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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Final Technical Report for NSF/DOE partnership grant 1004284/ER54905/SC0004660: 2011- 2013

Research has been performed on two topics: (1) Nonlinear time-dependent phenomena in the regime of electron magnetohydrodynamics (EMHD). (2) Plasma structures forming ”bubbles”. The latter are regions of localized density depletions or enhancements separated from the ambient plasma by sheaths or double layers. Three-dimensional magnetic null points can also be considered as ”magnetic bubbles”. Nonlinear EMHD fields are generated by pulsed magnetic loops. The magnetic field varies in time faster than an ion cyclotron period, hence interacts only with electrons. Space charge electric fields are created when magnetic forces try to separate electrons from ions. Electric and magnetic forces nearly balance to keep the plasma quasi neutral. This leads to rotational electron motions, i.e. Hall currents and associated magnetic perturbations. When the time scale falls between the ion and electron cyclotron period the magnetic perturbation is convected in the whistler mode. When the time-varying magnetic field exceeds the background magnetic field the whistler mode becomes highly nonlinear since its properties depend on the total magnetic field. When the time-varying magnetic field creates magnetic null points or null lines EMHD breaks down. Electrons can be accelerated in null lines which dissipated magnetic energy like in magnetic reconnection. Energized electrons with anisotropic distributions create secondary whistler instabilities. These effects have been observed in field topologies resembling spheromaks and field-reversed configurations (FRCs). The whistler mode propagation near magnetic nulls is not trivial when the field gradients and wavelength are comparable. Ray tracing is inappropriate when the WKB approximation breaks down. Experiments have been started to map wave propagation on curved field lines with null points. Initial results show that whistlers in highly nonuniform fields have highly oblique phase velocities but the energy flow remains dominantly along the field. Density bubbles are created by anode discharges, typically forming spherical ”fireballs” bounded by a double layer. In a magnetized plasma the shape becomes cylindrical, in nonuniform magnetic fields many asymmetric fireball shapes are possible. Due to electron energization the fireball plasma has higher density and temperature than the ambient plasma. The double layer can also be produced by a high transparency positively biased grid. Instabilities arise from the electron transit through the plasma bubble and the double layer. When a permanent magnet is biased positively the electrons perform ExB drifts in the equatorial plane between the poles. Electron drift modes are excited with a high spectral contents. Similarly, a negatively biased magnet produces a disk-shaped plasma bubble. The cold magnet emits secondary electrons due to the impact of energetic ions. Such magnetron discharges exhibit a variety of instabilities which affect the cross-field transport and sputtering applications. This report will summarize the main effects observed and provide references to more detailed publications.

, nonlinear time-dependent phenomena↗
NASA NTRS1994

A study of M31, M87, NGC 253, and M82 in high-energy gamma rays

The data from the Energetic Gamma Ray Experiment Telescope (EGRET) all-sky survey are examined for emission from the nearby galaxies M31, M87, NGC 253, and M82 in the high-energy (E greater than MeV) gamma-ray range. No significant emission is observed from any of these galaxies. The derived upper limits for all four galaxies are consistent with that expected from cosmic-ray interactions. For M87, the combination of the high-energy gamma-ray and radio data point to a lower limit of 7 microG for the magnetic field in the disk and 4 microG for the magnetic field in the halo, consistent with equipartition arguments. A study of NGC 253 and M82 show that EGRET observations do not sufficiently constrain the mean cosmic-ray energy density to provide more details on the cosmic-ray distribution in starburst galaxies.

