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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,261 records · Page 70
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DOE OSTI2016

Is the bulk mode conversion important in high density helicon plasma?

In a high-density helicon plasma production process, a contribution of Trivelpiece-Gould (TG) wave for surface power deposition is widely accepted. The TG wave can be excited either due to an abrupt density gradient near the plasma edge (surface conversion) or due to linear mode conversion from the helicon wave in a density gradient in the bulk region (bulk mode conversion). By numerically solving the boundary value problem of linear coupling between the helicon and the TG waves in a background with density gradient, we show that the efficiency of the bulk mode conversion strongly depends on the dissipation included in the plasma, and the bulk mode conversion is important when the dissipation is small. Also, by performing FDTD simulation, we show the time evolution of energy flux associated with the helicon and the TG waves.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

A high-efficiency relativistic magnetron with the filled dielectric

Relativistic magnetron (RM) is a popular high power microwave device. Filling the resonant cavities with the low-loss dielectric is a well-proven technology that improves the performance of RM. In order to enhance the power efficiency, a dielectric-filled relativistic magnetron (DFRM) is presented and investigated numerically with KARAT code in this paper. The simulation results indicate that the maximum power efficiency is enhanced from 50.0% in the conventional RM to 56.0% in the DFRM when the applied voltage and the magnetic field are 280 kV and 0.5 T, respectively. Besides, the simulation results are analyzed and discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Picosecond lasing in ytterbium fibre laser with nonlinear optical loop mirror: experiment and numerical simulation

The operation regimes of a pulsed all-normal-dispersion polarisation-maintaining fibre laser with a nonlinear optical loop mirror are studied. The use of polarisation-maintaining fibres ensures polarisation and temperature stability of output radiation. The lasing and instability thresholds of the pulsed laser are determined experimentally. A spectral filter placed in the cavity makes it possible to change the centre wavelength of laser radiation within the range 1.02 – 1.05 μm with a spectral full width at half maximum of 2 nm. The average output power is 7 mW, which corresponds to a pulse energy of 0.8 nJ. The autocorrelation function width of the output pulse is 50 ps. The minimum pulse duration achieved after compression by an external pair of diffraction gratings is 1.8 ps. The dynamics of the temporal and spectral parameters of laser pulses is studied using mathematical simulation based on numerical solution of the nonlinear Schrödinger equation. The simulation results coincide with experimental data with a high accuracy. (lasers)

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Monitoring of chlorin-based photosensitiser localisation with dual-wavelength fluorescence imaging: numerical simulations

An approach to the estimation of chlorin-based photosensitiser localisation with dual-wavelength fluorescence imaging after topical application or intravenous injection is developed based on numerical simulations. Fluorescence imaging, which is commonly used for the monitoring of photosensitiser distribution within tissue, does not allow for its depth-resolved imaging. Chlorin-based photosensitisers feature two peaks in their absorption spectra corresponding to 402 and 662 nm, which allows for fluorescence excitation at two wavelengths with significantly different light penetration depths into biotissues. The ratio of fluorescence signals detected upon irradiation at different excitation wavelengths is shown to provide the information about photosensitiser localisation. The cases of exponential in-depth decay of the photosensitiser concentration and uniform photosensitiser distribution within the layer at a certain depth within the medium are analysed. These cases simulate topical application of photosensitiser and its intravenous injection and further accumulation in tumour, respectively. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

