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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 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Generation of dissipative solitons in an actively mode-locked ultralong fibre laser

A single-pulse actively mode-locked fibre laser with a cavity length exceeding 1 km has been developed and investigated for the first time. This all-fibre erbium-doped laser has a normal intracavity dispersion and generates dissipative 8-ns solitons with a fundamental repetition rate of 163.8 kHz; the energy per pulse reaches 34 nJ. The implemented mode locking, based on the use of intracavity intensity modulator, provides self-triggering and high stability of pulsed lasing. A possibility of continuous tuning of the centre lasing wavelength in the range of 1558 - 1560 nm without any tunable spectral selective elements in the cavity is demonstrated. The tuning occurs when controlling the modulation signal frequency due to the forced change in the pulse repetition time (group delay) under the conditions of intracavity chromatic dispersion. (laser optics 2012)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Multi-pulse operation of a dissipative soliton fibre laser based on nonlinear polarisation rotation

We report an experimental observation of multiple dissipative soliton (DS) operation states in an all-normal-dispersion passively mode-locked Yb-doped fibre laser, including DS bound and oscillating states. In the bound state, multiple DSs up to 11 can coexist in the cavity. In the oscillating state, the DSs' movements are not purely random and three typical states are generalised and illustrated. A single-pulse mode-locked state is established at a high pump power by carefully adjusting the polarisation controllers. The broad spectrum indicates that it may be noise-like pulses, which can serve as a pump to generate a supercontinuum. (control of laser radiation parameters)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Generation of light and dark soliton trains in a dissipative four-wave mixing, mode-locked fibre ring laser

We consider a model of a dissipative four-wave mixing, mode-locked fibre ring laser with an intracavity interferometer. The necessary conditions required for mode locking are presented. A pulse train generation is numerically simulated at different repetition rates and gain levels. Admissible ranges of values, for which successful mode locking is possible, are found. It is shown that in the case of normal dispersion of the resonator, a laser with an intracavity interferometer can generate a train of pulses with an energy much greater than that in the case of anomalous dispersion. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Amplification of dissipative solitons with a polarisation-maintaining tapered fibre amplifier

The generation of strongly chirped dissipative solitons offers great opportunities for generating high-power laser pulses of short duration. We demonstrate the possibility of amplifying such pulses in a polarisation-maintaining tapered fibre having 32 μm mode diameter at the output end. Highly chirped pulses with a repetition rate of 14.3 MHz and a duration of 10 ps at 1055 nm centre wavelength are amplified in an all-fibre circuit to 2.11 μJ energy and compressed by diffraction gratings to a duration of ∼297 fs. Reducing the pulse repetition rate to 1 MHz allows a 16.5 μJ output pulse energy to be achieved without manifestations of SRS and self-phase modulation in the spectrum. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Study on harmonic generation regimes of Raman dissipative solitons in an external fibre cavity in a spectral region of 1.3 μm

We have studied the harmonic generation regimes in a pulsed SRS laser with synchronous pumping. The use of phosphosilicate polarisation-maintaining fibre made it possible to obtain stable generation of Raman dissipative solitons (RDS’s) with a wavelength of 1.3 μm, and a fold increase in the length of the external cavity relative to that of the pump laser cavity by a dispersion-shifted fibre allowed us to raise the threshold for the transition to noise-like pulses and to increase the RDS energy up to 2.5 nJ with an estimated duration of a transform-limited pulse of about 200 fs. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

On the existence of maximal semidefinite invariant subspaces for J-dissipative operators

For a certain class of operators we present some necessary and sufficient conditions for a J-dissipative operator in a Krein space to have maximal semidefinite invariant subspaces. We investigate the semigroup properties of restrictions of the operator to these invariant subspaces. These results are applied to the case when the operator admits a matrix representation with respect to the canonical decomposition of the space. The main conditions are formulated in terms of interpolation theory for Banach spaces. Bibliography: 25 titles.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2025

Equilibrium-gated pattern formation: How molecular dissociation thermodynamics drive emergent behavior in dissipative polymeric systems

