42,413 research outputs found
Investigation of electrodynamic stabilization and control of long orbiting tethers
An algorithm for using electric currents to control pendular oscillations induced by various perturbing forces on the Skyhook wire is considered. Transverse and vertical forces on the tether; tether instability modes and causes during retrieval by space shuttle; simple and spherical pendulum motion and vector damping; and current generation and control are discussed. A computer program for numerical integration of the in-plane and out-of-plane displacements of the tether vs time was developed for heuristic study. Some techniques for controlling instabilities during payload retrieval and methods for employing the tether for launching satellites from the space shuttle are considered. Derivations and analyses of a general nature used in all of the areas studied are included
A Modified "Bottom-up" Thermalization in Heavy Ion Collisions
In the initial stage of the bottom-up picture of thermalization in heavy ion
collisions, the gluon distribution is highly anisotropic which can give rise to
plasma instability. This has not been taken account in the original paper. It
is shown that in the presence of instability there are scaling solutions, which
depend on one parameter, that match smoothly onto the late stage of bottom-up
when thermalization takes place.Comment: 8 pages and 1 embedded figure, talk presented at the Workshop on
"Quark-Gluon Plasma Thermalization", Vienna, Austria, 10-12 August 200
On the thermodynamic framework of generalized coupled thermoelastic-viscoplastic-damage modeling
A complete potential based framework using internal state variables is put forth for the derivation of reversible and irreversible constitutive equations. In this framework, the existence of the total (integrated) form of either the (Helmholtz) free energy or the (Gibbs) complementary free energy are assumed a priori. Two options for describing the flow and evolutionary equations are described, wherein option one (the fully coupled form) is shown to be over restrictive while the second option (the decoupled form) provides significant flexibility. As a consequence of the decoupled form, a new operator, i.e., the Compliance operator, is defined which provides a link between the assumed Gibb's and complementary dissipation potential and ensures a number of desirable numerical features, for example the symmetry of the resulting consistent tangent stiffness matrix. An important conclusion reached, is that although many theories in the literature do not conform to the general potential framework outlined, it is still possible in some cases, by slight modifications of the used forms, to restore the complete potential structure
Explicit robust schemes for implementation of a class of principal value-based constitutive models: Theoretical development
The issue of developing effective and robust schemes to implement a class of the Ogden-type hyperelastic constitutive models is addressed. To this end, explicit forms for the corresponding material tangent stiffness tensors are developed, and these are valid for the entire deformation range; i.e., with both distinct as well as repeated principal-stretch values. Throughout the analysis the various implications of the underlying property of separability of the strain-energy functions are exploited, thus leading to compact final forms of the tensor expressions. In particular, this facilitated the treatment of complex cases of uncoupled volumetric/deviatoric formulations for incompressible materials. The forms derived are also amenable for use with symbolic-manipulation packages for systematic code generation
Electron transfer through a multiterminal quantum ring: magnetic forces and elastic scattering effects
We study electron transport through a semiconductor quantum ring with one
input and two output terminals for an elastic scatterer present within one of
the arms of the ring. We demonstrate that the scatterer not only introduces
asymmetry in the transport probability to the two output leads but also reduces
the visibility of the Aharonov-Bohm conductance oscillations. This reduction
occurs in spite of the phase coherence of the elastic scattering and is due to
interruption of the electron circulation around the ring by the potential
defect. The results are in a qualitative agreement with a recent experiment by
Strambini et al. [Phys. Rev. B {\bf 79}, 195443 (2009)]. We also indicate that
the magnetic symmetry of the sum of conductance of both the output leads as
obtained in the experiment can be understood as resulting from the invariance
of backscattering to the input lead with respect to the magnetic field
orientation.Comment: submitted to PR
Adaptive reflection and focusing of Bose-Einstein condensates
We report adjustable magnetic `bouncing' and focusing of a dilute Rb
Bose gas. Both the condensate production and manipulation are realised using a
particularly straight-forward apparatus. The bouncing region is comprised of
approximately concentric ellipsoidal magnetic equipotentials with a centre that
can be adjusted vertically. We extend, and discuss the limitations of, simple
Thomas-Fermi and Monte-Carlo theoretical models for the bouncing, which at
present find close agreement with the condensate's evolution. Very strong
focusing has been inferred and the observation of atomic matter-wave
diffraction should be possible. Prospects look bright for applications in
matter-wave atom-optics, due to the very smooth nature of the mirror
Weak-triplet, color-octet scalars and the CDF dijet excess
We extend the standard model to include a weak-triplet and color-octet
scalar. This `octo-triplet' field consists of three particles, two charged and
one neutral, whose masses and renormalizable interactions depend only on two
new parameters. The charged octo-triplet decay into a W boson and a gluon is
suppressed by a loop factor and an accidental cancellation. Thus, the main
decays of the charged octo-triplet may occur through higher-dimensional
operators, mediated by a heavy vectorlike fermion, into quark pairs. For an
octo-triplet mass below the t\bar{b} threshold, the decay into Wb\bar{b} or
Wb\bar{s} through an off-shell top quark has a width comparable to that into
c\bar{s} or c\bar{b}. Pair production with one octo-triplet decaying into two
jets and the other decaying into a W and two soft b jets may explain the
dijet-plus-W excess reported by the CDF Collaboration. Using a few kinematic
distributions, we compare two mechanisms of octo-triplet pair production:
through an s-channel coloron and through the coupling to gluons. The
higher-dimensional operators that allow dijet decays also lead to CP violation
in B_s - \bar B_s mixing.Comment: 18 pages. New CDF kinematic distributions using 7.3 fb^{-1} compared
to both resonant and gluon-induced pair production of octets. Corrections in
Section 3.1. Comment on the D0 Wjj result included in Section 3.3.
Implications for LHC expanded in Section 3.
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