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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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All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

High-Fidelity Modeling Approach for Coupling Thermal, Fluid, and Neutronic Analysis in an NTP Engine

Nuclear thermal propulsion (NTP), with its increased efficiency compared to traditional rocket engines, is a potentially enabling technology for space missions of higher complexity, such as delivering heavy machinery or crew to the moon or other planets. Thus, exploring modeling approaches that can effectively couple the thermal, fluid, and neutronic behaviors of NTP reactors is essential to mitigate cost and schedule challenges with hardware development. This paper details one such approach which utilizes Cardinal to unify reactor physics analysis performed using OpenMC, heat transfer analysis performed using the MOOSE framework, and thermal hydraulic analysis performed using the MOOSE Thermal Hydraulic Module. This approach is demonstrated with a proof of concept that focuses on a legacy reactor geometry from the historic NERVA program. This preliminary model found an overprediction in, when compared to the reference data, axial power peaking and the temperatures of both solids and fluids by roughly 20%. It is clear, however, that the model conserved a number of broad trends, and a few model refinements should reduce the observed error. This approach has definite merit and should enable the production of informative results for more modern NTP systems. This work is a first step in creating a transient reactor model that can be updated across a sequence of time steps based on continually fluctuating, time-dependent boundary conditions.

Nuclear Thermal Propulsion↗
NASA NTRS1982

Low-luminosity accretion onto magnetized neutron stars

The behavior of matter accreting at low rates (M is less than 10 to the 16th g/s) onto the polar caps of a highly magnetized (B = 10 to the 12th G) neutron star is investigated. Flow solutions are found for the case in which the matter undergoes a stationary collisionless shock. It is found that the cyclotron emission is the dominant energy loss mechanism and can yield continuum spectra resembling those observed from X-ray pulsars. A number of relations among the accretion rate, the surface magnetic field, the shock height, and the characteristic electron and ion temperatures are obtained. For magnetic fields greater than 10 to the 12th G, typical values of KTe are several times the cyclotron energy at the surface of the neutron star. When the field drops below 10 to the 12th G, the electrons become very hot and emit gamma-rays.

Langer, S. H.↗
NASA NTRS1997

The Nature and Evolutionary History of GRO J1744-28

GRO J1744-28 is the first known X-ray source to display bursts, periodic pulsations, and quasi-periodic oscillations. This source may thus provide crucial clues that will lead to an understanding of the differences in the nature of the X-ray variability from various accreting neutron stars. The orbital period is 11.8 days, and the measured mass function of 1.31 x 10(exp -4) solar mass is one of the smallest among all known binaries. If we assume that the donor star is a low-mass giant transferring matter through the inner Lagrange point, then we can show that its mass is lower than approximately 0.7 solar mass and probably closer to 0.25 solar mass. Higher mass, but unevolved, donor stars are shown to be implausible. We also demonstrate that the current He core mass of the donor star lies in the range of 0.20-0.25 solar mass. Thus, this system is most likely in the final stages of losing its hydrogen-rich envelope, with only a small amount of mass remaining in the envelope. If this picture is correct, then GRO J1744-28 may well represent the closest observational link that we have between the low-mass X-ray binaries and recycled binary pulsars in wide orbits. We have carried out a series of binary evolution calculations and explored, both systematically and via a novel Monte Carlo approach, the range of initial system parameters and input physics that can lead to the binary parameters of the present-day GRO J1744-28 system. The input parameters include both the initial total mass and the core mass of the donor star, the neutron-star mass, the strength of the magnetic braking, the mass-capture fraction, and the specifics of the core mass/radius relation for giants. Through these evolution calculations, we compute probability distributions for the current binary system parameters (i.e., the total mass, core mass, radius, luminosity, and K-band magnitude of the donor star, the neutron star mass, the orbital inclination angle, and the semimajor axis of the binary). Our calculations yield the following values for the GRO J1744-28 system parameters (with 95% confidence limits in parentheses): donor star mass: 0.24 solar mass (0.2-0.7 solar mass); He core mass of the donor star: 0.22 solar mass (0.20-0.25 solar mass); neutron-star mass: 1.7 solar mass (1.39-1.96 solar mass); orbital inclination angle: 18deg (7deg-22deg); semi- major axis: 64 lt-s (60-67 lt-s); radius of the donor star: 6.2 solar radius(6-9 solar radius); luminosity of donor star: 23 solar luminosity (15-49 solar luminosity), and long-term mass transfer rate at the current epoch: 5 x 10(exp -10)solar mass/yr (2 x 10(exp -10) to 5 x 10(exp -9)solar mass/yr). We deduce that the magnetic field of the underlying neutron star lies in the range of approximately 1.8 x 10(exp 11)G to approximately 7 x 10(exp 11)G, with a most probable value of 2.7 x 10(exp 11)G. This is evidently sufficiently strong to funnel the accretion flow onto the magnetic polar caps and suppress the thermonuclear flashes that would otherwise give rise to the type 1 X-ray bursts observed in most X-ray bursters. We present a simple paradigm for magnetic accreting neutron stars where X-ray pulsars, GRO J1744-28, the Rapid Burster, and the type 1 X-ray bursters may form a continuum of possible behaviors among accreting neutron stars, with the strength of the neutron-star magnetic field serving as a crucial parameter that determines the mode of X-ray variability from a given object.