Emergent patterns in biological systems arise through dissipative processes that balance reaction and transport phenomena, producing highly functional properties from self-regulating mechanisms. Synthetic fabrication, by contrast, often relies on user-controlled, multistep methods that lack the self-organizing capabilities of natural systems. Inspired by nature, we sought chemical systems that integrate strongly coupled reaction and transport phenomena, identifying frontal ring-opening metathesis polymerization (FROMP) as a method capable of creating diverse forms and functions through reactive processing. By employing discrete molecular initiators, FROMP allows precise control of key reaction steps—inhibition, initiation, and propagation. Using an integrated computational and experimental framework, we uncover how near-equilibrium inhibition dynamics, coupled with far-from-equilibrium reaction kinetics, drive pattern formation in frontally polymerized synthetic materials. We propose the concept of equilibrium-gated pattern formation, demonstrating how initiator chemistry can be tuned to achieve programmable macroscale properties. Our study reveals a surprising insight: Emergent behavior in FROMP systems arises from the inhibition-dominated regime of resin composition, expanding prior observations that such behavior is confined to a narrow compositional space near the boundary between front quenching and uniform front propagation. We identify a broader compositional window, far from the quenching regime, where emergent behavior reliably manifests. This expanded design space significantly enhances the operational flexibility of reactive systems and their capacity for self-organization. Furthermore, these insights provide a roadmap for designing bioinspired materials with self-organizing capabilities, unlocking possibilities in synthetic manufacturing.

Architected Polymers↗
DOE OSTI2021

A modified many-body dissipative particle dynamics model for mesoscopic fluid simulation: methodology, calibration, and application for hydrocarbon and water

The many-body dissipative particle dynamics (mDPD) is a prominent mesoscopic multiphase model for fluid transport in mesoconfinement. However, it has been a long-standing challenge for mDPD (and other multiphase-enabled DPD models) to accurately predict real-fluid static and dynamic properties simultaneously. We have developed a modified mDPD model to overcome the issue and a rigorous calibration approach that uses reference data, including experimental and/or molecular dynamics (MD) simulations to parameterise the modified mDPD for real fluids. We choose heptane as a representative example of hydrocarbon in source rocks to demonstrate the model's capability to accurately predict the equation of state (EOS), free surface tension, diffusivity, and viscosity. Our timing test shows that the modified mDPD is 400–500 times faster than its MD counterpart for simulating bulk heptane in equivalent volumes. To further demonstrate the robustness of the model, we revisited the benchmark problem of mesoscopic modelling of water, in which all the previous works on DPD reported only a limited portion of the water properties. Here, we show that the modified mDPD can provide accurate modelling of water static and dynamic properties and an EOS that matches the experimental data to a large range of confinement pressure.

42 ENGINEERING↗
DOE OSTI2012

ON THE DYNAMICS AND TIDAL DISSIPATION RATE OF THE WHITE DWARF IN 4U 1820-30

It has been suggested that the 170 day period in the light curve of the low-mass X-ray binary 4U 1820-30 arises from the presence of a third body with a large inclination to the binary orbit. We show that this long-period motion arises if the system is librating around the stable fixed point in a Kozai resonance. We demonstrate that mass transfer drives the system toward this fixed point and calculate, both analytically and via numerical integrations, that the period of libration is of order 170 days when the mutual inclination is near the Kozai critical value. The non-zero eccentricity of the binary, combined with tidal dissipation, implies that the rate of change of the binary period would be slower than, or even of opposite sign to, that implied by standard mass transfer models. If the 170 day period results from libration, then, contrary to appearances, the orbital period of the inner binary is increasing with time; in that case, (e/0.009){sup 2} Q/k{sub 2} {approx}> 2.5 Multiplication-Sign 10{sup 9}, where k{sub 2} Almost-Equal-To 0.01 is the tidal Love number and e = 0.009 is the fiducial eccentricity of the inner binary. It appears unlikely that the observed negative period derivative results from the smaller than expected (but positive) value of P-dot combined with the previously suggested acceleration of the system in the gravitational field of the host globular cluster NGC 6624. The discrepancy between the observed and the expected period derivative requires further investigation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Stellar velocity dispersion in dissipative galaxy mergers with star formation

In order to better understand stellar dynamics in merging systems, such as NGC 6240, we examine the evolution of central stellar velocity dispersion (σ{sub *}) in dissipative galaxy mergers using a suite of binary disk merger simulations that include feedback from stellar formation and active galactic nuclei (AGNs). We find that σ{sub *} undergoes the same general stages of evolution that were observed in our previous dissipationless simulations: coherent oscillation, then phase mixing, followed by dynamical equilibrium. We also find that measurements of σ{sub *} that are based only upon the youngest stars in simulations consistently yield lower values than measurements based upon the total stellar population. This finding appears to be consistent with the so-called 'σ{sub *} discrepancy', observed in real galaxies. We note that quasar-level AGN activity is much more likely to occur when σ{sub *} is near its equilibrium value rather than during periods of extreme σ{sub *}. Finally, we provide estimates of the scatter inherent in measuring σ{sub *} in ongoing mergers.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Longitudinal magnetohydrodynamics oscillations in dissipative, cooling coronal loops

