727 research outputs found
The Conformal Einstein Field Equations with Massless Vlasov Matter
We prove the stability of de Sitter space-time as a solution to the
Einstein-Vlasov system with massless particles. The semi-global stability of
Minkowski space-time is also addressed. The proof relies on conformal
techniques, namely Friedrich's conformal Einstein field equations. We exploit
the conformal invariance of the massless Vlasov equation on the cotangent
bundle and adapt Kato's local existence theorem for symmetric hyperbolic
systems to prove a long enough time of existence for solutions of the evolution
system implied by the Vlasov equation and the conformal Einstein field
equations.Comment: 27 pages. To appear in Ann. Inst. Fourie
Performance Mapping Studies in Redox Flow Cells
Pumping power requirements in any flow battery system constitute a direct parasitic energy loss. It is therefore useful to determine the practical lower limit for reactant flow rates. Through the use of a theoretical framework based on electrochemical first principles, two different experimental flow mapping techniques were developed to evaluate and compare electrodes as a function of flow rate. For the carbon felt electrodes presently used in NASA-Lewis Redox cells, a flow rate 1.5 times greater than the stoichiometric rate seems to be the required minimum
Zero-energy states and fragmentation of spin in the easy-plane antiferromagnet on a honeycomb lattice
The core of the vortex in the Neel order parameter for an easy-plane
antiferromagnet on honeycomb lattice is demonstrated to bind two zero-energy
states. Remarkably, a single electron occupying this mid-gap band has its spin
fragmented between the two sublattices: Whereas it yields a vanishing total
magnetization it shows a finite Neel order, orthogonal to the one of the
assumed background. The requisite easy-plane anisotropy may be introduced by a
magnetic field parallel to the graphene layer, for example. The results are
relevant for spin-1/2 fermions on graphene's or optical honeycomb lattice, in
the strongly interacting regime.Comment: 4 pages; cosmetic changes; published versio
NASA Redox cell stack shunt current, pumping power, and cell performance tradeoffs
The NASA Redox energy storage system is under active technology development. The hardware undergoing laboratory testing is either 310 sq. cm. or 929 sq. cm. (0.33 sq. ft. or 1.0 sq. ft. per cell active area with up to 40 individual cells connected to make up a modular cell stack. This size of hardware allows rather accurate projections to be made of the shunt power/pump power tradeoffs. The modeling studies that were completed on the system concept are reviewed along with the approach of mapping the performance of Redox cells over a wide range of flow rates and depths of discharge of the Redox solutions. Methods are outlined for estimating the pumping and shunt current losses for any type of cell and stack combination. These methods are applicable to a variety of pumping options that are present with Redox systems. The results show that a fully developed Redox system has acceptable parasitic losses when using a fixed flow rate adequate to meet the worst conditions of current density and depth of discharge. These losses are reduced by about 65 percent if variable flow schedules are used. The exact value of the overall parasitics will depend on the specific system requirements of current density, voltage limits, charge, discharge time, etc
Design principles for nickel-hydrogen cells and batteries
Nickel-hydrogen cells and, more recently, bipolar batteries have been built by a variety of organizations. The design principles that have been used by the technology group at the NASA Lewis Research Center draw upon their extensive background in separator technology, alkaline fuel cell technology, and several alkaline cell technology areas. These design principles have been incorporated into both the more contemporary individual pressure vessel (IPV) designs that were pioneered by other groups, as well as the more recent bipolar battery designs using active cooling that are being developed at NASA Lewis Research Center and under contract. These principles are rather straightforward applications of capillary force formalisms, coupled with the slowly developing data base resulting from careful post test analyses. The objective of this overall effort is directed towards the low-Earth-orbit (LEO) application where the cycle life requirements are much more severe than the geosynchronous-orbit (GEO) application. A summary of the design principles employed is presented along with a discussion of the recommendations for component pore sizes and pore size distributions, as well as suggested materials of construction. These will be made based on our experience in these areas to show how these design principles have been translated into operating hardware
Trapped ion emulation of electric dipole moment of neutral relativistic particles
The electric dipole moments of various neutral elementary particles, such as
neutron, neutrinos, certain hypothetical dark matter particles and others, are
predicted to exist by the standard model of high energy physics and various
extensions of it. However, the predicted values are beyond the present
experimental capabilities. We propose to simulate and emulate the electric
dipole moment of neutral relativistic particles and the ensuing effects in the
presence of electrostatic field by emulation of an extended Dirac equation in
ion traps
States of the Dirac equation in confining potentials
We study the Dirac equation in confining potentials with pure vector
coupling, proving the existence of metastable states with longer and longer
lifetimes as the non-relativistic limit is approached and eventually merging
with continuity into the Schr\"odinger bound states. We believe that the
existence of these states could be relevant in high energy model construction
and in understanding possible resonant scattering effects in systems like
Graphene. We present numerical results for the linear and the harmonic cases
and we show that the the density of the states of the continuous spectrum is
well described by a sum of Breit-Wigner lines. The width of the line with
lowest positive energy, as expected, reproduces very well the Schwinger pair
production rate for a linear potential: we thus suggest a different way of
obtaining informations on the pair production in unbounded, non uniform
electric fields, where very little is known.Comment: 4 page
Symmetry Breaking of Relativistic Multiconfiguration Methods in the Nonrelativistic Limit
The multiconfiguration Dirac-Fock method allows to calculate the state of
relativistic electrons in atoms or molecules. This method has been known for a
long time to provide certain wrong predictions in the nonrelativistic limit. We
study in full mathematical details the nonlinear model obtained in the
nonrelativistic limit for Be-like atoms. We show that the method with sp+pd
configurations in the J=1 sector leads to a symmetry breaking phenomenon in the
sense that the ground state is never an eigenvector of L^2 or S^2. We thereby
complement and clarify some previous studies.Comment: Final version, to appear in Nonlinearity. Nonlinearity (2010) in
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Zitterbewegung of relativistic electrons in a magnetic field and its simulation by trapped ions
One-electron 3+1 and 2+1 Dirac equations are used to calculate the motion of
a relativistic electron in a vacuum in the presence of an external magnetic
field. First, calculations are carried on an operator level and exact
analytical results are obtained for the electron trajectories which contain
both intraband frequency components, identified as the cyclotron motion, as
well as interband frequency components, identified as the trembling motion
(Zitterbewegung, ZB). Next, time-dependent Heisenberg operators are used for
the same problem to compute average values of electron position and velocity
employing Gaussian wave packets. It is shown that the presence of a magnetic
field and the resulting quantization of the energy spectrum has pronounced
effects on the electron Zitterbewegung: it introduces intraband frequency
components into the motion, influences all the frequencies and makes the motion
stationary (not decaying in time) in case of the 2+1 Dirac equation. Finally,
simulations of the 2+1 Dirac equation and the resulting electron ZB in the
presence of a magnetic field are proposed and described employing trapped ions
and laser excitations. Using simulation parameters achieved in recent
experiments of Gerritsma and coworkers we show that the effects of the
simulated magnetic field on ZB are considerable and can certainly be observed.Comment: 19 pages, 9 figures, published versio
The Dirac equation without spinors
In the first part of the paper we give a tensor version of the Dirac
equation. In the second part we formulate and analyse a simple model equation
which for weak external fields appears to have properties similar to those of
the 2--dimensional Dirac equation.Comment: 20 pages. Submitted for publication in the proceedings of the
conference `Functional analysis, partial differential equations and
applications', Rostock (Germany) 31 August--4 September 199
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