Sreekumar, P.↗
DOE OSTI2014

Effects of turbulence on cosmic ray propagation in protostars and young star/disk systems

The magnetic fields associated with young stellar objects are expected to have an hour-glass geometry, i.e., the magnetic field lines are pinched as they thread the equatorial plane surrounding the forming star but merge smoothly onto a background field at large distances. With this field configuration, incoming cosmic rays experience both a funneling effect that acts to enhance the flux impinging on the circumstellar disk and a magnetic mirroring effect that acts to reduce that flux. To leading order, these effects nearly cancel out for simple underlying magnetic field structures. However, the environments surrounding young stellar objects are expected to be highly turbulent. This paper shows how the presence of magnetic field fluctuations affects the process of magnetic mirroring, and thereby changes the flux of cosmic rays striking circumstellar disks. Turbulence has two principle effects: (1) the (single) location of the magnetic mirror point found in the absence of turbulence is replaced with a wide distribution of values. (2) The median of the mirror point distribution moves outward for sufficiently large fluctuation amplitudes (roughly when δB/B {sub 0} > 0.2 at the location of the turbulence-free mirror point); the distribution becomes significantly non-Gaussian in this regime as well. These results may have significant consequences for the ionization fraction of the disk, which in turn dictates the efficiency with which disk material can accrete onto the central object. A similar reduction in cosmic ray flux can occur during the earlier protostellar stages; the decrease in ionization can help alleviate the magnetic braking problem that inhibits disk formation.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2000

Submillimeter Polarimetric Observations of the Galactic Center

We report the first detection of polarized submillimeter emission from the Sagittarius A region at the Galactic center. We observed three separate 2' x 2' fields: one centered on the circumnuclear disk, one centered on the peak of the molecular cloud M -0.02-0.07 (also called the "50 kilometer per second cloud"), and one centered on the peak of the molecular cloud M -0.13-0.08 (also called the "20 kilometer per second cloud"). Linear polarization at lambda = 350 micrometers was detected in each of the three regions, at a total of 106 distinct sky positions. In the circumnuclear disk, the projected magnetic field directions that we infer from our measurements are similar to those inferred from previous far-infrared (lambda = 100 micrometers) polarimetry. In the "curved ridge" region of M -0.02-0.07 that has been compressed by the expansion of Sgr A East, our results show clearly the effects of this compression on the magnetic field. In M -0.13-0.08, we observe what appears to be a stretched magnetic field, as expected for this tidally sheared cloud. It has been suggested that a "finger-like extension" or streamer" from M -0.13-0.08 is falling into the circumnuclear disk. We tentatively interpret a flaring of magnetic field lines that we see in M -0.13-0.08 as evidence that the entire cloud has a velocity component in the Galactic eastern direction, i.e., toward the circumnuclear disk. Further observations are needed to test this interpretation. We argue that polarimetry of dust emission provides a promising tool for obtaining new information on the complex dynamics of neutral gas in the Galactic center.

Novak, G.↗
NASA NTRS1996

A media maniac's guide to removable mass storage media

This paper addresses at a high level, the many individual technologies available today in the removable storage arena including removable magnetic tapes, magnetic floppies, optical disks and optical tape. Tape recorders represented below discuss logitudinal, serpantine, logitudinal serpantine,and helical scan technologies. The magnetic floppies discussed will be used for personal electronic in-box applications.Optical disks still fill the role for dense long-term storage. The media capacities quoted are for native data. In some cases, 2 KB ASC2 pages or 50 KB document images will be referenced.

Kempster, Linda S.↗
DOE OSTI2022

Variability and Spectral Characteristics of Three Flaring Gamma-Ray Quasars Observed by VERITAS and Fermi-LAT

