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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Magnetic field sensor based on cascaded microfiber coupler with magnetic fluid

A kind of magnetic field sensor based on cascaded microfiber coupler with magnetic fluid is proposed and experimentally demonstrated. The magnetic fluid is utilized as the cladding of the fused regions of the cascaded microfiber coupler. As the interference valley wavelength of the sensing structure is sensitive to the ambient variation, considering the magnetic-field-dependent refractive index of magnetic fluid, the proposed structure is employed for magnetic field sensing. The effective coupling length for each coupling region of the as-fabricated cascaded microfiber coupler is 6031 μm. The achieved sensitivity is 125 pm/Oe, which is about three times larger than that of the previously similar structure based on the single microfiber coupler. Experimental results indicate that the sensing sensitivity can be easily improved by increasing the effective coupling length or cascading more microfiber couplers. The proposed magnetic field sensor is attractive due to its low cost, immunity to electromagnetic interference, as well as high sensitivity, which also has the potentials in other tunable all-fiber photonic devices, such as filter.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Fully anharmonic infrared cascade spectra of polycyclic aromatic hydrocarbons

The infrared (IR) emission of polycyclic aromatic hydrocarbons (PAHs) permeates our universe; astronomers have detected the IR signatures of PAHs around many interstellar objects. The IR emission of interstellar PAHs differs from their emission as seen under conditions on Earth as they emit through a collisionless cascade down through their excited vibrational states from high internal energies. The difficulty in reproducing interstellar conditions in the laboratory results in a reliance on theoretical techniques. However, the size and complexity of PAHs require careful consideration when producing the theoretical spectra. In this work, we outline the theoretical methods necessary to lead to fully theoretical IR cascade spectra of PAHs including: an anharmonic second order vibrational perturbation theory treatment, the inclusion of Fermi resonances through polyads, and the calculation of anharmonic temperature band shifts and broadenings (including resonances) through a Wang-Landau approach. We also suggest a simplified scheme to calculate vibrational emission spectra that retain the essential characteristics of the full IR cascade treatment and can directly transform low temperature absorption spectra in IR cascade spectra. Finally, we show that past astronomical models were in error in assuming a 15 cm -1 correction was needed to account for anharmonic emission effects.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Cascaded frequency doublers for broadband laser radiation

A new scheme of a cascaded converter of the first harmonic of broadband cw laser radiation into the second harmonic (SH) with compensation for the group walk-off in cascades is proposed and investigated. The conditions under which high conversion coefficients of broadband ({approx}33 cm{sup -1}) single-mode fibre laser radiation with low peak power ({approx}300 W) into the SH are determined for frequency doublers based on the most promising LBO crystal. Conversion of cw radiation with an average power of 300 W and efficiency {eta} = 4.5 % into the SH is obtained in a single LBO crystal. Effect of coherent addition of SH radiation excited in different cascades is demonstrated for two- and three-stage schemes. The expected conversion efficiencies, calculated disregarding loss but taking into account real aberrations of elements, are 18 % and 38 %, respectively. The effect of pumping depletion begins to manifest itself in the third cascade of a three-stage converter; it may reduce the latter value to {approx}30 %. (nonlinear optical phenomena)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Development and study of high-power quantum-cascade lasers emitting at 4.5 – 4.6 μm

Quantum-cascade room-temperature 4.5 – 4.6 μm lasers with different numbers of quantum cascades are developed and studied. It is shown that losses at the metallised sidewalls of the ridge waveguide considerably increase the threshold current density. It is demonstrated that the current density needed to overcome the internal and outcoupling losses in lasers with 30 quantum cascades is an order of magnitude lower than in lasers with 15 quantum cascades. (lasers)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