TURBULENCE AND RADIO MINI-HALOS IN THE SLOSHING CORES OF GALAXY CLUSTERS

A number of relaxed, cool-core galaxy clusters exhibit diffuse, steep-spectrum radio sources in their central regions, known as radio mini-halos. It has been proposed that the relativistic electrons responsible for the emission have been reaccelerated by turbulence generated by the sloshing of the cool core gas. We present a high-resolution MHD simulation of gas sloshing in a galaxy cluster coupled with subgrid simulations of relativistic electron acceleration to test this hypothesis. Our simulation shows that the sloshing motions generate turbulence on the order of {delta}v {approx} 50-200 km s{sup -1} on spatial scales of {approx}50-100 kpc and below in the cool core region within the envelope of the sloshing cold fronts, whereas outside the cold fronts, there is negligible turbulence. This turbulence is potentially strong enough to reaccelerate relativistic electron seeds (with initial {gamma} {approx} 100-500) to {gamma} {approx} 10{sup 4} via damping of magnetosonic waves and non-resonant compression. The seed electrons could remain in the cluster from, e.g., past active galactic nucleus activity. In combination with the magnetic field amplification in the core, these electrons then produce diffuse radio synchrotron emission that is coincident with the region bounded by the sloshing cold fronts, as indeed observed in X-rays and the radio. The result holds for different initial spatial distributions of pre-existing relativistic electrons. The power and the steep spectral index ({alpha} Almost-Equal-To 1-2) of the resulting radio emission are consistent with observations of mini-halos, though the theoretical uncertainties of the acceleration mechanisms are high. We also produce simulated maps of inverse-Compton hard X-ray emission from the same population of relativistic electrons.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

PHOTOSPHERIC EMISSION AS THE DOMINANT RADIATION MECHANISM IN LONG-DURATION GAMMA-RAY BURSTS

We present the results of a set of numerical simulations of long-duration gamma-ray burst jets associated with massive, compact stellar progenitors. The simulations extend to large radii and allow us to locate the region in which the peak frequency of the advected radiation is set before the radiation is released at the photosphere. Light curves and spectra are calculated for different viewing angles as well as different progenitor structures and jet properties. We find that the radiation released at the photosphere of matter-dominated jets is able to reproduce the observed Amati and energy-Lorentz factor correlations. Our simulations also predict a correlation between the burst energy and the radiative efficiency of the prompt phase, consistent with observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE LARGE-SCALE MAGNETIC FIELDS OF THIN ACCRETION DISKS

Large-scale magnetic field threading an accretion disk is a key ingredient in the jet formation model. The most attractive scenario for the origin of such a large-scale field is the advection of the field by the gas in the accretion disk from the interstellar medium or a companion star. However, it is realized that outward diffusion of the accreted field is fast compared with the inward accretion velocity in a geometrically thin accretion disk if the value of the Prandtl number P{sub m} is around unity. In this work, we revisit this problem considering the angular momentum of the disk to be removed predominantly by the magnetically driven outflows. The radial velocity of the disk is significantly increased due to the presence of the outflows. Using a simplified model for the vertical disk structure, we find that even moderately weak fields can cause sufficient angular momentum loss via a magnetic wind to balance outward diffusion. There are two equilibrium points, one at low field strengths corresponding to a plasma-beta at the midplane of order several hundred, and one for strong accreted fields, {beta} {approx} 1. We surmise that the first is relevant for the accretion of weak, possibly external, fields through the outer parts of the disk, while the latter one could explain the tendency, observed in full three-dimensional numerical simulations, of strong flux bundles at the centers of disk to stay confined in spite of strong magnetororational instability turbulence surrounding them.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE MASS OF THE MILKY WAY AND M31 USING THE METHOD OF LEAST ACTION

We constrain the most likely range of masses for the Milky Way (MW) and M31 using an application of the numerical action method (NAM) that optimizes the fit to observed parameters over a large ensemble of NAM-generated solutions. Our 95% confidence level mass ranges, 1.5-4.5 × 10{sup 12} M{sub ☉} for MW and 1.5-5.5 × 10{sup 12} M{sub ☉} for M31, are consistent with the upper range of estimates from other methods and suggests that a larger proportion of the total mass becomes detectable when the peculiar motions of many nearby satellites are taken into account in the dynamical analysis. We test the method against simulated Local Group catalogs extracted from the Millennium Run to confirm that mass predictions are consistent with actual galaxy halo masses.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Cosmic ray sampling of a clumpy interstellar medium

