363 research outputs found
The fabrication and surface tolerance measurements of the JPL clear aperture microwave antenna
Present ground station microwave antennas of the Deep Space Network are of the symmetric dual reflector (cassegrainian) type. An investigation is being made of alternative high-performance offset antenna designs which have a clear aperture (no reflector or structural blockage) with shaped reflector surfaces. A 1.5-m, 32-GHz clear aperture model was built for experimental studies. The unique processes of fabrication, surface measurement, and alignment are described
Floatation of Tunnel in Liquefiable Soil
Underground structures such as tunnels have a lower unit weight than the surrounding soil and are commonly deemed to be susceptible to floatation in liquefiable soil. In the process of floatation, the tunnel has to possess ample buoyancy force to shear and carry the overlying soil upwards. This is aided by soil liquefaction resulting from the increase in water pressure with number of earthquake loading cycles. With onset of liquefaction, effective stress decreases which lead to a reduction in the shear strength of soil, hence assisting the floatation of tunnel. Conversely, the total stress exerted by the overburden soil suppresses the process. A series of centrifuge tests were conducted to investigate the floatation of tunnels in liquefiable sand deposits. This paper discusses the initiation and cessation of the floatation as well as the floatation susceptibility of varying depths of tunnels
Nonintegrable Interaction of Ion-Acoustic and Electromagnetic Waves in a Plasma
In this paper we re-examine the one-dimensional interaction of
electromagnetic and ion acoustic waves in a plasma. Our model is similar to one
solved by Rao et al. (Phys. Fluids, vol. 26, 2488 (1983)) under a number of
analytical approximations. Here we perform a numerical investigation to examine
the stability of the model. We find that for slightly over dense plasmas, the
propagation of stable solitary modes can occur in an adiabatic regime where the
ion acoustic electric field potential is enslaved to the electromagnetic field
of a laser. But if the laser intensity or plasma density increases or the laser
frequency decreases, the adiabatic regime loses stability via a transition to
chaos. New asymptotic states are attained when the adiabatic regime no longer
exists. In these new states, the plasma becomes rarefied, and the laser field
tends to behave like a vacuum field.Comment: 19 pages, REVTeX, 6 ps figures, accepted for publication in Phys.
Rev.
Intermittent chaos driven by nonlinear Alfvén waves
International audienceWe investigate the relevance of chaotic saddles and unstable periodic orbits at the onset of intermittent chaos in the phase dynamics of nonlinear Alfvén waves by using the Kuramoto-Sivashinsky (KS) equation as a model for phase dynamics. We focus on the role of nonattracting chaotic solutions of the KS equation, known as chaotic saddles, in the transition from weak chaos to strong chaos via an interior crisis and show how two of these unstable chaotic saddles can interact to produce the plasma intermittency observed in the strongly chaotic regimes. The dynamical systems approach discussed in this work can lead to a better understanding of the mechanisms responsible for the phenomena of intermittency in space plasmas
A model of CP Violation from Extra Dimension
We construct a realistic model of CP violation in which CP is broken in the
process of dimensional reduction and orbifold compactification from a five
dimensional theories with gauge symmetry. CP
violation is a result of the Hosotani type gauge configuration in the higher
dimension.Comment: 5 page
Lagrangian chaos in an ABC--forced nonlinear dynamo
The Lagrangian properties of the velocity field in a magnetized fluid are
studied using three-dimensional simulations of a helical magnetohydrodynamic
dynamo. We compute the attracting and repelling Lagrangian coherent structures,
which are dynamic lines and surfaces in the velocity field that delineate
particle transport in flows with chaotic streamlines and act as transport
barriers. Two dynamo regimes are explored, one with a robust coherent mean
magnetic field and one with intermittent bursts of magnetic energy. The
Lagrangian coherent structures and the statistics of finite--time Lyapunov
exponents indicate that the stirring/mixing properties of the velocity field
decay as a linear function of the magnetic energy. The relevance of this study
for the solar dynamo problem is discussed
Flux dependent MeV self-ion- induced effects on Au nanostructures: Dramatic mass transport and nano-silicide formation
We report a direct observation of dramatic mass transport due to 1.5 MeV Au2+
ion impact on isolated Au nanostructures of an average size 7.6 nm and a height
6.9 nm that are deposited on Si (111) substrate under high flux (3.2x10^10 to
6.3x10^12 ions cm-2 s-1) conditions. The mass transport from nanostructures
found to extend up to a distance of about 60 nm into the substrate, much beyond
their size. This forward mass transport is compared with the recoil
implantation profiles using SRIM simulation. The observed anomalies with theory
and simulations are discussed. At a given energy, the incident flux plays a
major role in mass transport and its re-distribution. The mass transport is
explained on the basis of thermal effects and creation of rapid diffusion paths
at nano-scale regime during the course of ion irradiation. The unusual mass
transport is found to be associated with the formation of gold silicide
nanoalloys at sub-surfaces. The complexity of the ion-nanostructure interaction
process has been discussed with a direct observation of melting (in the form of
spherical fragments on the surface) phenomena. The transmission electron
microscopy, scanning transmission electron microscopy and Rutherford
backscattering spectroscopy methods have been used.Comment: 16 pages, 6 Figure
KAOSS: turbulent, but disc-like kinematics in dust-obscured star-forming galaxies at z ∼ 1.3–2.6
We present spatially resolved kinematics of 27 ALMA-identified dust-obscured star-forming galaxies (DSFGs) at z ∼ 1.3–2.6, as traced by Hα emission using VLT/KMOS near-infrared integral field spectroscopy from the ‘KMOS-ALMA Observations of Submillimetre Sources’ (KAOSS) Large Programme. We derive Hα rotation curves and velocity dispersion profiles for the DSFGs, and find that among the 27 sources with bright, spatially extended Hα emission, 24 display evidence for disc-like kinematics. We measure a median inclination-corrected velocity at 2.2 Rd of vrot = 190 ± 40 km s−1 and intrinsic velocity dispersion of σ0 = 87 ± 6 km s−1 for these disc-like sources. The kinematics yield median circular velocities of vcirc = 230 ± 20 km s−1 and dynamical masses within 2Re (∼ 7 kpc radius) of Mdyn = (1.1 ± 0.2) × 1011 M⊙. Compared to less actively star-forming galaxies, KAOSS DSFGs are both faster rotating with higher intrinsic velocity dispersions, but have similar vrot/σ0 ratios, median v/σ0 = 2.5 ± 0.5. We suggest that the kinematics of the DSFGs are primarily rotation supported but with a non-negligible contribution from pressure support, which may be driven by star formation or mergers/interactions. We estimate the normalization of the stellar mass Tully–Fisher relation (sTFR) for the disc-like DSFGs and compare it with local studies, finding no evolution at fixed slope between z ∼ 2 and z ∼ 0. Finally, we show that the kinematic properties of the DSFG population are consistent with them evolving into massive early-type galaxies, the dominant z ∼ 0 population at these masses
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