MAGNETIC ENERGY CASCADE IN SPHERICAL GEOMETRY. I. THE STELLAR CONVECTIVE DYNAMO CASE

We present a method to characterize the spectral transfers of magnetic energy between scales in simulations of stellar convective dynamos. The full triadic transfer functions are computed thanks to analytical coupling relations of spherical harmonics based on the Clebsch-Gordan coefficients. The method is applied to mean field {alpha}{Omega} dynamo models as benchmark tests. From a physical standpoint, the decomposition of the dynamo field into primary and secondary dynamo families proves very instructive in the {alpha}{Omega} case. The same method is then applied to a fully turbulent dynamo in a solar convection zone, modeled with the three-dimensional MHD Anelastic Spherical Harmonics code. The initial growth of the magnetic energy spectrum is shown to be non-local. It mainly reproduces the kinetic energy spectrum of convection at intermediate scales. During the saturation phase, two kinds of direct magnetic energy cascades are observed in regions encompassing the smallest scales involved in the simulation. The first cascade is obtained through the shearing of the magnetic field by the large-scale differential rotation that effectively cascades magnetic energy. The second is a generalized cascade that involves a range of local magnetic and velocity scales. Non-local transfers appear to be significant, such that the net transfers cannot be reduced to the dynamics of a small set of modes. The saturation of the large-scale axisymmetric dipole and quadrupole is detailed. In particular, the dipole is saturated by a non-local interaction involving the most energetic scale of the magnetic energy spectrum, which points to the importance of the magnetic Prandtl number for large-scale dynamos.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

TEST FOR WAVEVECTOR ANISOTROPIES IN PLASMA TURBULENCE CASCADES

The frequency and wavevector matching conditions in nonlinear three-wave coupling are used to test whether the forward cascade of plasma turbulence may lead to wavevector anisotropies in a homogeneous, collisionless, magnetized plasma. Linear kinetic theory at {beta}{sub p} = 0.01, 0.10, and 1.0 is used to determine the frequency-wavenumber dispersion of three normal modes: long-wavelength Alfven-cyclotron waves, long-wavelength magnetosonic waves, and intermediate-wavelength magnetosonic-whistler waves. Using linear dispersion in the nonlinear matching conditions, the test predicts with one exception that forward cascades are favored by fluctuations propagating nearly perpendicular to the background magnetic field B{sub o}. This is consistent with the typical development of wavevector anisotropies with k >> k{sub Parallel-To} (subscripts refer to directions perpendicular and parallel to B{sub o}, respectively) in computer simulations of the forward cascade of various types of plasma turbulence. The exception is that, at {beta}{sub p} = 1.0, the test predicts that the cascade of long-wavelength magnetosonic waves should be favored by modes at k {approx} k{sub Parallel-To }.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ON THE POLAR CAP CASCADE PAIR MULTIPLICITY OF YOUNG PULSARS

We study the efficiency of pair production in polar caps of young pulsars under a variety of conditions to estimate the maximum possible multiplicity of pair plasma in pulsar magnetospheres. We develop a semi-analytic model for calculation of cascade multiplicity which allows efficient exploration of the parameter space and corroborate it with direct numerical simulations. Pair creation processes are considered separately from particle acceleration in order to assess different factors affecting cascade efficiency, with acceleration of primary particles described by recent self-consistent non-stationary model of pair cascades. We argue that the most efficient cascades operate in the curvature radiation/synchrotron regime, the maximum multiplicity of pair plasma in pulsar magnetospheres is ∼few × 10{sup 5}. The multiplicity of pair plasma in magnetospheres of young energetic pulsars weakly depends on the strength of the magnetic field and the radius of curvature of magnetic field lines and has a stronger dependence on pulsar inclination angle. This result questions assumptions about very high pair plasma multiplicity in theories of pulsar wind nebulae.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Identifying the electron–positron cascade regimes in high-intensity laser-matter interactions

Strong-field quantum electrodynamics predicts electron-seeded electron–positron pair cascades when the electric field in the rest-frame of the seed electron approaches the Sauter–Schwinger field, i.e. $\eta ={E}_{{\rm{RF}}}/{E}_{S}\sim 1$. Electrons in the focus of next generation multi-PW lasers are expected to reach this threshold. We identify three distinct cascading regimes in the interaction of counter-propagating, circularly-polarised laser pulses with a thin foil by performing a comprehensive scan over the laser intensity (from 10 23 to 5 × 10 24 W cm –2 ) and initial foil target density (from 10 26 to 10 31 m –3 ). For low densities and intensities the number of pairs grows exponentially. If the intensity and target density are high enough the number density of created pairs reaches the relativistically-corrected critical density, the pair plasma efficiently absorbs the laser energy (through radiation reaction) and the cascade saturates. If the initial density is too high, such that the initial target is overdense, the cascade is suppressed by the skin effect. Here, we derive a semi-analytical model which predicts that dense pair plasmas are endemic features of these interactions for intensities above 10 24 W cm –2 provided the target's relativistic skin-depth is longer than the laser wavelength. Further, it shows that pair production is maximised in near-critical-density targets, providing a guide for near-term experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

