65,033 research outputs found
Space shuttle electrical power generation and reactant supply system
The design philosophy and development experience of fuel cell power generation and cryogenic reactant supply systems are reviewed, beginning with the state of technology at the conclusion of the Apollo Program. Technology advancements span a period of 10 years from initial definition phase to the most recent space transportation system (STS) flights. The development program encompassed prototype, verification, and qualification hardware, as well as post-STS-1 design improvements. Focus is on the problems encountered, the scientific and engineering approaches employed to meet the technological challenges, and the results obtained. Major technology barriers are discussed, and the evolving technology development paths are traced from their conceptual beginnings to the fully man-rated systems which are now an integral part of the shuttle vehicle
Approach to Equilibrium for a Forced Burgers Equation
We show that approach to equilibrium in certain forced Burgers equations is
implied by a decay estimate on a suitable intrinsic semigroup estimate, and we
verify this estimate in a variety of cases including a periodic force.Comment: To appear in Journal of Evolution Equation
Quark Models of Baryon Masses and Decays
The application of quark models to the spectra and strong and electromagnetic
couplings of baryons is reviewed. This review focuses on calculations which
attempt a global description of the masses and decay properties of baryons,
although recent developments in applying large N_c QCD and lattice QCD to the
baryon spectrum are described. After outlining the conventional
one-gluon-exchange picture, models which consider extensions to this approach
are contrasted with dynamical quark models based on Goldstone-boson exchange
and an algebraic collective-excitation approach. The spectra and
electromagnetic and strong couplings that result from these models are compared
with the quantities extracted from the data and each other, and the impact of
various model assumptions on these properties is emphasized. Prospects for the
resolution of the important issues raised by these comparisons are discussed.Comment: 91 page review article; 25 figures, 9 tables; submitted to Progress
in Particle and Nuclear Physic
Hamilton's theory of turns revisited
We present a new approach to Hamilton's theory of turns for the groups
SO(3) and SU(2) which renders their properties, in particular their
composition law, nearly trivial and immediately evident upon inspection.
We show that the entire construction can be based on binary rotations rather
than mirror reflections.Comment: 7 pages, 4 figure
The triton in a finite volume
Understanding the volume dependence of the triton binding energy is an
important step towards lattice simulations of light nuclei. We calculate the
triton binding energy in a finite cubic box with periodic boundary conditions
to leading order in the pionless effective field theory. Higher order
corrections are estimated and the proper renormalization of our results is
verified explicitly. We present results for the physical triton as well as for
the pion-mass dependence of the triton spectrum near the ``critical'' pion
mass, Mpi_c ~ 197 MeV, where chiral effective field theory suggests that the
nucleon-nucleon scattering lengths in the singlet- and triplet-channels diverge
simultaneously. An extension of the Luescher formula to the three-body system
is implicit in our results.Comment: 11 pages, 4 figure
Perturbed vortex lattices and the stability of nucleated topological phases
We study the stability of nucleated topological phases that can emerge when
interacting non-Abelian anyons form a regular array. The studies are carried
out in the context of Kitaev's honeycomb model, where we consider three
distinct types of perturbations in the presence of a lattice of Majorana mode
binding vortices -- spatial anisotropy of the vortices, dimerization of the
vortex lattice and local random disorder. While all the nucleated phases are
stable with respect to weak perturbations of each kind, strong perturbations
are found to result in very different behavior. Anisotropy of the vortices
stabilizes the strong-pairing like phases, while dimerization can recover the
underlying non-Abelian phase. Local random disorder, on the other hand, can
drive all the nucleated phases into a gapless thermal metal state. We show that
all these distinct behavior can be captured by an effective staggered
tight-binding model for the Majorana modes. By studying the pairwise
interactions between the vortices, i.e. the amplitudes for the Majorana modes
to tunnel between vortex cores, the locations of phase transitions and the
nature of the resulting states can be predicted. We also find that due to
oscillations in the Majorana tunneling amplitude, lattices of Majorana modes
may exhibit a Peierls-like instability, where a dimerized configuration is
favored over a uniform lattice. As the nature of the nucleated phases depends
only on the Majorana tunneling, our results apply also to other system
supporting localized Majorana mode arrays, such as Abrikosov lattices in p-wave
superconductors, Wigner crystals in Moore-Read fractional quantum Hall states
or arrays of topological nanowires.Comment: 13 pages, 4 pages of appendices, 24 figures. Published versio
Gauge Symmetry and Gravito-Electromagnetism
A tensor description of perturbative Einsteinian gravity about an arbitrary
background spacetime is developed. By analogy with the covariant laws of
electromagnetism in spacetime, gravito-electromagnetic potentials and fields
are defined to emulate electromagnetic gauge transformations under
substitutions belonging to the gauge symmetry group of perturbative
gravitation. These definitions have the advantage that on a flat background,
with the aid of a covariantly constant timelike vector field, a subset of the
linearised gravitational field equations can be written in a form that is fully
analogous to Maxwell's equations (without awkward factors of 4 and extraneous
tensor fields). It is shown how the remaining equations in the perturbed
gravitational system restrict the time dependence of solutions to these
equations and thereby prohibit the existence of propagating vector fields. The
induced gravito-electromagnetic Lorentz force on a test particle is evaluated
in terms of these fields together with the torque on a small gyroscope. It is
concluded that the analogy of perturbative gravity to Maxwell's description of
electromagnetism can be valuable for (quasi-)stationary gravitational phenomena
but that the analogy has its limitations.Comment: 29 pages no-fig
Data-aided carrier tracking loops
Power in composite signal sidebands is used to enhance signal-to-noise ratio in carrier tracking loop, thereby reducing radio loss and decreasing probability of receiver error. By adding quadrature channel to phase-lock-loop detector circuit of receiver, dc component can be fed back into carrier tracking loop
A nonlinear-coherence receiver
Mathematical analysis and detailed study of generic model for coherent receiver has demonstrated that nonlinear coherence between given biphase-modulated input signal and supplied reference signal can be used in receivers to improve telecommunication systems
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