How cosmic rays sample the multi-phase interstellar medium (ISM) in starburst galaxies has important implications for many science goals, including evaluating the cosmic ray calorimeter model for these systems, predicting their neutrino fluxes, and modeling their winds. Here, we use Monte Carlo simulations to study cosmic ray sampling of a simple, two-phase ISM under conditions similar to those of the prototypical starburst galaxy M82. The assumption that cosmic rays sample the mean density of the ISM in the starburst region is assessed over a multi-dimensional parameter space where we vary the number of molecular clouds, the galactic wind speed, the extent to which the magnetic field is tangled, and the cosmic ray injection mechanism. We evaluate the ratio of the emissivity from pion production in molecular clouds to the emissivity that would be observed if the cosmic rays sampled the mean density, and seek areas of parameter space where this ratio differs significantly from unity. The assumption that cosmic rays sample the mean density holds over much of parameter space; however, this assumption begins to break down for high cloud density, injection close to the clouds, and a very tangled magnetic field. We conclude by evaluating the extent to which our simulated starburst region behaves as a proton calorimeter and constructing the time-dependent spectrum of a burst of cosmic rays.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Preventing star formation in early-type galaxies with late-time stellar heating

We revisit previous suggestions that the heating provided by the winds of dying low-mass stars plays an important role in preventing star formation in quiescent galaxies. At the end of their asymptotic giant branch phase, intermediate and low-mass stars eject their envelopes rapidly in a super-wind phase, usually giving rise to planetary nebulae. In spheroidal galaxies with high stellar velocity dispersions, the interaction of these ejected envelopes with the ambient diffuse gas can lead to significant, isotropic and steady-state heating that scales as M-dot {sub *}σ{sub *}{sup 2}. We show that cooling of the central regions of the hot diffuse halo gas can be delayed for a Hubble time for halos more massive than ∼10{sup 12.5} M{sub ⊙} at 0<z<2, although stellar heating alone is unlikely to forestall cooling in the most massive clusters at z = 0. This mechanism provides a natural explanation for the strong trend of galaxy quiescence with stellar surface density and velocity dispersion. In addition, since the ejected material will thermalize to kT ∼ σ{sub ∗}{sup 2}, this mechanism provides an explanation for the observed similarity between the central temperature of the hot diffuse gas and σ{sub ∗}{sup 2}, a result that is not trivial in light of the short inferred cooling times of the hot gas. The main uncertainty in this analysis is the ultimate fate of the stellar ejecta. Preventing accumulation of the ejecta in the central regions may require energy input from another source, such as Type Ia supernovae. Detailed simulations of the interaction of the stellar wind with the ambient gas are required to better quantify the net effect of AGB heating.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Relativistic weak lensing from a fully non-linear cosmological density field

In this paper we examine cosmological weak lensing on non-linear scales and show that there are Newtonian and relativistic contributions and that the latter can also be extracted from standard Newtonian simulations. We use the post-Friedmann formalism, a post-Newtonian type framework for cosmology, to derive the full weak-lensing deflection angle valid on non-linear scales for any metric theory of gravity. We show that the only contributing term that is quadratic in the first order deflection is the expected Born correction and lens-lens coupling term. We use this deflection angle to analyse the vector and tensor contributions to the E- and B- mode cosmic shear power spectra. In our approach, once the gravitational theory has been specified, the metric components are related to the matter content in a well-defined manner. Specifying General Relativity, we write down a complete set of equations for a GR+ΛCDM universe for computing all of the possible lensing terms from Newtonian N-body simulations. We illustrate this with the vector potential and show that, in a GR+ΛCDM universe, its contribution to the E-mode is negligible with respect to that of the conventional Newtonian scalar potential, even on non-linear scales. Thus, under the standard assumption that Newtonian N-body simulations give a good approximation of the matter dynamics, we show that the standard ray tracing approach gives a good description for a ΛCDM cosmology.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Neutrinos from cosmic ray interactions in the Sun