A NEW MODEL FOR GAMMA-RAY CASCADES IN EXTRAGALACTIC MAGNETIC FIELDS

Very high energy (VHE, E {approx}> 100 GeV) gamma rays emitted by extragalactic sources, such as blazars, initiate electromagnetic cascades in the intergalactic medium. The cascade photons arrive at the Earth with angular and temporal distributions correlated with the extragalactic magnetic field (EGMF). We have developed a new semi-analytical model of the cascade properties which is more accurate than previous analytic approaches and faster than full Monte Carlo simulations. Within its range of applicability, our model can quickly generate cascade spectra for a variety of source emission models, EGMF strengths, and assumptions about the source livetime. In this Letter, we describe the properties of the model and demonstrate its utility by exploring the gamma-ray emission from the blazar RGB J0710+591. In particular, we predict, under various scenarios, the VHE and high-energy (100 MeV {approx}< E {approx}< 300 GeV) fluxes detectable with the VERITAS and Fermi Large Area Telescope observatories. We then develop a systematic framework for comparing the predictions to published results, obtaining constraints on the EGMF strength. At a confidence level of 95%, we find the lower limit on the EGMF strength to be {approx}2 x 10{sup -16} G if no limit is placed on the livetime of the source or {approx}3 x 10{sup -18} G if the source livetime is limited to the past {approx}3 years during which Fermi observations have taken place.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2026

Primary defect production from molecular dynamics simulations of high-energy displacement cascades in NbMoTaW alloys

In this work, we report on large-scale molecular-dynamics (MD) simulations of displacement cascades in equiatomic NbMoTaW alloys at PKA energies ranging from 0.15 to 150 keV. We find defect production to be strongly dependent on recoil energy, scaling sublinearly up to 10 keV, and linearly thereafter. We find the sublinear regime to be defined by low values of surviving Frenkel pairs, typically found as isolated point defects or small defect clusters, while at higher recoil energies dense cascades become more frequent, leading to splitting into subcascades and the production of relatively large prismatic-dislocation loops with ⟨111⟩ and ⟨001⟩ Burgers vectors. These loops immobilize large fractions of defects, leading to a rapid growth of the number of surviving defects in the linear regime. We also anneal post-cascade defect configurations using object-kinetic Monte Carlo (OKMC) simulations to account for intracascade recombination on time scales not accessible to MD simulations. Cascade annealing is strongly temperature dependent, with the OKMC simulations only showing significant recovery at 1000 K but not below. Our results are in general agreement with existing published data for refractory concentrated alloys.

Zhou, Xinran [Lawrence Berkeley National Laborator↗
DOE OSTI2011

Quantum correlations and violation of the Bell inequality induced by an external field in a two-photon radiative cascade

We study the polarization-dependent second-order correlation of a pair of photons emitted in a four-level radiative cascade driven by an external field. It is found that the quantum correlations of the emitted photons, degraded by the energy splitting of the intermediate levels in the radiative cascade, can be efficiently revived by a far-detuned external field. The physics of this revival is linked to an induced Stark shift and the formation of dressed states in the system by the nonresonant external field. Furthermore, we investigated the competition between the effect of the coherent external field and incoherent dephasing of the intermediate levels. We find that the degradation of quantum correlations due to the incoherent dephasing can be contained for small dephasing with the external field. We also studied the nonlocality of the correlations by evaluating the Bell inequality in the linear polarization basis for the radiative cascade. We find that the Bell parameter decreases rapidly with increase in the intermediate-level energy splitting or incoherent dephasing rate to the extent that there is no violation. However, the presence of an external field leads to control over the degrading mechanisms and preservation of nonlocal correlation among the photons. This in turn can induce a violation of Bell's inequality in the radiative cascade for arbitrary intermediate-level splitting and small incoherent dephasing.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Classical model of the upper bounds of the cascading contribution to the second hyperpolarizability

