3,683 research outputs found
Spacecraft drag-free technology development: On-board estimation and control synthesis
Estimation and control methods for a Drag-Free spacecraft are discussed. The functional and analytical synthesis of on-board estimators and controllers for an integrated attitude and translation control system is represented. The framework for detail definition and design of the baseline drag-free system is created. The techniques for solution of self-gravity and electrostatic charging problems are applicable generally, as is the control system development
Minimal PD-sets for codes associated with the graphs Qm2, m even
Please read abstract in the article.The National Research Foundation of South Africahttp://link.springer.com/journal/2002021-12-08hj2021Mathematics and Applied Mathematic
Helicopter simulation validation using flight data
A joint NASA/Army effort to perform a systematic ground-based piloted simulation validation assessment is described. The best available mathematical model for the subject helicopter (UH-60A Black Hawk) was programmed for real-time operation. Flight data were obtained to validate the math model, and to develop models for the pilot control strategy while performing mission-type tasks. The validated math model is to be combined with motion and visual systems to perform ground based simulation. Comparisons of the control strategy obtained in flight with that obtained on the simulator are to be used as the basis for assessing the fidelity of the results obtained in the simulator
Fast-ignition design transport studies: realistic electron source, integrated PIC-hydrodynamics, imposed magnetic fields
Transport modeling of idealized, cone-guided fast ignition targets indicates
the severe challenge posed by fast-electron source divergence. The hybrid
particle-in-cell [PIC] code Zuma is run in tandem with the
radiation-hydrodynamics code Hydra to model fast-electron propagation, fuel
heating, and thermonuclear burn. The fast electron source is based on a 3D
explicit-PIC laser-plasma simulation with the PSC code. This shows a quasi
two-temperature energy spectrum, and a divergent angle spectrum (average
velocity-space polar angle of 52 degrees). Transport simulations with the
PIC-based divergence do not ignite for > 1 MJ of fast-electron energy, for a
modest 70 micron standoff distance from fast-electron injection to the dense
fuel. However, artificially collimating the source gives an ignition energy of
132 kJ. To mitigate the divergence, we consider imposed axial magnetic fields.
Uniform fields ~50 MG are sufficient to recover the artificially collimated
ignition energy. Experiments at the Omega laser facility have generated fields
of this magnitude by imploding a capsule in seed fields of 50-100 kG. Such
imploded fields are however more compressed in the transport region than in the
laser absorption region. When fast electrons encounter increasing field
strength, magnetic mirroring can reflect a substantial fraction of them and
reduce coupling to the fuel. A hollow magnetic pipe, which peaks at a finite
radius, is presented as one field configuration which circumvents mirroring.Comment: 16 pages, 17 figures, submitted to Phys. Plasma
Cigarette smoking and urinary oestrogen excretion in premenopausal and post-menopausal women.
Cigarette smoking is associated with a reduction in the risk for endometrial cancer in post-menopausal women and it has been suggested that this is because smoking has an anti-oestrogenic effect. To investigate this, concentrations of oestrone, oestradiol and oestriol were measured in 24 h urine samples from 167 premenopausal women (53 smokers, 114 non-smokers) and 200 post-menopausal women (54 smokers, 146 non-smokers). Among premenopausal women there were no significant differences in oestrogen excretion between smokers and non-smokers. Among post-menopausal women, geometric mean excretion rates for oestrone and oestradiol did not differ significantly between groups, but oestriol excretion was 19% lower (95% confidence interval -34% to -1%) in smokers than in non-smokers. This may partly explain the reduced risk for endometrial cancer among post-menopausal smokers
Quantum Logic for Trapped Atoms via Molecular Hyperfine Interactions
We study the deterministic entanglement of a pair of neutral atoms trapped in
an optical lattice by coupling to excited-state molecular hyperfine potentials.
Information can be encoded in the ground-state hyperfine levels and processed
by bringing atoms together pair-wise to perform quantum logical operations
through induced electric dipole-dipole interactions. The possibility of
executing both diagonal and exchange type entangling gates is demonstrated for
two three-level atoms and a figure of merit is derived for the fidelity of
entanglement. The fidelity for executing a CPHASE gate is calculated for two
87Rb atoms, including hyperfine structure and finite atomic localization. The
main source of decoherence is spontaneous emission, which can be minimized for
interaction times fast compared to the scattering rate and for sufficiently
separated atomic wavepackets. Additionally, coherent couplings to states
outside the logical basis can be constrained by the state dependent trapping
potential.Comment: Submitted to Physical Review
A Storage Ring for Neutral Atoms
We have demonstrated a storage ring for ultra-cold neutral atoms. Atoms with
mean velocities of 1 m/s corresponding to kinetic energies of ~100 neV are
confined to a 2 cm diameter ring by magnetic forces produced by two
current-carrying wires. Up to 10^6 atoms are loaded at a time in the ring, and
7 revolutions are clearly observed. Additionally, we have demonstrated multiple
loading of the ring and deterministic manipulation of the longitudinal velocity
distribution of the atoms using applied laser pulses. Applications of this ring
include large area atom interferometers and cw monochromatic atomic beam
generation.Comment: 4 pages, 5 figure
Ground state properties of a one-dimensional condensate of hard core bosons in a harmonic trap
The exact N-particle ground state wave function for a one-dimensional
condensate of hard core bosons in a harmonic trap is employed to obtain
accurate numerical results for the one-particle density matrix, occupation
number distribution of the natural orbitals, and momentum distribution. Our
results show that the occupation of the lowest orbital varies as N^{0.59}, in
contrast to N^{0.5} for a spatially uniform system, and N for a true BEC.Comment: 10 pages, 6 figures, submitted to Phys. Rev.
Propagation of Bose-Einstein condensates in a magnetic waveguide
Gaseous Bose-Einstein condensates of 2-3 million atoms were loaded into a
microfabricated magnetic trap using optical tweezers. Subsequently, the
condensates were released into a magnetic waveguide and propagated 12 mm.
Single-mode propagation was observed along homogeneous segments of the
waveguide. Inhomogeneities in the guiding potential arose from geometric
deformations of the microfabricated wires and caused strong transverse
excitations. Such deformations may restrict the waveguide physics that can be
explored with propagating condensates.Comment: 5 pages, 4 figure
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