalized dark states (GDS).
It turns out that GDS do exist for all optical transitions ,
including bright transitions and with a
half-integer, for which the CPT effect is absent in a classical field. We
propose an idea to use an optically thick medium with a transition
with a half-integer as a ''quantum filter'', which transmits only
a quantum light.Comment: revtex4, twocolumn, 6 pages, including 1 figur
A connection with parallel totally skew-symmetric torsion on a class of almost hypercomplex manifolds with Hermitian and anti-Hermitian metrics
The subject of investigations are the almost hypercomplex manifolds with
Hermitian and anti-Hermitian (Norden) metrics. A linear connection D is
introduced such that the structure of these manifolds is parallel with respect
to D and its torsion is totally skew-symmetric. The class of the nearly Kaehler
manifolds with respect to the first almost complex structure is of special
interest. It is proved that D has a D-parallel torsion and is weak if it is not
flat. Some curvature properties of these manifolds are studied.Comment: 18 page
Variability in the extreme helium star LSS 5121
We report a photometric and spectroscopic study of the hot extreme helium
star LSS 5121. We found photometric variability, but no period was evident in
its periodogram. This is consistent with the previous proposal, based on
spectral line variations, that LSS 5121 is a non-radial pulsator similar to
other hot extreme helium stars.Comment: 5 pages, 6 figure
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