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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 1,099 records · Page 61
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

New approaches to provide feedback from nuclear and covariance data adjustment for effective improvement of evaluated nuclear data files

A critical examination of the role of uncertainty assessment, target accuracies, role of integral experiment for validation and, consequently, of data adjustments methods is underway since several years at OECD-NEA, the objective being to provide criteria and practical approaches to use effectively the results of sensitivity analyses and cross section adjustments for feedback to evaluators and experimentalists in order to improve without ambiguities the knowledge of neutron cross sections, uncertainties, and correlations to be used in a wide range of applications and to meet new requirements and constraints for innovative reactor and fuel cycle system design. An approach will be described that expands as much as possible the use in the adjustment procedure of selected integral experiments that provide information on “elementary” phenomena, on separated individual physics effects related to specific isotopes or on specific energy ranges. An application to a large experimental data base has been performed and the results are discussed in the perspective of new evaluation projects like the CIELO initiative.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2014

Magnetic turbulence and pressure gradient feedback effect of the 1/2 mode soft-hard magnetohydrodynamic limit in large helical device

The aim of this study was to analyze the feedback process between the magnetic turbulence and the pressure gradients in Large Helical Device (LHD) inward-shifted configurations as well as its role in the transition between the soft-hard magnetohydrodynamic (MHD) regimes for instabilities driven by the mode 1/2 in the middle plasma. In the present paper, we summarize the results of two simulations with different Lundquist numbers, S=2.5×10{sup 5} and 10{sup 6}, assuming a plasma in the slow reconnection regime. The results for the high Lundquist number simulation show that the magnetic turbulence and the pressure gradient in the middle plasma region of LHD are below the critical value to drive the transition to the hard MHD regime, therefore only relaxations in the soft MHD limit are triggered (1/2 sawtooth-like events) [Phys. Plasmas 19, 082512 (2012)]. In the case of the simulation with low Lundquist number, the system reaches the hard MHD limit and a plasma collapse is observed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Generation of “gigantic” ultra-short microwave pulses based on passive mode-locking effect in electron oscillators with saturable absorber in the feedback loop

A periodic train of powerful ultrashort microwave pulses can be generated in electron oscillators with a non-linear saturable absorber installed in the feedback loop. This method of pulse formation resembles the passive mode-locking widely used in laser physics. Nevertheless, there is a specific feature in the mechanism of pulse amplification when consecutive energy extraction from different fractions of a stationary electron beam takes place due to pulse slippage over the beam caused by the difference between the wave group velocity and the electron axial velocity. As a result, the peak power of generated “gigantic” pulses can exceed not only the level of steady-state generation but also, in the optimal case, the power of the driving electron beam.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Temporal feedback control of high-intensity laser pulses to optimize ultrafast heating of atomic clusters

We describe how active feedback routines can be applied at a limited repetition rate (5 Hz) to optimize high-power (>10 TW) laser interactions with clustered gases. Optimization of x-ray production from an argon cluster jet, using a genetic algorithm, approximately doubled the measured energy through temporal modification of the 150 mJ driving laser pulse. This method achieved an increased radiation yield through exploration of a multi-dimensional parameter space, without requiring detailed a priori knowledge of the complex cluster dynamics. The optimized laser pulses exhibited a slow rising edge to the intensity profile, which enhanced the laser energy coupling into the cluster medium, compared to the optimally compressed FWHM pulse (40 fs). This report suggests that this technique can be more widely utilized for control of intense pulsed secondary radiation from petawatt-class laser systems.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2024

Vector-based feedback of continuous wave radiofrequency compression cavity for ultrafast electron diffraction

The temporal resolution of ultrafast electron diffraction at weakly relativistic beam energies (⁠≲100 keV) suffers from space-charge induced electron pulse broadening. We describe the implementation of a radio frequency (RF) cavity operating in the continuous wave regime to compress high repetition rate electron bunches from a 40.4 kV DC photoinjector for ultrafast electron diffraction applications. Active stabilization of the RF amplitude and phase through a feedback loop based on the demodulated in-phase and quadrature components of the RF signal is demonstrated. This scheme yields 144 ± 19 fs RMS temporal resolution in pump–probe studies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Dynamics of homogeneous cavitation with pressure feedback