We investigate whether microscopic cascading of second-order nonlinearities of two molecules in the side-by-side configuration can lead to a third-order molecular nonlinear-optical response that exceeds the fundamental limit. We find that for large values of the second hyperpolarizability, the side-by-side configuration has a cascading contribution that lowers the direct contribution. However, we do find that there is a cascading contribution to the second hyperpolarizability when there is no direct contribution. Thus, while cascading can never lead to a larger nonlinear-optical response than for a single molecule with the same number of electrons, it may provide design flexibility in making large third-order susceptibility materials when the molecular second hyperpolarizability vanishes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Quantum mechanical model of the upper bounds of the cascading contribution to the second hyperpolarizability

Microscopic cascading of second-order nonlinearities between two molecules has been proposed to yield an enhanced third-order molecular nonlinear-optical response. In this contribution, we investigate the two-molecule cascaded second hyperpolarizability and show that it will never exceed the fundamental limit of a single molecule with the same number of electrons as the two-molecule system. We show the apparent divergence behavior of the cascading contribution to the second hyperpolarizability vanishes when properly taking into account the intermolecular interactions. Although cascading can never lead to a larger nonlinear-optical response than a single molecule, it provides alternative molecular design configurations for creating materials with large third-order susceptibilities that may be difficult to design into a single molecule.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

geant4 hadronic cascade models analysis of proton and charged pion transverse momentum spectra from p + Cu and Pb collisions at 3, 8, and 15 GeV/c

We describe how various hadronic cascade models, which are implemented in the geant4 toolkit, describe proton and charged pion transverse momentum spectra from p + Cu and Pb collisions at 3, 8, and 15 GeV/c, recently measured in the hadron production (HARP) experiment at CERN. The Binary, ultrarelativistic quantum molecular dynamics (UrQMD) and modified FRITIOF (FTF) hadronic cascade models are chosen for investigation. The first two models are based on limited (Binary) and branched (UrQMD) binary scattering between cascade particles which can be either a baryon or meson, in the three-dimensional space of the nucleus, while the latter (FTF) considers collective interactions between nucleons only, on the plane of impact parameter. It is found that the slow (p{sub T}{<=}0.3 GeV/c) proton spectra are quite sensitive to the different treatments of cascade pictures, while the fast (p{sub T}>0.3 GeV/c) proton spectra are not strongly affected by the differences between the FTF and UrQMD models. It is also shown that the UrQMD and FTF combined with Binary (FTFB) models could reproduce both proton and charged pion spectra from p + Cu and Pb collisions at 3, 8, and 15 GeV/c with the same accuracy.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Vulnerability and cosusceptibility determine the size of network cascades

In a network, a local disturbance can propagate and eventually cause a substantial part of the system to fail in cascade events that are easy to conceptualize but extraordinarily difficult to predict. Furthermore, we develop a statistical framework that can predict cascade size distributions by incorporating two ingredients only: the vulnerability of individual components and the cosusceptibility of groups of components (i.e., their tendency to fail together). Using cascades in power grids as a representative example, we show that correlations between component failures define structured and often surprisingly large groups of cosusceptible components. Aside from their implications for blackout studies, these results provide insights and a new modeling framework for understanding cascades in financial systems, food webs, and complex networks in general.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2013

Quantum-engineered interband cascade photovoltaic devices

Quantum-engineered multiple stage photovoltaic (PV) devices are explored based on InAs/GaSb/AlSb interband cascade (IC) structures. These ICPV devices employ multiple discrete absorbers that are connected in series by widebandgap unipolar barriers using type-II heterostructure interfaces for facilitating carrier transport between cascade stages similar to IC lasers. The discrete architecture is beneficial for improving the collection efficiency and for spectral splitting by utilizing absorbers with different bandgaps. As such, the photo-voltages from each individual cascade stage in an ICPV device add together, creating a high overall open-circuit voltage, similar to conventional multi-junction tandem solar cells. Furthermore, photo-generated carriers can be collected with nearly 100% efficiency in each stage. This is because the carriers travel over only a single cascade stage, designed to be shorter than a typical diffusion length. The approach is of significant importance for operation at high temperatures where the diffusion length is reduced. Here, we will present our recent progress in the study of ICPV devices, which includes the demonstration of ICPV devices at room temperature and above with narrow bandgaps (e.g. 0.23 eV) and high open-circuit voltages. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