Cosmic rays hitting the solar atmosphere generate neutrinos that interact and oscillate in the Sun and oscillate on the way to Earth. These neutrinos could potentially be detected with neutrino telescopes and will be a background for searches for neutrinos from dark matter annihilation in the Sun. We calculate the flux of neutrinos from these cosmic ray interactions in the Sun and also investigate the interactions near a detector on Earth that give rise to muons. We compare this background with both regular Earth-atmospheric neutrinos and signals from dark matter annihilation in the Sun. Our calculation is performed with an event-based Monte Carlo approach that should be suitable as a simulation tool for experimental collaborations. Our program package is released publicly along with this paper.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

AN EXTREME GRAVITATIONALLY REDSHIFTED IRON LINE AT 4.8 KeV IN Mrk 876

X-ray spectral lines at unforeseen energies are important because they can shed light on the extreme physical conditions of the environment around the supermassive black holes of active galactic nuclei (AGNs). Mrk 876 displays such a line at 4.80{sub −0.04}{sup +0.05} rest-frame energy. A possible interpretation of its origin can be found in the hotspot scenario. In this scenario, the primary radiation from a flare in the hot corona of an AGN illuminates a limited portion of the accretion disk that emits by fluorescence. In this context, the line can represent an extreme gravitationally redshifted Fe line originating on the accretion disk below six gravitational radii from a rotating supermassive black hole. The correct estimate of the line significance requires a dedicated approach. Based on an existing rigorous approach, we have performed extensive Monte Carlo simulations. We determine that the line is a real feature at a ∼99% confidence level.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

TU-F-BRE-09: Towards the Establishment of Dosimetric References in Small Fields Using the New Concept of Dose-Area Product

Purpose: To establish dosimetric references of absorbed dose in water in radiation fields smaller than 2 cm used in radiotherapy thanks to a new methodology based on the use of dosimeters larger than the field size. Methods: A new graphite calorimeter was constructed with a large sensitive volume (diameter of the core: 30 mm). This primary dosimeter was fully characterized and compared to previous LNE-LNHB graphite calorimeters in a 60Co large field. A specially designed graphite parallel-plate ionization chamber with a 30 mm collecting electrode was also assembled and tested. Measurements were then conducted in two 6 MV small circular fields of 2 cm and 1 cm diameter respectively, using the new concept of dose-area product instead of punctual dose commonly used in radiotherapy. Results: The dose rate established in a large 60Co field with the new calorimeter is in agreement within 0.4% with previous calorimeters. The ionization chamber shows good characteristics except for a 0.06% drift per hour in water. The ratio of calorimetric against ionometric measurements in the 2 cm diameter field is 1.1% higher than the one in the 1 cm diameter field (with respectively 0.30% and 1.03% type A uncertainty for each field). Conclusion: Results presented here highlight the possibility of measuring dose-area products in small fields with a graphite calorimeter and a parallel-plate ionization chamber. Measurements in a 0.75 cm diameter field are already underway to confirm the trend observed in the 2 cm and 1 cm diameter fields. The last step to establish precise dosimetric references in small fields is to calculate correction factors thanks to Monte Carlo simulations.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2015

SU-E-T-656: Quantitative Analysis of Proton Boron Fusion Therapy (PBFT) in Various Conditions

Purpose: Three alpha particles are concomitant of proton boron interaction, which can be used in radiotherapy applications. We performed simulation studies to determine the effectiveness of proton boron fusion therapy (PBFT) under various conditions. Methods: Boron uptake regions (BURs) of various widths and densities were implemented in Monte Carlo n-particle extended (MCNPX) simulation code. The effect of proton beam energy was considered for different BURs. Four simulation scenarios were designed to verify the effectiveness of integrated boost that was observed in the proton boron reaction. In these simulations, the effect of proton beam energy was determined for different physical conditions, such as size, location, and boron concentration. Results: Proton dose amplification was confirmed for all proton beam energies considered (< 96.62%). Based on the simulation results for different physical conditions, the threshold for the range in which proton dose amplification occurred was estimated as 0.3 cm. Effective proton boron reaction requires the boron concentration to be equal to or greater than 14.4 mg/g. Conclusion: We established the effects of the PBFT with various conditions by using Monte Carlo simulation. The results of our research can be used for providing a PBFT dose database.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2016