Theoretical studies of homogeneous cavitation have largely been based on the classical nucleation theory. However, existing cavitation models cannot adequately describe its dynamics at nanosecond timescale, which has been called for in other fields. We develop a model coupling nucleation kinetics with cavity growth and pressure feedback as saturation mechanisms. Our numerical studies reveal the exponential dependence of cavitation characteristics such as saturation cavity density and most probable cavity radius on model parameters: Tolman length and initial pressure. This work also sheds light on the possibility of accurately determining Tolman length, whose value has a large spread in the literature.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

FBG array-based random distributed feedback Raman fibre laser

We have demonstrated the possibility of making a Raman laser with a random distributed feedback ensured by an array of 57 fibre Bragg gratings with random phases and amplitudes inscribed in a 13-m-long polarisation-maintaining passive fibre. The linearly polarised cw output power at a wavelength of 1092.3 nm reaches 5.7 W. The laser linewidth at the highest output power is about 80 pm. In the nearthreshold regime, we observe single-frequency lasing with a linewidth under 100 kHz as determined by self-heterodyne measurements. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Generation of a broad-area laser diode in an asymmetrical V-cavity possessing a spectrally nonselective feedback mirror

Generation of a broad-area laser diode is experimentally studied in an external asymmetrical V-cavity comprising an inclined output feedback mirror (FBM) that is nonselective with respect to spectrum and phase. The study is aimed at finding the ranges of a pump current and FBM inclination angles, in which the laser beam is narrow-directed. Laser radiation spectra are measured and FBM angles are juxtaposed with the angles of the output laser beam and the average period of filament positions in the laser diode. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

High-coupling distributed feedback lasers for the 1.55 μm spectral region

A technique for the development of high-coupling distributed-feedback (DFB) lasers for the 1.55 μm spectral region is described. The lasers demonstrate stable single-frequency generation in a broad temperature range with a maximum side-mode suppression ratio of more than 30 dB. (letters)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Accelerating the discovery of novel magnetic materials using machine learning–guided adaptive feedback

Magnetic materials are essential for energy generation and information devices, and they play an important role in advanced technologies and green energy economies. Currently, the most widely used magnets contain rare earth (RE) elements. An outstanding challenge of notable scientific interest is the discovery and synthesis of novel magnetic materials without RE elements that meet the performance and cost goals for advanced electromagnetic devices. Here, we report our discovery and synthesis of an RE-free magnetic compound, Fe 3 CoB 2 , through an efficient feedback framework by integrating machine learning (ML), an adaptive genetic algorithm, first-principles calculations, and experimental synthesis. Magnetic measurements show that Fe 3 CoB 2 exhibits a high magnetic anisotropy ( K 1 = 1.2 MJ/m 3 ) and saturation magnetic polarization ( J s = 1.39 T), which is suitable for RE-free permanent-magnet applications. Our ML-guided approach presents a promising paradigm for efficient materials design and discovery and can also be applied to the search for other functional materials.

36 MATERIALS SCIENCE↗
DOE OSTI2013

THE CIRCUMGALACTIC MEDIUM OF MASSIVE GALAXIES AT z {approx} 3: A TEST FOR STELLAR FEEDBACK, GALACTIC OUTFLOWS, AND COLD STREAMS

