9,506 research outputs found
Rotational CARS application to simultaneous and multiple-point temperature and concentration determination in a turbulent flow
Coherent anti-Stokes Raman scattering (CARS) from the pure rotational Raman lines of N2 is employed to measure the instantaneous (approximately 10 ns) rotational temperature of N2 gas at room temperature and below with good spatial resolution (0.2 x 0.2 x 3.0 cu mm). A broad bandwidth dye laser is used to obtain the entire rotational spectrum from a single laser pulse; the CARS signal is then dispersed by a spectrograph and recorded on an optical multichannel analyzer. A best fit temperature is found in several seconds with the aid of a computer for each experimental spectrum by a least squares comparison with calculated spectra. The model used to calculate the theoretical spectra incorporates the temperature and pressure dependence of the pressure-broadened rotational Raman lines, includes the nonresonant background susceptibility, and assumes that the pump laser has a finite linewidth. Temperatures are fit to experimental spectra recorded over the temperature range of 135 to 296 K, and over the pressure range of .13 to 15.3 atm
Hyperfine characterization and coherence lifetime extension in Pr3+:La2(WO4)3
Rare-earth ions in dielectric crystals are interesting candidates for storing
quantum states of photons. A limiting factor on the optical density and thus
the conversion efficiency is the distortion introduced in the crystal by doping
elements of one type into a crystal matrix of another type. Here, we
investigate the system Pr3+:La2(WO4)3, where the similarity of the ionic radii
of Pr and La minimizes distortions due to doping. We characterize the
praseodymium hyperfine interaction of the ground state (3H4) and one excited
state (1D2) and determine the spin Hamiltonian parameters by numerical analysis
of Raman-heterodyne spectra, which were collected for a range of static
external magnetic field strengths and orientations. On the basis of a crystal
field analysis, we discuss the physical origin of the experimentally determined
quadrupole and Zeeman tensor characteristics. We show the potential for quantum
memory applications by measuring the spin coherence lifetime in a magnetic
field that is chosen such that additional magnetic fields do not shift the
transition frequency in first order. Experimental results demonstrate a spin
coherence lifetime of 158 ms - almost three orders of magnitude longer than in
zero field.Comment: 14 pages, 6 figure
Digital Predistortion in Large-Array Digital Beamforming Transmitters
In this article, we propose a novel digital predistortion (DPD) solution that
allows to considerably reduce the complexity resulting from linearizing a set
of power amplifiers (PAs) in single-user large-scale digital beamforming
transmitters. In contrast to current state-of-the art solutions that assume a
dedicated DPD per power amplifier, which is unfeasible in the context of large
antenna arrays, the proposed solution only requires a single DPD in order to
linearize an arbitrary number of power amplifiers. To this end, the proposed
DPD predistorts the signal at the input of the digital precoder based on
minimizing the nonlinear distortion of the combined signal at the intended
receiver direction. This is a desirable feature, since the resulting emissions
in other directions get partially diluted due to less coherent superposition.
With this approach, only a single DPD is required, yielding great complexity
and energy savings.Comment: 8 pages, Accepted for publication in Asilomar Conference on Signals,
Systems, and Computer
Mössbauer Spectrometry
Mössbauer spectrometry gives electronic, magnetic, and structural information from within
materials. A Mössbauer spectrum is an intensity of γ-ray absorption versus energy for a
specific resonant nucleus such as ^(57)Fe or ^(119)Sn. For one nucleus to emit a Îł-ray and a second
nucleus to absorb it with efficiency, both nuclei must be embedded in solids, a phenomenon
known as the “Mössbauer effect.” Mössbauer spectrometry looks at materials from the
“inside out,” where “inside” refers to the resonant nucleus.
Mössbauer spectra give quantitative information on “hyperfine interactions,” which are small
energies from the interaction between the nucleus and its neighboring electrons. The three
hyperfine interactions originate from the electron density at the nucleus (the isomer shift),
the gradient of the electric field (the nuclear quadrupole splitting), and the unpaired electron
density at the nucleus (the hyperfine magnetic field). Over the years, methods have been
refined for using these three hyperfine interactions to determine valence and spin at the
resonant atom. Even when the hyperfine interactions are not easily interpreted, they can
often be used reliably as “fingerprints” to identify the different local chemical environments
of the resonant atom, usually with a good estimate of their fractional abundances. Mössbauer
spectrometry is useful for quantitative phase analyses or determinations of the concentrations
of resonant element in different phases, even when the phases are nanostructured or
amorphous.