Rappaport, S.↗
NASA NTRS2003

The Bursting Behavior of 4U 1728-34: Parameters of a Neutron Star and Geometry of a NS-disk system

We analyze a set of Type I X-ray bursts from the low mass X-ray binary 4U 1728-34, observed with Rossi X-ray Timing Explorer (RXTE). We infer the dependence of the neutron star (NS) mass and radius with respect to the assumed distance to the system using an analytical model of X-ray burst spectral formation. The model behavior clearly indicates that the burster atmosphere is helium-dominated. Our results strongly favor the soft equation of state (EOS) of NS for 4U 1728-34. We find that distance to the source should be within 4.5-5.0 kpc range. We obtain rather narrow constrains for the NS radius in 8.7-9.7 km range and interval 1.2-1.6 Ma for NS mass for this particular distance range. We uncover a temporal behavior of red-shift corrected burst flux for the radial expansion episodes and we put forth a dynamical evolution scenario for the NS accretion disk geometry during which an expanded envelope affects the accretion disk and increases the area of the neutron star exposed to the Earth observer. In the framework of this scenario we provide a new method for the estimation of the inclination angle which leads to the value of approximately 50 degrees for 4U 1728-34.

Shaposhnikov, Nickolai↗
NASA NTRS2002

Structural Fluctuations and Thermophysical Properties of Molten II-VI Compounds

The objectives of the project are to conduct ground-based experimental and theoretical research on the structural fluctuations and thermophysical properties of molten II-VI compounds to enhance the basic understanding of the existing flight experiments in microgravity materials science programs as well as to study the fundamental heterophase fluctuation phenomena in these melts by: 1) conducting neutron scattering analysis and measuring quantitatively the relevant thermophysical properties of the II-VI melts (such as viscosity, electrical conductivity, thermal diffusivity and density) as well as the relaxation characteristics of these properties to advance the understanding of the structural properties and the relaxation phenomena in these melts and 2) performing theoretical analyses on the melt systems to interpret the experimental results. All the facilities required for the experimental measurements have been procured, installed and tested. Thermal diffusivity of molten tellurium has been measured by a laser flash method in the temperature range of 500 C to 900 C. The measured diffusivity as a function of temperature agrees fairly well with published data. However, a relaxation phenomenon, which shows a slow drift of the measured thermal conductivity toward the equilibrium value after cooling of the melt, was observed for the first time. An apparatus based on the transient torque induced by a rotating magnetic field has been developed to determine the viscosity and electrical conductivity of semiconducting liquids. Viscosity measurements on molten tellurium showed a similar relaxation behavior to the measured diffusivity. The density and volume expansion coefficients for pure Te and HgTe melts were measured as a function of temperature using a pycnometric method. A density maximum was found for both melts but no relaxation behavior was observed. Neutron scattering experiments were performed on the HgTe and HgZnTe melts and the results on pair distribution showed better resolution than previously reported. A simple kinetic theory, which contains the formation reactions of Te polymerization that increases the molecular weight, was proposed to interpret the relaxation behavior of HgZnTe viscosity data.

Su, Ching-Hua↗
NASA NTRS1981

Giant glitches and pinned vorticity in the Vela and other pulsars

Previously unexamined regularities in the Vela pulsar timing data are noted, and explained by a theory of giant glitches in the Vela and other pulsars as the dynamic consequence of catastrophic unpinning events in the pinned crustal neutron superfluid. Postglitch behavior then results from the glitch-induced vortex creep. An implication of the theory is that the energy release in this glitch scenario need not appear instantaneously on the surface as heat.

Alpar, M. A.↗
NASA NTRS1982

Gamma-ray burst spectra

A review of recent results in gamma-ray burst spectroscopy is given. Particular attention is paid to the recent discovery of emission and absorption features in the burst spectra. These lines represent the strongest evidence to date that gamma-ray bursts originate on or near neutron stars. Line parameters give information on the temperature, magnetic field and possibly the gravitational potential of the neutron star. The behavior of the continuum spectrum is also discussed. A remarkably good fit to nearly all bursts is obtained with a thermal-bremsstrahlung-like continuum. Significant evolution is observed of both the continuum and line features within most events.