14 SOLAR ENERGY↗
DOE OSTI2014

A cascaded model of spectral distortions due to spectral response effects and pulse pileup effects in a photon-counting x-ray detector for CT

Purpose: Energy discriminating, photon-counting detectors (PCDs) are an emerging technology for computed tomography (CT) with various potential benefits for clinical CT. The photon energies measured by PCDs can be distorted due to the interactions of a photon with the detector and the interaction of multiple coincident photons. These effects result in distorted recorded x-ray spectra which may lead to artifacts in reconstructed CT images and inaccuracies in tissue identification. Model-based compensation techniques have the potential to account for the distortion effects. This approach requires only a small number of parameters and is applicable to a wide range of spectra and count rates, but it needs an accurate model of the spectral distortions occurring in PCDs. The purpose of this study was to develop a model of those spectral distortions and to evaluate the model using a PCD (model DXMCT-1; DxRay, Inc., Northridge, CA) and various x-ray spectra in a wide range of count rates. Methods: The authors hypothesize that the complex phenomena of spectral distortions can be modeled by: (1) separating them into count-rate independent factors that we call the spectral response effects (SRE), and count-rate dependent factors that we call the pulse pileup effects (PPE), (2) developing separate models for SRE and PPE, and (3) cascading the SRE and PPE models into a combined SRE+PPE model that describes PCD distortions at both low and high count rates. The SRE model describes the probability distribution of the recorded spectrum, with a photo peak and a continuum tail, given the incident photon energy. Model parameters were obtained from calibration measurements with three radioisotopes and then interpolated linearly for other energies. The PPE model used was developed in the authors’ previous work [K. Taguchi et al., “Modeling the performance of a photon counting x-ray detector for CT: Energy response and pulse pileup effects,” Med. Phys. 38(2), 1089–1102 (2011)]. The agreement between the x-ray spectra calculated by the cascaded SRE+PPE model and the measured spectra was evaluated for various levels of deadtime loss ratios (DLR) and incident spectral shapes, realized using different attenuators, in terms of the weighted coefficient of variation (COV{sub W}), i.e., the root mean square difference weighted by the statistical errors of the data and divided by the mean. Results: At low count rates, when DLR < 10%, the distorted spectra measured by the DXMCT-1 were in agreement with those calculated by SRE only, with COV{sub W}'s less than 4%. At higher count rates, the measured spectra were also in agreement with the ones calculated by the cascaded SRE+PPE model; with PMMA as attenuator, COV{sub W} was 5.6% at a DLR of 22% and as small as 6.7% for a DLR as high as 55%. Conclusions: The x-ray spectra calculated by the proposed model agreed with the measured spectra over a wide range of count rates and spectral shapes. The SRE model predicted the distorted, recorded spectra with low count rates over various types and thicknesses of attenuators. The study also validated the hypothesis that the complex spectral distortions in a PCD can be adequately modeled by cascading the count-rate independent SRE and the count-rate dependent PPE.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2018

Radiation of a Cascade near a Plane Interface and in a Planar Layer

In order to detect cosmic rays and ultrahigh-energy neutrinos, a number of experiments based on the detection of radio radiation of cascades initiated by these particles in dense media such as ground ice massifs or lunar regolith have been developed. In most of the experiments, radio radiation is detected at the emission to the atmosphere or cosmic space rather than in a dense medium. Consequently, it is necessary to calculate the radiation of a cascade taking into account an interface between two media. This problem is usually solved numerically by the Monte Carlo method. A simple analytical expression for a radiation field in the wave zone of the less dense medium has been obtained for the case of development of the cascade in the dense medium and the crossing of the interface between two media by radiation. The effect of the third, additional medium on the radiation field of the cascade has also been considered.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