Flat-spectrum radio quasars (FSRQs) are the most luminous blazars at GeV energies but only rarely emit detectable fluxes of TeV gamma rays, typically during bright GeV flares. We explore the gamma-ray variability and spectral characteristics of three FSRQs that have been observed at GeV and TeV energies by Fermi-LAT and VERITAS, making use of almost 100 hr of VERITAS observations spread over 10 yr: 3C 279, PKS 1222+216, and Ton 599. We explain the GeV flux distributions of the sources in terms of a model derived from a stochastic differential equation describing fluctuations in the magnetic field in the accretion disk and estimate the timescales of magnetic flux accumulation and stochastic instabilities in their accretion disks. We identify distinct flares using a procedure based on Bayesian blocks and analyze their daily and subdaily variability and gamma-ray energy spectra. Using observations from VERITAS, as well as Fermi, Swift, and the Steward Observatory, we model the broadband spectral energy distributions of PKS 1222+216 and Ton 599 during very high energy (VHE)–detected flares in 2014 and 2017, respectively, strongly constraining the jet Doppler factors and gamma-ray emission region locations during these events. Finally, we place theoretical constraints on the potential production of PeV-scale neutrinos during these VHE flares.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

High Speed Tribology of the Head-Disk Interface by Computer Simulation Final Report CRADA No. TC-297-92-C

The purpose of this project is to develop and validate a molecular dynamics model of nanoscale tribological processes occurring inside the head-disk interface of magnetic recording devices. Within the three years of the project the partners will: write a new molecular dynamics computer simulation code for growth, sliding and impact inside the head-disk interface; identify accurate models for chemical bonding within coatings used to protect magnetic surfaces, especially amorphous carbon thin films; simulate the growth of amorphous carbon thin films and analyse the resulting chemical bonding; simulate friction and wear processes relevant to the head-disk interface; and validate the model through comparison with AFM and nanoindenter studies of friction and wear of amorphous carbon thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Temperature fluctuations driven by magnetorotational instability in protoplanetary disks

The magnetorotational instability (MRI) drives magnetized turbulence in sufficiently ionized regions of protoplanetary disks, leading to mass accretion. The dissipation of the potential energy associated with this accretion determines the thermal structure of accreting regions. Until recently, the heating from the turbulence has only been treated in an azimuthally averaged sense, neglecting local fluctuations. However, magnetized turbulence dissipates its energy intermittently in current sheet structures. We study this intermittent energy dissipation using high resolution numerical models including a treatment of radiative thermal diffusion in an optically thick regime. Our models predict that these turbulent current sheets drive order-unity temperature variations even where the MRI is damped strongly by Ohmic resistivity. This implies that the current sheet structures where energy dissipation occurs must be well-resolved to correctly capture the flow structure in numerical models. Higher resolutions are required to resolve energy dissipation than to resolve the magnetic field strength or accretion stresses. The temperature variations are large enough to have major consequences for mineral formation in disks, including melting chondrules, remelting calcium-aluminum-rich inclusions, and annealing silicates; and may drive hysteresis: current sheets in MRI active regions could be significantly more conductive than the remainder of the disk.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1990

Fast, Capacious Disk Memory Device

Device for recording digital data on, and playing back data from, memory disks has high recording or playback rate and utilizes available recording area more fully. Two disks, each with own reading/writing head, used to record data at same time. Head on disk A operates on one of tracks numbered from outside in; head on disk B operates on track of same number in sequence from inside out. Underlying concept of device applicable to magnetic or optical disks.

Muller, Ronald M.↗
NASA NTRS1993

Magnetic bearings for a high-performance optical disk buffer

An optical disk buffer concept can provide gigabit-per-second data rates and terabit capacity through the use of arrays of solid state lasers applied to a stack of erasable/reusable optical disks. The RCA optical disk buffer has evoked interest by NASA for space applications. The porous graphite air bearings in the rotary spindle as well as those used in the linear translation of the read/write head would be replaced by magnetic bearings or mechanical (ball or roller) bearings. Based upon past experience, roller or ball bearings for the translation stages are not feasible. Unsatisfactory, although limited experience exists with ball bearing spindles also. Magnetic bearings, however, appear ideally suited for both applications. The use of magnetic bearings is advantageous in the optical disk buffer because of the absence of physical contact between the rotating and stationary members. This frictionless operation leads to extended life and reduced drag. The manufacturing tolerances that are required to fabricate magnetic bearings would also be relaxed from those required for precision ball and gas bearings. Since magnetic bearings require no lubricant, they are inherently compatible with a space (vacuum) environment. Magnetic bearings also allow the dynamics of the rotor/bearing system to be altered through the use of active control. This provides the potential for reduced vibration, extended regions of stable operation, and more precise control of position.