This paper investigates the effect of cooling on standing slow magnetosonic waves in coronal magnetic loops. The damping mechanism taken into account is thermal conduction that is a viable candidate for dissipation of slow magnetosonic waves in coronal loops. In contrast to earlier studies, here we assume that the characteristic damping time due to thermal conduction is not small, but arbitrary, and can be of the order of the oscillation period, i.e., a temporally varying plasma is considered. The approximation of low-beta plasma enables us to neglect the magnetic field perturbation when studying longitudinal waves and consider, instead, a one-dimensional motion that allows a reliable first insight into the problem. The background plasma temperature is assumed to be decaying exponentially with time, with the characteristic cooling timescale much larger than the oscillation period. This assumption enables us to use the WKB method to study the evolution of the oscillation amplitude analytically. Using this method we obtain the equation governing the oscillation amplitude. The analytical expressions determining the wave properties are evaluated numerically to investigate the evolution of the oscillation frequency and amplitude with time. The results show that the oscillation period increases with time due to the effect of plasma cooling. The plasma cooling also amplifies the amplitude of oscillations in relatively cool coronal loops, whereas, for very hot coronal loop oscillations the damping rate is enhanced by the cooling. We find that the critical point for which the amplification becomes dominant over the damping is in the region of 4 MK. These theoretical results may serve as impetus for developing the tools of solar magneto-seismology in dynamic plasmas.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Correlation of electron path lengths observed in the highly wound outer region of magnetic clouds with the slab fraction of magnetic turbulence in the dissipation range

Three magnetic cloud events, in which solar impulsive electron events occurred in their outer region, are employed to investigate the difference of path lengths L {sub 0eIII} traveled by non-relativistic electrons from their release site near the Sun to the observer at 1 AU, where L {sub 0eIII} = v {sub l} × (t {sub l} – t {sub III}), v {sub l} and t {sub l} being the velocity and arrival time of electrons in the lowest energy channel (∼27 keV) of the Wind/3DP/SST sensor, respectively, and t {sub III} being the onset time of type III radio bursts. The deduced L {sub 0eIII} value ranges from 1.3 to 3.3 AU. Since a negligible interplanetary scattering level can be seen in both L {sub 0eIII} > 3 AU and ∼1.2 AU events, the difference in L {sub 0eIII} could be linked to the turbulence geometry (slab or two-dimensional) in the solar wind. By using the Wind/MFI magnetic field data with a time resolution of 92 ms, we examine the turbulence geometry in the dissipation range. In our examination, ∼6 minutes of sampled subintervals are used in order to improve time resolution. We have found that, in the transverse turbulence, the observed slab fraction is increased with an increasing L {sub 0eIII} value, reaching ∼100% in the L {sub 0eIII} > 3 AU event. Our observation implies that when only the slab spectral component exists, magnetic flux tubes (magnetic surfaces) are closed and regular for a very long distance along the transport route of particles.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Internal energy dissipation of gamma-ray bursts observed with Swift: Precursors, prompt gamma-rays, extended emission, and late X-ray flares

We jointly analyze the gamma-ray burst (GRB) data observed with Burst Alert Telescope (BAT) and X-ray Telescope on board the Swift mission to present a global view on the internal energy dissipation processes in GRBs, including precursors, prompt gamma-ray emission, extended soft gamma-ray emission, and late X-ray flares. The Bayesian block method is utilized to analyze the BAT light curves to identify various emission episodes. Our results suggest that these emission components likely share the same physical origin, which is the repeated activation of the GRB central engine. What we observe in the gamma-ray band may be a small part of more extended underlying activities. The precursor emission, which is detected in about 10% of Swift GRBs, is preferably detected in those GRBs that have a massive star core-collapse origin. The soft extended emission tail, on the other hand, is preferably detected in those GRBs that have a compact star merger origin. Bright X-ray emission is detected during the BAT quiescent phases prior to subsequent gamma-ray peaks, implying that X-ray emission may be detectable prior the BAT trigger time. Future GRB alert instruments with soft X-ray capability are essential for revealing the early stages of GRB central engine activities, and shedding light on jet composition and the jet launching mechanism in GRBs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Gamma-ray emission in dissipative pulsar magnetospheres: from theory to Fermi observations