Most Mössbauer spectra are acquired with simple laboratory equipment and a radioisotope
source, but the recent development of synchrotron instrumentation now allow for measurements
on small 10 µm samples, which may be exposed to extreme environments of pressure
and temperature. Other capabilities include measurements of the vibrational spectra of the
resonant atoms, and coherent scattering and diffraction of nuclear radiation.
This article is not a review of the field, but an instructional reference that explains principles
and practices, and gives the working materials scientist a basis for evaluating whether or not
Mössbauer spectrometry may be useful for a research problem. A few representative
materials studies are presented
A turn-key Concept for active cancellation of Global Positioning System L3 Signal
We present a concept, developed at the National Astronomy and Ionosphere
Center (NAIC) at Arecibo, Puerto Rico, for active suppression of Global
Positioning System (GPS) signals in the 305 m dish radio receiver path prior to
backend processing. The subsystem does not require an auxiliary antenna and is
intended for easy integration with radio telescope systems with a goal of being
a turnkey addition to virtually any facility. Working with actual sampled
signal data, we have focused on the detection and cancellation of the GPS L3
signal at 1381.05 MHz which, during periodic test modes and particularly during
system-wide tests, interfere with observations of objects in a range of
redshifts that includes the Coma supercluster, for example. This signal can
dynamically change modulation modes and our scheme is capable of detecting
these changes and applying cancellation or sending a blanking signal, as
appropriate. The subsystem can also be adapted to GPS L1 (1575.42 MHz), L2C
(1227.6 MHz), and others. A follow-up is underway to develop a prototype to
deploy and evaluate at NAIC.Comment: Presented at the RFI mitigation workshop, 29-31 March 2010,
Groningen, the Netherlands. Accepted for publication by the Proceedings of
Scienc
A proposed study of multiple scattering through clouds up to 1 THz
A rigorous computation of the electromagnetic field scattered from an atmospheric liquid water cloud is proposed. The recent development of a fast recursive algorithm (Chew algorithm) for computing the fields scattered from numerous scatterers now makes a rigorous computation feasible. A method is presented for adapting this algorithm to a general case where there are an extremely large number of scatterers. It is also proposed to extend a new binary PAM channel coding technique (El-Khamy coding) to multiple levels with non-square pulse shapes. The Chew algorithm can be used to compute the transfer function of a cloud channel. Then the transfer function can be used to design an optimum El-Khamy code. In principle, these concepts can be applied directly to the realistic case of a time-varying cloud (adaptive channel coding and adaptive equalization). A brief review is included of some preliminary work on cloud dispersive effects on digital communication signals and on cloud liquid water spectra and correlations
Fast Spectral Variability from Cygnus X-1
We have developed an algorithm that, starting from the observed properties of
the X-ray spectrum and fast variability of an X-ray binary allows the
production of synthetic data reproducing observables such as power density
spectra and time lags, as well as their energy dependence. This allows to
reconstruct the variability of parameters of the energy spectrum and to reduce
substantially the effects of Poisson noise, allowing to study fast spectral
variations. We have applied the algorithm to Rossi X-ray Timing Explorer data
of the black-hole binary Cygnus X-1, fitting the energy spectrum with a
simplified power law model. We recovered the distribution of the power law
spectral indices on time-scales as low as 62 ms as being limited between 1.6
and 1.8. The index is positively correlated with the flux even on such
time-scales.Comment: 14 pages, 19 figures, accepted by MNRA
Misalignment diagnosis of a planetary gearbox based on vibration analysis
As a critical power transmission system, planetary gearbox is widely used in many industrial important machines such as wind turbines, aircraft turbine engines, helicopters. Early fault detection and diagnosis of the gearbox will help to prevent unexpected breakdowns of this important equip-ment. Misalignment is one of the major operating problems in the planetary gearbox which may be caused by inadequate system integration, variable operating conditions and differences of elastic deformations in the system. In this paper, the effect of varying degrees of installation misalignment of planetary gearbox are investigated based on vibration measurements using spectrum analysis and modulation signal bispectrum (MSB) analysis. It has shown that the misalignment can be diagnosed in the low frequency range in which the adverse effect due to co-occurrence of amplitude modula-tion and frequency modulation (AM-FM) effect is low compared with the components around meshing frequencies. Moreover, MSB produces a more accurate and reliable diagnosis in that it gives correct indication of the fault severity and location for all operating conditions. In contrast, spectrum can produce correct results for some of the operating conditions. Keywords: Planetary gearbox, Condition Monitoring, Misalignment, Modulation signal bispectrum
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