Experimental determination of the PTW 60019 microDiamond dosimeter active area and volume

Purpose: Small field output correction factors have been studied by several research groups for the PTW 60019 microDiamond (MD) dosimeter, by comparing the response of such a device with both reference dosimeters and Monte Carlo simulations. A general good agreement is observed for field sizes down to about 1 cm. However, evident inconsistencies can be noticed when comparing some experimental results and Monte Carlo simulations obtained for smaller irradiation fields. This issue was tentatively attributed by some authors to unintentional large variations of the MD active surface area. The aim of the present study is a nondestructive experimental determination of the MD active surface area and active volume. Methods: Ten MD dosimeters, one MD prototype, and three synthetic diamond samples were investigated in the present work. 2D maps of the MD response were recorded under scanned soft x-ray microbeam irradiation, leading to an experimental determination of the device active surface area. Profiles of the device responses were measured as well. In order to evaluate the MD active volume, the thickness of the diamond sensing layer was independently evaluated by capacitance measurements and alpha particle detection experiments. The MD sensitivity, measured at the PTW calibration laboratory, was also used to calculate the device active volume thickness. Results: An average active surface area diameter of (2.19 ± 0.02) mm was evaluated by 2D maps and response profiles of all the MDs. Average active volume thicknesses of (1.01 ± 0.13) μm and (0.97 ± 0.14) μm were derived by capacitance and sensitivity measurements, respectively. The obtained results are well in agreement with the nominal values reported in the manufacturer dosimeter specifications. A homogeneous response was observed over the whole device active area. Besides the one from the device active volume, no contributions from other components of the housing nor from encapsulation materials were observed in the 2D response maps. Conclusions: The obtained results demonstrate the high reproducibility of the MD fabrication process. The observed discrepancies among the output correction factors reported by several authors for MD response in very small fields are very unlikely to be ascribed to unintentional variations of the device active surface area and volume. It is the opinion of the authors that the role of the volume averaging as well as of other perturbation effects should be separately investigated instead, both experimentally and by Monte Carlo simulations, in order to better clarify the behaviour of the MD response in very small fields.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

Self-focusing of profiled ultrashort-wavelength laser beams in air

We report on the results of laboratory experiments of filamentation of sharply focused gigawatt femtosecond laser radiation passed through various aperture diaphragms in air. For the multiple filamentation regime, the dependences of the length and spatial structure of the filamentation region on the initial beam profile are established. It is found that light beam profiling by a diaphragm leads in some cases to a displacement of the filamentation region and to repeated self-focusing of radiation behind the linear focal waist. In the beam of the same power in the absence of a diaphragm and in the regime of the formation of a single filament, this effect terminates in front of the geometrical focus. The experimental results are illustrated by numerical simulation data.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

SIMULATIONS OF ENERGETIC PARTICLES INTERACTING WITH DYNAMICAL MAGNETIC TURBULENCE

We explore the transport of energetic particles in interplanetary space by using test-particle simulations. In previous work such simulations have been performed by using either magnetostatic turbulence or undamped propagating plasma waves. In the current paper we simulate for the first time particle transport in dynamical turbulence. To do so we employ two models, namely the damping model of dynamical turbulence and the random sweeping model. We compute parallel and perpendicular diffusion coefficients and compare our numerical findings with solar wind observations. We show that good agreement can be found between simulations and the Palmer consensus range for both dynamical turbulence models if the ratio of turbulent magnetic field and mean field is δB/B{sub 0} = 0.5.

79 ASTRONOMY AND ASTROPHYSICS↗