12,723 research outputs found
Some NASA contributions to the use of plasma jet technology in chemical processing
NASA contributions to use of plasma jet technology in chemical processin
Space-Varying Coefficient Models for Brain Imaging
The methodological development and the application in this paper originate from diffusion tensor imaging (DTI), a powerful nuclear magnetic resonance technique enabling diagnosis and monitoring of several diseases as well as reconstruction of neural pathways. We reformulate the current analysis framework of separate voxelwise regressions as a 3d space-varying coefficient model (VCM) for the entire set of DTI images recorded on a 3d grid of voxels. Hence by allowing to borrow strength from spatially adjacent voxels, to smooth noisy observations, and to estimate diffusion tensors at any location within the brain, the three-step cascade of standard data processing is overcome simultaneously. We conceptualize two VCM variants based on B-spline basis functions: a full tensor product approach and a sequential approximation, rendering the VCM numerically and computationally feasible even for the huge dimension of the joint model in a realistic setup. A simulation study shows that both approaches outperform the standard method of voxelwise regressions with subsequent regularization. Due to major efficacy, we apply the sequential method to a clinical DTI data set and demonstrate the inherent ability of increasing the rigid grid resolution by evaluating the incorporated basis functions at intermediate points. In conclusion, the suggested fitting methods clearly improve the current state-of-the-art, but ameloriation of local adaptivity remains desirable
Convective line shifts for the Gaia RVS from the CIFIST 3D model atmosphere grid
To derive space velocities of stars along the line of sight from wavelength
shifts in stellar spectra requires accounting for a number of second-order
effects. For most stars, gravitational redshifts, convective blueshifts, and
transverse stellar motion are the dominant contributors. We provide theoretical
corrections for the net velocity shifts due to convection expected for the
measurements from the Gaia Radial Velocity Spectrometer (RVS). We used a set of
three-dimensional time-dependent simulations of stellar surface convection
computed with CO5BOLD to calculate spectra of late-type stars in the Gaia RVS
range and to infer the net velocity offset that convective motions will induce
in radial velocities derived by cross-correlation. The net velocity shifts
derived by cross-correlation depend both on the wavelength range and spectral
resolution of the observations. Convective shifts for Gaia RVS observations are
less than 0.1 km/s for late-K-type stars, and they increase with stellar mass,
reaching about 0.3 km/s or more for early F-type dwarfs. This tendency is the
result of an increase with effective temperature in both temperature and
velocity fluctuations in the line-forming region. Our simulations also indicate
that the net RVS convective shifts can be positive (i.e. redshifts) in some
cases. Overall, the blueshifts weaken slightly with increasing surface gravity,
and are enhanced at low metallicity. Gravitational redshifts amount up to 0.7
km/s and dominate convective blueshifts for dwarfs, but become much weaker for
giants.Comment: 13 pages, to appear in A&A; model fluxes available from
ftp://leda.as.utexas.edu/pub/callende/Gaia3D and soon from CD
Accounting for Convective Blue-Shifts in the Determination of Absolute Stellar Radial Velocities
For late-type non-active stars, gravitational redshifts and convective
blueshifts are the main source of biases in the determination of radial
velocities. If ignored, these effects can introduce systematic errors of the
order of ~ 0.5 km/s. We demonstrate that three-dimensional hydrodynamical
simulations of solar surface convection can be used to predict the convective
blue-shifts of weak spectral lines in solar-like stars to ~ 0.070 km/s. Using
accurate trigonometric parallaxes and stellar evolution models, the
gravitational redshifts can be constrained with a similar uncertainty, leading
to absolute radial velocities accurate to better than ~ 0.1 km/s.Comment: To appear in the proceedings of the Joint Discussion 10, IAU General
Assembly, Rio de Janeiro, August 10-11, 200
Gas turbine ceramic-coated-vane concept with convection-cooled porous metal core
Analysis and flow experiments on a ceramic-coated-porous-metal vane concept indicated the feasibility, from a heat transfer standpoint, of operating in a high-temperature (2500 F) gas turbine cascade facility. The heat transfer and pressure drop calculations provided a basis for selecting the ceramic layer thickness (to 0.08 in.), which was found to be the dominant factor in the overall heat transfer coefficient. Also an approximate analysis of the heat transfer in the vane trailing edge revealed that with trailing-edge ejection the ceramic thickness could be reduced to (0.01 in.) in this portion of the vane
The Three-Nucleon System Near the N-d Threshold
The three-nucleon system is studied at energies a few hundred keV above the
N-d threshold. Measurements of the tensor analyzing powers and
for p-d elastic scattering at keV are presented
together with the corresponding theoretical predictions. The calculations are
extended to very low energies since they are useful for extracting the p-d
scattering lengths from the experimental data. The interaction considered here
is the Argonne V18 potential plus the Urbana three-nucleon potential. The
calculation of the asymptotic D- to S-state ratio for H and He, for
which recent experimental results are available, is also presented.Comment: Latex, 11 pages, 2 figures, to be published in Phy.Lett.
On a generalization of Jacobi's elliptic functions and the Double Sine-Gordon kink chain
A generalization of Jacobi's elliptic functions is introduced as inversions
of hyperelliptic integrals. We discuss the special properties of these
functions, present addition theorems and give a list of indefinite integrals.
As a physical application we show that periodic kink solutions (kink chains) of
the double sine-Gordon model can be described in a canonical form in terms of
generalized Jacobi functions.Comment: 18 pages, 9 figures, 3 table
Ground state of a confined Yukawa plasma
The ground state of an externally confined one-component Yukawa plasma is
derived analytically. In particular, the radial density profile is computed.
The results agree very well with computer simulations on three-dimensional
spherical Coulomb crystals. We conclude in presenting an exact equation for the
density distribution for a confinement potential of arbitrary geometry.Comment: 5 pages, 4 figure
Radio frequency pulsed-gate charge spectroscopy on coupled quantum dots
Time-resolved electron dynamics in coupled quantum dots is directly observed
by a pulsed-gate technique. While individual gate voltages are modulated with
periodic pulse trains, average charge occupations are measured with a nearby
quantum point contact as detector. A key component of our setup is a sample
holder optimized for broadband radio frequency applications. Our setup can
detect displacements of single electrons on time scales well below a
nanosecond. Tunneling rates through individual barriers and relaxation times
are obtained by using a rate equation model. We demonstrate the full
characterization of a tunable double quantum dot using this technique, which
could also be used for coherent charge qubit control
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