We present new results on the kinematics, thermal and ionization state, and spatial distribution of metal-enriched gas in the circumgalactic medium (CGM) of massive galaxies at redshift {approx}3, using the Eris suite of cosmological hydrodynamic ''zoom-in'' simulations. The reference run adopts a blastwave scheme for supernova feedback that produces large-scale galactic outflows, a star formation recipe based on a high gas density threshold, metal-dependent radiative cooling, and a model for the diffusion of metals and thermal energy. The effect of the local UV radiation field is added in post-processing. The CGM (defined as all gas at R > 0.2 R{sub vir} = 10 kpc, where R{sub vir} is the virial radius) contains multiple phases having a wide range of physical conditions, with more than half of its heavy elements locked in a warm-hot component at T > 10{sup 5} K. Synthetic spectra, generated by drawing sightlines through the CGM, produce interstellar absorption-line strengths of Ly{alpha}, C II, C IV, Si II, and Si IV as a function of the galactocentric impact parameter (scaled to the virial radius) that are in broad agreement with those observed at high redshift by Steidel et al. The covering factor of absorbing material declines less rapidly with impact parameter for Ly{alpha} and C IV compared to C II, Si IV, and Si II, with Ly{alpha} remaining strong (W{sub Ly{alpha}} > 300 mA) to {approx}> 5 R{sub vir} = 250 kpc. Only about one third of all the gas within R{sub vir} is outflowing. The fraction of sightlines within one virial radius that intercept optically thick, N{sub H{sub I}}>10{sup 17.2} cm{sup -2} material is 27%, in agreement with recent observations by Rudie et al. Such optically thick absorption is shown to trace inflowing ''cold'' streams that penetrate deep inside the virial radius. The streams, enriched to metallicities above 0.01 solar by previous episodes of star formation in the main host and in nearby dwarfs, are the origin of strong (N{sub C{sub II}}>10{sup 13} cm{sup -2}) C II absorption with a covering factor of 22% within R{sub vir} and 10% within 2 R{sub vir}. Galactic outflows do not cause any substantial suppression of the cold accretion mode. The central galaxy is surrounded by a large O VI halo, with a typical column density N{sub O{sub VI}} {approx}> 10{sup 14} cm{sup -2} and a near unity covering factor maintained all the way out to 150 kpc. This matches the trends recently observed in star-forming galaxies at low redshift by Tumlinson et al. Our zoom-in simulations of this single system appear then to reproduce quantitatively the complex baryonic processes that determine the exchange of matter, energy, and metals between galaxies and their surroundings.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

ANOMALOUS CO{sub 2} ICE TOWARD HOPS-68: A TRACER OF PROTOSTELLAR FEEDBACK

We report the detection of a unique CO{sub 2} ice band toward the deeply embedded, low-mass protostar HOPS-68. Our spectrum, obtained with the Infrared Spectrograph on board the Spitzer Space Telescope, reveals a 15.2 {mu}m CO{sub 2} ice bending mode profile that cannot be modeled with the same ice structure typically found toward other protostars. We develop a modified CO{sub 2} ice profile decomposition, including the addition of new high-quality laboratory spectra of pure, crystalline CO{sub 2} ice. Using this model, we find that 87%-92% of the CO{sub 2} is sequestered as spherical, CO{sub 2}-rich mantles, while typical interstellar ices show evidence of irregularly shaped, hydrogen-rich mantles. We propose that (1) the nearly complete absence of unprocessed ices along the line of sight is due to the flattened envelope structure of HOPS-68, which lacks cold absorbing material in its outer envelope, and possesses an extreme concentration of material within its inner (10 AU) envelope region and (2) an energetic event led to the evaporation of inner envelope ices, followed by cooling and re-condensation, explaining the sequestration of spherical, CO{sub 2} ice mantles in a hydrogen-poor mixture. The mechanism responsible for the sublimation could be either a transient accretion event or shocks in the interaction region between the protostellar outflow and envelope. The proposed scenario is consistent with the rarity of the observed CO{sub 2} ice profile, the formation of nearly pure CO{sub 2} ice, and the production of spherical ice mantles. HOPS-68 may therefore provide a unique window into the protostellar feedback process, as outflows and heating shape the physical and chemical structure of protostellar envelopes and molecular clouds.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

ACCRETION ONTO BLACK HOLES FROM LARGE SCALES REGULATED BY RADIATIVE FEEDBACK. III. ENHANCED LUMINOSITY OF INTERMEDIATE-MASS BLACK HOLES MOVING AT SUPERSONIC SPEEDS

In this third paper of a series, we study the growth and luminosity of black holes (BHs) in motion with respect to their surrounding medium. We run a large set of two-dimensional axis-symmetric simulations to explore a large parameter space of initial conditions and formulate an analytical model for the accretion. Contrary to the case without radiation feedback, the accretion rate increases with increasing BH velocity v{sub bh} reaching a maximum value at v{sub bh} = 2c{sub s,in} {approx} 50 km s{sup -1}, where c{sub s,in} is the sound speed inside the ''cometary-shaped'' H II region around the BH, before decreasing as v{sub bh}{sup -3} when the ionization front (I-front) becomes R-type (rarefied) and the accretion rate approaches the classical Bondi-Hoyle-Lyttleton solution. The increase of the accretion rate with v{sub bh} is produced by the formation of a D-type (dense) I-front preceded by a standing bow shock that reduces the downstream gas velocity to transonic values. There is a range of densities and velocities where the dense shell is unstable producing periodic accretion rate peaks which can significantly increase the detectability of intermediate-mass BHs. We find that the mean accretion rate for a moving BH is larger than that of a stationary BH of the same mass if the medium temperature is T{sub {infinity}} < 10{sup 4} K. This result could be important for the growth of seed BHs in the multi-phase medium of the first galaxies and for building an early X-ray background that may affect the formation of the first galaxies and the reionization process.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Toward a comprehensive model for feedback by active galactic nuclei: New insights from M87 observations by LOFAR, Fermi, and H.E.S.S

