1,858 research outputs found
Measurements of the cosmic background radiation
Maps of the large scale structure (theta is greater than 6 deg) of the cosmic background radiation covering 90 percent of the sky are now available. The data show a very strong 50-100 sigma (statistical error) dipole component, interpreted as being due to our motion, with a direction of alpha = 11.5 + or - 0.15 hours, sigma = -5.6 + or - 2.0 deg. The inferred direction of the velocity of our galaxy relative to the cosmic background radiation is alpha = 10.6 + or - 0.3 hours, sigma = -2.3 + or - 5 deg. This is 44 deg from the center of the Virgo cluster. After removing the dipole component, the data show a galactic signature but no apparent residual structure. An autocorrelation of the residual data, after substraction of the galactic component from a combined Berkeley (3 mm) and Princeton (12 mm) data sets, show no apparent structure from 10 to 180 deg with a rms of 0.01 mK(sup 2). At 90 percent confidence level limit of .00007 is placed on a quadrupole component
Global Distribution of Water Vapor and Cloud Cover--Sites for High Performance THz Applications
Absorption of terahertz radiation by atmospheric water vapor is a serious
impediment for radio astronomy and for long-distance communications.
Transmission in the THz regime is dependent almost exclusively on atmospheric
precipitable water vapor (PWV). Though much of the Earth has PWV that is too
high for good transmission above 200 GHz, there are a number of dry sites with
very low attenuation. We performed a global analysis of PWV with
high-resolution measurements from the Moderate Resolution Imaging Spectrometer
(MODIS) on two NASA Earth Observing System (EOS) satellites over the year of
2011. We determined PWV and cloud cover distributions and then developed a
model to find transmission and atmospheric radiance as well as necessary
integration times in the various windows. We produced global maps over the
common THz windows for astronomical and satellite communications scenarios.
Notably, we show that up through 1 THz, systems could be built in excellent
sites of Chile, Greenland and the Tibetan Plateau, while Antarctic performance
is good to 1.6 THz. For a ground-to-space communication link up through 847
GHz, we found several sites in the Continental United States where mean
atmospheric attenuation is less than 40 dB; not an insurmountable challenge for
a link.Comment: 15 pages, 23 figure
QSO 0130-4021: A third QSO showing a low Deuterium to Hydrogen Abundance Ratio
We have discovered a third quasar absorption system which is consistent with
a low deuterium to hydrogen abundance ratio, D/H = 3.4 times 10^-5. The z ~ 2.8
partial Lyman limit system towards QSO 0130-4021 provides the strongest
evidence to date against large D/H ratios because the H I absorption, which
consists of a single high column density component with unsaturated high order
Lyman series lines, is readily modeled -- a task which is more complex in other
D/H systems. We have obtained twenty-two hours of spectra from the HIRES
spectrograph on the W.M. Keck telescope, which allow a detailed description of
the Hydrogen. We see excess absorption on the blue wing of the H I Lyman alpha
line, near the expected position of Deuterium. However, we find that Deuterium
cannot explain all of the excess absorption, and hence there must be
contamination by additional absorption, probably H I. This extra H I can
account for most or all of the absorption at the D position, and hence D/H = 0
is allowed. We find an upper limit of D/H < 6.7 times 10^-5 in this system,
consistent with the value of D/H ~ 3.4 times 10^-5 deduced towards QSO
1009+2956 and QSO 1937-1009 by Burles and Tytler (1998a, 1998b). This
absorption system shows only weak metal line absorption, and we estimate [Si/H]
< -2.6 -- indicating that the D/H ratio of the system is likely primordial. All
four of the known high redshift absorption line systems simple enough to
provide useful limits on D are consistent with D/H = 3.4 +/- 0.25 times 10^-5.
