10,405 research outputs found
Unstable coronal loops : numerical simulations with predicted observational signatures
We present numerical studies of the nonlinear, resistive magnetohydrodynamic
(MHD) evolution of coronal loops. For these simulations we assume that the
loops carry no net current, as might be expected if the loop had evolved due to
vortex flows. Furthermore the initial equilibrium is taken to be a cylindrical
flux tube with line-tied ends. For a given amount of twist in the magnetic
field it is well known that once such a loop exceeds a critical length it
becomes unstableto ideal MHD instabilities. The early evolution of these
instabilities generates large current concentrations. Firstly we show that
these current concentrations are consistent with the formation of a current
sheet. Magnetic reconnection can only occur in the vicinity of these current
concentrations and we therefore couple the resistivity to the local current
density. This has the advantage of avoiding resistive diffusion in regions
where it should be negligible. We demonstrate the importance of this procedure
by comparison with simulations based on a uniform resistivity. From our
numerical experiments we are able to estimate some observational signatures for
unstable coronal loops. These signatures include: the timescale of the loop
brightening; the temperature increase; the energy released and the predicted
observable flow speeds. Finally we discuss to what extent these observational
signatures are consistent with the properties of transient brightening loops.Comment: 13 pages, 9 figure
A test of the CPL parameterization for rapid dark energy equation of state transitions
We test the robustness and flexibility of the Chevallier-Polarski-Linder
(CPL) parameterization of the Dark Energy equation of state in recovering a four-parameter step-like fiducial model. We
constrain the parameter space region of the underlying fiducial model where the
CPL parameterization offers a reliable reconstruction. It turns out that non
negligible biases leak into the results for recent () rapid transitions,
but that CPL yields a good reconstruction in all other cases. The presented
analysis is performed with supernova Ia data as forecasted for a space mission
like SNAP/JDEM, combined with future expectations for the CMB shift parameter
and the BAO parameter .Comment: 8 pages, 6 ps figure
On local invariants of pure three-qubit states
We study invariants of three-qubit states under local unitary
transformations, i.e. functions on the space of entanglement types, which is
known to have dimension 6. We show that there is no set of six independent
polynomial invariants of degree less than or equal to 6, and find such a set
with maximum degree 8. We describe an intrinsic definition of a canonical state
on each orbit, and discuss the (non-polynomial) invariants associated with it.Comment: LateX, 13 pages. Minor typoes corrected. Published versio
Redox reactions with empirical potentials: Atomistic battery discharge simulations
Batteries are pivotal components in overcoming some of today's greatest
technological challenges. Yet to date there is no self-consistent atomistic
description of a complete battery. We take first steps toward modeling of a
battery as a whole microscopically. Our focus lies on phenomena occurring at
the electrode-electrolyte interface which are not easily studied with other
methods. We use the redox split-charge equilibration (redoxSQE) method that
assigns a discrete ionization state to each atom. Along with exchanging partial
charges across bonds, atoms can swap integer charges. With redoxSQE we study
the discharge behavior of a nano-battery, and demonstrate that this reproduces
the generic properties of a macroscopic battery qualitatively. Examples are the
dependence of the battery's capacity on temperature and discharge rate, as well
as performance degradation upon recharge.Comment: 14 pages, 10 figure
Optimal Entanglement Enhancement for Mixed States
We consider the actions of protocols involving local quantum operations and
classical communication (LQCC) on a single system consisting of two separated
qubits. We give a complete description of the orbits of the space of states
under LQCC and characterise the representatives with maximal entanglement of
formation. We thus obtain a LQCC entanglement concentration protocol for a
single given state (pure or mixed) of two qubits which is optimal in the sense
that the protocol produces, with non-zero probability, a state of maximal
possible entanglement of formation. This defines a new entanglement measure,
the maximum extractable entanglement.Comment: Final version: to appear in Phys. Rev. Let
Classification of n-qubit states with minimum orbit dimension
The group of local unitary transformations acts on the space of n-qubit pure
states, decomposing it into orbits. In a previous paper we proved that a
product of singlet states (together with an unentangled qubit for a system with
an odd number of qubits) achieves the smallest possible orbit dimension, equal
to 3n/2 for n even and (3n + 1)/2 for n odd, where n is the number of qubits.
In this paper we show that any state with minimum orbit dimension must be of
this form, and furthermore, such states are classified up to local unitary
equivalence by the sets of pairs of qubits entangled in singlets.Comment: 15 pages, latex, revision 2, conclusion added, some proofs shortene
Cold trapped atoms detected with evanescent waves
We demonstrate the in situ detection of cold 87 Rb atoms near a dielectric
surface using the absorption of a weak, resonant evanescent wave. We have used
this technique in time of flight experiments determining the density of atoms
falling on the surface. A quantitative understanding of the measured curve was
obtained using a detailed calculation of the evanescent intensity distribution.
We have also used it to detect atoms trapped near the surface in a
standing-wave optical dipole potential. This trap was loaded by inelastic
bouncing on a strong, repulsive evanescent potential. We estimate that we trap
1.5 x 10 4 atoms at a density 100 times higher than the falling atoms.Comment: 5 pages, 3 figure
Surge Detection In An Industrial Axial Flow Compressor.
LecturePg. 83-88Compressor surge can be a disruptive and potentially destructive phenomena in turbocompressors. A surge detection scheme utilized in shop and field testing of a 29,600 hp (22 MW) industrial axial compressor is described. Dynamic pressure transducers were installed in the compressor casing to monitor rotor blade-to-blade dynamic pressure changes indicative of rotating stall cells. Rotor shaft radial vibration was also monitored to correlate with the occurrence of surge. The onset of surge could clearly be identified in both the shop performance testing and actual field operation. Low frequency, subsynchronous stall cells were detected prior to an actual flow breakdown and flow reversal. The detection of incipient surge allowed corrective measures to be taken to prevent the compressor from surging. The instrumentation used to detect incipient surge and the results of the compressor testing are described
- …