Feedback by active galactic nuclei (AGNs) appears to be critical in balancing radiative cooling of the low-entropy gas at the centers of galaxy clusters and in mitigating the star formation of elliptical galaxies. New observations of M87 enable us to put forward a comprehensive model for the physical heating mechanism. Low-frequency radio observations by LOFAR revealed the absence of fossil cosmic-ray (CR) electrons in the radio halo surrounding M87. This puzzle can be resolved by accounting for the CR release from the radio lobes and the subsequent mixing of CRs with the dense ambient intracluster gas, which thermalizes the electrons on a timescale similar to the radio halo age of 40 Myr. Hadronic interactions of similarly injected CR protons with the ambient gas should produce an observable gamma-ray signal in accordance with the steady emission of the low state of M87 detected by Fermi and H.E.S.S. Hence, we normalize the CR population to the gamma-ray emission, which shows the same spectral slope as the CR injection spectrum probed by LOFAR, thereby supporting a common origin. We show that CRs, which stream at the Alfvén velocity with respect to the plasma rest frame, heat the surrounding thermal plasma at a rate that balances that of radiative cooling on average at each radius. However, the resulting global thermal equilibrium is locally unstable and allows for the formation of the observed cooling multi-phase medium through thermal instability. Provided that CR heating balances cooling during the emerging 'cooling flow', the collapse of the majority of the gas is halted around 1 keV—in accordance with X-ray data. We show that both the existence of a temperature floor and the similar radial scaling of the heating and cooling rates are generic predictions of the CR heating model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Satellites of radio AGN in SDSS: Insights into agn triggering and feedback

We study the effects of radio jets on galaxies in their vicinity (satellites) and the role of satellites in triggering radio-loud active galactic nuclei (AGNs). The study compares the aggregate properties of satellites of a sample of 7220 radio AGNs at z < 0.3 (identified by Best and Heckman from the SDSS and NVSS+FIRST surveys) to the satellites of a control sample of radio-quiet galaxies, which are matched in redshift, color, luminosity, and axis ratio, as well as by environment type: field galaxies, cluster members, and brightest cluster galaxies (BCGs). Remarkably, we find that radio AGNs exhibit on average a 50% excess (17σ significance) in the number of satellites within 100 kpc even though the cluster membership was controlled (e.g., radio BCGs have more satellites than radio-quiet BCGs, etc.). Satellite excess is not confirmed for high-excitation sources, which are only 2% of radio AGN. Extra satellites may be responsible for raising the probability for hot gas AGN accretion via tidal effects or may otherwise enhance the intensity or duration of the radio-emitting phase. Furthermore, we find that the incidence of radio AGNs among potential hosts (massive ellipticals) is similar for field galaxies and for non-BCG cluster members, suggesting that AGN fueling depends primarily on conditions in the host halo rather than the parent, cluster halo. Regarding feedback, we find that radio AGNs, either high or low excitation, have no detectable effect on star formation in their satellites, as neither induced star formation nor star formation quenching is present in more than ∼1% of radio AGN.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE INTERSTELLAR MEDIUM AND FEEDBACK IN THE PROGENITORS OF THE COMPACT PASSIVE GALAXIES AT z ∼ 2