Teegarden, B. J.↗
NASA NTRS2003

Unraveling the Cooling Trend of the Soft Gamma Repeater, SGR162-41

SGR 1627-41 was discovered in 1998 after a single active episode which lasted approx. 6 weeks. We report here our monitoring results of the decay trend of the persistent X-ray luminosity of the source during the last 5 years. We find an initial temporal power law decay with index 0.47, reaching a piateau which is followed by a sharp (factor of ten) flux decline approx. 800 days after the source activation. The source spectrum is best described during the entire period by a single power law with high absorption (N(sub H) = 9.0(7) x 10(exp 22)/sq cm); the spectral index, however, varies dramatically between 2.2-3.8 spanning the entire range for all known SGR sources. We discuss the cooling behavior of the neutron star assuming a deep crustal heating initiated by the burst activity of the source during 1998.

Kouveliotou, C.↗
NASA NTRS2003

Unraveling the Cooling Trend of the Soft Gamma Repeater SGR 1627-41

SGR 1627-41 was discovered in 1998 after a single active episode that lasted approximately 6 weeks. We report here our monitoring results of the decay trend of the persistent X-ray luminosity of the source during the last 5 years. We find an initial temporal power-law decay with index 0.47, reaching a plateau that is followed by a sharp (factor of 10) flux decline approximately 800 days after the source activation. The source spectrum is best described during the entire period by a single power law with high absorption [N(sub H) = 9.0(7) x 10(exp 22) per square centimeter]; the spectral index, however, varies dramatically between 2.2 and 3.8 spanning the entire range for all known soft gamma repeater sources. We discuss the cooling behavior of the neutron star assuming a deep crustal heating initiated by the burst activity of the source during 1998.

Kouveliotou, C.↗
NASA NTRS2021

The X-Ray Pulsar XTE J1858+034 Observed with NuSTAR and Fermi/GBM: Spectral and Timing Characterization plus a Cyclotron Line

Accreting X-ray pulsars (XRPs) undergo luminous X-ray outbursts during which the spectral and timing behavior of the neutron star can be studied in detail. We analyze a NuSTAR observation of the XRP XTE J1858+034 during its outburst in 2019. The spectrum is fit with a phenomenological, a semi-empirical and a physical spectral model. A candidate cyclotron line is found at 48 keV, implying a magnetic field of 5.4×1012G at the site of emission. This is also supported by the physical best-fit model. A nominal Gaia distance of 1.55 kpc is available for a star that is close to – although not coincident with – the previously proposed optical counterpart but, based on Fermi Gamma-ray Burst Monitor data, the standard disk accretion-torque theory allowed us to infer a distance of 10.9±1.0 kpc. Pulse profiles are single-peaked and show a pulsed fraction that is strongly energy-dependent at least up to 40keV.

C Malacaria↗
NASA NTRS2002

Characteristics of Transients in the Galactic Bulge

Biweekly X-ray observations of the Galactic Bulge with the Rossi X-Ray Timing Explorer Proportional Counter Array for 5 years have built up statistics for different kinds of source behavior for low-mass binaries with neutron stars. The relation between the characteristic behaviors is not known. A typical transient may rise fast and decay exponentially in brightness and repeat this cycle after short (1/2 yr) or long intervals (20 yr). But an X-ray source also may be steady for an extended period (years) before the disk can become quiescent. It fades in the same way as do the transients which immediately decay. These sources have been reported to be very cool in quiescence; either not substantially heated by outbursts, or cooled very quickly. There are 3 sources active during the last 5 years which were long lasting transients which turned off and 1 which turned on. We will compare their X-ray characteristics in outburst, and in so far as available with Chandra, in quiescence.

Swank, Jean H.↗
NASA NTRS1993

Successive X-ray bursts from accreting neutron stars

The evolution of a neutron star undergoing a series of thermonuclear flashes in its accreted hydrogen-rich layer has been numerically followed to determine the effects of the history of the neutron star's thermal and compositional structure on the properties of the emitted X-ray bursts. Burst characteristics are studied for a range of mass accretion rates, CNO abundances in the accreted matter, and initial thermal states of the underlying neutron star core; the bursts exhibit erratic behavior for low CNO metal abundances and cool neutron star cores, with the burst recurrence time scales varying by 1-2 orders of magnitude. There is typically a continued presence of a substantial amount of unburnt hydrogen in the accreted layer throughout the series of the X-ray burst events. Convective mixing during the quiescent phase leads to the inward transport of helium to high densities and eventually to the initiation of the next outburst. The resulting bursts can be weak and, in such cases, are characterized by short recurrence time scales (1-2 hr), low peak luminosities (0.1-0.2 times the Eddington value), and low alpha-values (about 20).

Taam, Ronald E.↗