We compute the patterns of γ-ray emission due to curvature radiation in dissipative pulsar magnetospheres. Our ultimate goal is to construct macrophysical models that are able to reproduce the observed γ-ray light curve phenomenology recently published in the Second Fermi Pulsar Catalog. We apply specific forms of Ohm's law on the open field lines using a broad range for the macroscopic conductivity values that result in solutions ranging, from near-vacuum to near-force-free. Using these solutions, we generate model γ-ray light curves by calculating realistic trajectories and Lorentz factors of radiating particles under the influence of both the accelerating electric fields and curvature radiation reaction. We further constrain our models using the observed dependence of the phase lags between the radio and γ-ray emission on the γ-ray peak separation. We perform a statistical comparison of our model radio-lag versus peak-separation diagram and the one obtained for the Fermi standard pulsars. We find that for models of uniform conductivity over the entire open magnetic field line region, agreement with observations favors higher values of this parameter. We find, however, significant improvement in fitting the data with models that employ a hybrid form of conductivity, specifically, infinite conductivity interior to the light cylinder and high but finite conductivity on the outside. In these models the γ-ray emission is produced in regions near the equatorial current sheet but modulated by the local physical properties. These models have radio lags near the observed values and statistically best reproduce the observed light curve phenomenology. Additionally, they also produce GeV photon cut-off energies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Tidal dissipation in a homogeneous spherical body. I. Methods

A formula for the tidal dissipation rate in a spherical body is derived from first principles to correct some mathematical inaccuracies found in the literature. The development is combined with the Darwin-Kaula formalism for tides. Our intermediate results are compared with those by Zschau and Platzman. When restricted to the special case of an incompressible spherical planet spinning synchronously without libration, our final formula can be compared with the commonly used expression from Peale and Cassen. However, the two turn out to differ, as in our expression the contributions from all Fourier modes are positive-definite, which is not the case with the formula from Peale and Cassen. Examples of the application of our expression for the tidal damping rate are provided in the work by Makarov and Efroimsky (Paper II) published back to back with the current paper.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Dissipation and reconnection in boundary-driven reduced magnetohydrodynamics

We study the statistics of coherent current sheets, the population of X-type critical points, and reconnection rates in a coronal loop geometry, via numerical simulations of reduced magnetohydrodynamic turbulence. Current sheets and sites of reconnection (magnetic X-points) are identified in two-dimensional planes of the three-dimensional simulation domain. The geometry of the identified current sheets—including area, length, and width—and the magnetic dissipation occurring in the current sheets are statistically characterized. We also examine the role of magnetic reconnection, by computing the reconnection rates at the identified X-points and investigating their association with current sheets.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

STATISTICAL AND SCALING FEATURES OF FLUCTUATIONS IN THE DISSIPATION RANGE DURING A RECONNECTION EVENT

Reconnection events in space plasmas are accompanied by the occurrence of large-amplitude turbulent fluctuations of the magnetic and electric field, covering a wide range of temporal and spatial scales. Here, we study the scaling and statistical features of magnetic and electric field fluctuations below the ion-gyroperiod (i.e., in the dissipation domain) by carefully investigating the occurrence of local or global scaling features during a reconnection event studied by Eastwood et al . Our results point toward the presence of a global scale invariance, i.e., a mono-fractal nature, of fluctuations above the ion-cyclotron frequency and at spatial scales near the ion-inertial length.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Testing dissipative magnetosphere model light curves and spectra with FERMI pulsars

We explore the emission properties of a dissipative pulsar magnetosphere model introduced by Kalapotharakos et al. comparing its high-energy light curves and spectra, due to curvature radiation, with data collected by the Fermi LAT. The magnetosphere structure is assumed to be near the force-free solution. The accelerating electric field, inside the light cylinder (LC), is assumed to be negligible, while outside the LC it rescales with a finite conductivity (σ). In our approach we calculate the corresponding high-energy emission by integrating the trajectories of test particles that originate from the stellar surface, taking into account both the accelerating electric field components and the radiation reaction forces. First, we explore the parameter space assuming different value sets for the stellar magnetic field, stellar period, and conductivity. We show that the general properties of the model are in a good agreement with observed emission characteristics of young γ-ray pulsars, including features of the phase-resolved spectra. Second, we find model parameters that fit each pulsar belonging to a group of eight bright pulsars that have a published phase-resolved spectrum. The σ values that best describe each of the pulsars in this group show an increase with the spin-down rate ( E-dot ) and a decrease with the pulsar age, expected if pair cascades are providing the magnetospheric conductivity. Finally, we explore the limits of our analysis and suggest future directions for improving such models.

79 ASTRONOMY AND ASTROPHYSICS↗