Quenched galaxies at z > 2 are nearly all very compact relative to z ∼ 0, suggesting a physical connection between high stellar density and efficient, rapid cessation of star-formation. We present rest-frame UV spectra of Lyman-break galaxies (LBGs) at z ∼ 3 selected to be candidate progenitors of the quenched galaxies at z ∼ 2 based on their compact rest-frame-optical sizes and high Σ{sub SFR}. We compare their UV properties to those of more extended LBGs of similar mass and star-formation rate (non-candidates). We find that candidate progenitors have faster bulk interstellar medium (ISM) gas velocities and higher equivalent widths of interstellar absorption lines, implying larger velocity spread among absorbing clouds. Candidates deviate from the relationship between equivalent widths of Lyα and interstellar absorption lines in that their Lyα emission remains strong despite high interstellar absorption, possibly indicating that the neutral H I fraction is patchy, such that Lyα photons can escape. We detect stronger C IV P-Cygni features (emission and absorption) and He II emission in candidates, indicative of larger populations of metal-rich Wolf-Rayet stars compared to non-candidates. The faster bulk motions, broader spread of gas velocity, and Lyα properties of candidates are consistent with their ISM being subject to more energetic feedback than non-candidates. Together with their larger metallicity (implying more evolved star-formation activity) this leads us to propose, if speculatively, that they are likely to quench sooner than non-candidates, supporting the validity of selection criteria used to identify them as progenitors of z ∼ 2 passive galaxies. We propose that massive, compact galaxies undergo more rapid growth of their stellar mass content, perhaps because the gas accretion mechanisms are different, and quench sooner than normally sized LBGs at these (early) epochs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Classical bulges, supermassive black holes, and AGN feedback: Extension to low-mass galaxies

The empirical model of Lu et al. for the relation between star formation rate and halo mass growth is adopted to predict the classical bulge mass (M{sub cb})–total stellar mass (M{sub ⋆}) relation for central galaxies. The assumption that the supermassive black hole (SMBH) mass (M{sub BH}) is directly proportional to the classical bulge mass, with the proportionality given by that for massive galaxies, predicts a M{sub BH}−M{sub ⋆} relation that matches well the observed relation for different types of galaxies. In particular, the model reproduces the strong transition at M{sub ⋆}=10{sup 10.5}−10{sup 11} M{sub ⊙}, below which M{sub BH} drops rapidly with decreasing M{sub ⋆}. Our model predicts a new sequence at M{sub ⋆}<10{sup 10.5} M{sub ⊙}, where M{sub BH}∝M{sub ⋆} but the amplitude is a factor of ∼50 lower than the amplitude of the sequence at M{sub ⋆}>10{sup 11} M{sub ⊙}. If all SMBHs grow through similar quasar modes with a feedback efficiency of a few percent, then the energy produced in low-mass galaxies at redshift z≳2 can heat the circumgalactic medium up to a specific entropy level that is required to prevent excessive star formation in low-mass dark matter halos.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Supernova feedback and the hot gas filling fraction of the interstellar medium

Supernovae (SNe), the most energetic stellar feedback mechanism, are crucial for regulating the interstellar medium (ISM) and launching galactic winds. We explore how supernova remnants (SNRs) create a multiphase medium by performing three-dimentional hydrodynamical simulations at various SN rates, S, and ISM average densities, n-bar . The evolution of an SNR in a self-consistently generated three-phase ISM is qualitatively different from that in a uniform or a two-phase warm/cold medium. By traveling faster and further in the low-density hot phase, the domain of an SNR increases by >10{sup 2.5}. Varying n-bar and S, we find that a steady state can only be achieved when the hot gas volume fraction f{sub V,hot}≲0.6±0.1. Above that level, overlapping SNRs render connecting topology of the hot gas, and the ISM is subjected to thermal runaway. Photoelectric heating (PEH) has a surprisingly strong impact on f{sub V,hot}. For n-bar ≳3 cm{sup −3}, a reasonable PEH rate is able to suppress the thermal runaway. Overall, we determine the critical SN rate for the onset of thermal runaway to be S{sub crit}=200( n-bar /1 cm{sup −3}){sup k}(E{sub SN}/10{sup 51} erg){sup −1} kpc{sup −3} Myr{sup −1}, where k = (1.2, 2.7) for n-bar ⩽1 and >1 cm{sup −3}, respectively. We present a fitting formula of the ISM pressure P( n-bar ,S), which can be used as an effective equation of state in cosmological simulations. Despite the five orders of magnitude span of ( n-bar ,S), the average Mach number varies little: M ≈ 0.5 ± 0.2, 1.2 ± 0.3, and 2.3 ± 0.9 for the hot, warm, and cold phases, respectively.

79 ASTRONOMY AND ASTROPHYSICS↗