Conversely, this QSO provides the third case which is inconsistent with much
larger values.Comment: 18 pages, 5 figures, submitted to Ap
Galois covers of the open p-adic disc
This paper investigates Galois branched covers of the open -adic disc and
their reductions to characteristic . Using the field of norms functor of
Fontaine and Wintenberger, we show that the special fiber of a Galois cover is
determined by arithmetic and geometric properties of the generic fiber and its
characteristic zero specializations. As applications, we derive a criterion for
good reduction in the abelian case, and give an arithmetic reformulation of the
local Oort Conjecture concerning the liftability of cyclic covers of germs of
curves.Comment: 19 pages; substantial organizational and expository changes; this is
the final version corresponding to the official publication in Manuscripta
Mathematica; abstract update
Analytical Model for the Magnetic Field Distribution in a Flux Modulation Superconducting Machine
International audienceThis paper presents a theoretical analysis of an axial field machine using High Temperature Superconductors (HTS) wires and bulks. The air-gap magnetic field obtained with the HTS coil and modulated by the HTS bulks is predicted by two 2D axisymmetric models. Analytical models are based on the solution of Laplace's equation by the separation of variable method. The torque is obtained by a quick numerical integration of the Laplace force that acts on the armature winding. The proposed model is compared with 3D finite element simulations and good agreement is obtained. This model can be used with an optimization design procedure with a large reduction of the computational time
Construction and Test of a Flux Modulation Superconducting Machine for Aircraft
International audienceThe increasing of drives towards More Electric Aircraft (MEA) or the development of electric propulsion aircraft calls for MW-class electrical machines with more compact and power dense designs. One way is to explore the use of superconducting materials to create a high magnetic field in order to reduce the mass of ferromagnetic components. This paper presents the construction and the test of a brushless axial flux superconducting machine. The brushless topology satisfies the aeronautics industry requirements in terms of maintenance, while the axial configuration ensures an optimal use of the anisotropic HTS tapes. The machine is classed as partially superconducting, only the inductor is made with superconducting materials. A special design concerning the use of a stationary cryostat is presented. This improvement reduces significantly the electromagnetic air-gap length. A 50kW prototype is manufactured with a minimal mass objective. The prototype constitutes a first step to a scale-up MW-class machine design
Resolving the emission transition dipole moments of single doubly-excited seeded nanorods via heralded defocused imaging
Semiconductor nanocrystal emission polarization is a crucial probe of
nanocrystal physics and an essential factor for nanocrystal-based technologies.
While the transition dipole moment of the lowest excited state to ground state
transition is well characterized, the dipole moment of higher multiexcitonic
transitions is inaccessible via most spectroscopy techniques. Here, we realize
direct characterization of the doubly-excited state relaxation transition
dipole by heralded defocused imaging. Defocused imaging maps the dipole
emission pattern onto a fast single-photon avalanche diode detector array,
allowing the post-selection of photon pairs emitted from the biexciton-exciton
emission cascade and resolving the differences in transition dipole moments.
Type-I1/2 seeded nanorods exhibit higher anisotropy of the biexciton-to-exciton
transition compared to the exciton-to-ground state transition. In contrast,
type-II seeded nanorods display a reduction of biexciton emission anisotropy.
These findings are rationalized in terms of an interplay between transient
dynamics of the refractive index and the excitonic fine structure
A Spin Modulated Telescope to Make Two Dimensional CMB Maps
We describe the HEMT Advanced Cosmic Microwave Explorer (HACME), a balloon
borne experiment designed to measure sub-degree scale Cosmic Microwave
Background anisotropy over hundreds of square degrees, using a unique two
dimensional scanning strategy. A spinning flat mirror that is canted relative
to its spin axis modulates the direction of beam response in a nearly
elliptical path on the sky. The experiment was successfully flown in February
of 1996, achieving near laboratory performance for several hours at float
altitude. A map free of instrumental systematic effects is produced for a 3.5
hour observation of 630 square degrees, resulting in a flat band power upper
limit of (l(l+1)C_l/2 pi)^0.5 < 77 microK at l = 38 (95% confidence). The
experiment design, flight operations and data, including atmospheric effects
and noise performance, are discussed.Comment: 4 pages, 3 figure
Magnetically geared induction machines
A wound-rotor induction machine is artfully coupled to a magnetic gear to achieve a high-torque-density drive system called magnetically geared induction machine (MaGIM). The high-speed rotor of MaGIM is common to both the machine and gear sides. A rotating diode rectifier electrically links the machine's wound rotor and a dc boost winding on the gear side to increase the torque-transmission capabilities of the overall system. The first investigations on a 100 kW-120 r/min MaGIM are promising, since an increase in torque of ∼ 15% could be obtained by inserting the diode rectifier. For fixed speed applications, this induction-machine-based system can be directly supplied from the main
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