Hockney, Richard↗
DOE OSTI2016

A MODEL FOR (QUASI-)PERIODIC MULTIWAVELENGTH PHOTOMETRIC VARIABILITY IN YOUNG STELLAR OBJECTS

We present radiation transfer models of rotating young stellar objects (YSOs) with hot spots in their atmospheres, inner disk warps, and other three-dimensional effects in the nearby circumstellar environment. Our models are based on the geometry expected from magneto-accretion theory, where material moving inward in the disk flows along magnetic field lines to the star and creates stellar hot spots upon impact. Due to rotation of the star and magnetosphere, the disk is variably illuminated. We compare our model light curves to data from the Spitzer YSOVAR project to determine if these processes can explain the variability observed at optical and mid-infrared wavelengths in young stars. We focus on those variables exhibiting “dipper” behavior that may be periodic, quasi-periodic, or aperiodic. We find that the stellar hot-spot size and temperature affects the optical and near-infrared light curves, while the shape and vertical extent of the inner disk warp affects the mid-IR light curve variations. Clumpy disk distributions with non-uniform fractal density structure produce more stochastic light curves. We conclude that magneto-accretion theory is consistent with certain aspects of the multiwavelength photometric variability exhibited by low-mass YSOs. More detailed modeling of individual sources can be used to better determine the stellar hot-spot and inner disk geometries of particular sources.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Thermal effects on transducer material for heat assisted magnetic recording application

Heat Assisted Magnetic Recording (HAMR) is a promising technology for next generation hard disk drives with significantly increased data recording capacities. In HAMR, an optical near-field transducer (NFT) is used to concentrate laser energy on a magnetic recording medium to fulfill the heat assist function. The key components of a NFT are transducer material, cladding material, and adhesion material between the cladding and the transducer materials. Since transducer materials and cladding materials have been widely reported, this paper focuses on the adhesion materials between the Au transducer and the Al{sub 2}O{sub 3} cladding material. A comparative study for two kinds of adhesion material, Ta and Cr, has been conducted. We found that Ta provides better thermal stability to the whole transducer than Cr. This is because after thermal annealing, chromium forms oxide material at interfaces and chromium atoms diffuse remarkably into the Au layer and react with Au to form Au alloy. This study also provides insights on the selection of adhesion material for HAMR transducer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

SPITZER OBSERVATIONS OF LONG-TERM INFRARED VARIABILITY AMONG YOUNG STELLAR OBJECTS IN CHAMAELEON I

Infrared variability is common among young stellar objects, with surveys finding daily to weekly fluctuations of a few tenths of a magnitude. Space-based observations can produce highly sampled infrared light curves, but are often limited to total baselines of about 1 month due to the orientation of the spacecraft. Here we present observations of the Chameleon I cluster, whose low declination makes it observable by the Spitzer Space Telescope over a 200-day period. We observe 30 young stellar objects with a daily cadence to better sample variability on timescales of months. We find that such variability is common, occurring in ∼80% of the detected cluster members. The change in [3.6]–[4.5] color over 200 days for many of the sources falls between that expected for extinction and fluctuations in disk emission. With our high cadence and long baseline we can derive power spectral density curves covering two orders of magnitude in frequency and find significant power at low frequencies, up to the boundaries of our 200-day survey. Such long timescales are difficult to explain with variations driven by the interaction between the disk and stellar magnetic field, which has a dynamical timescale of days to weeks. The most likely explanation is either structural or temperature fluctuations spread throughout the inner ∼0.5 au of the disk, suggesting that the intrinsic dust structure is highly dynamic.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Magnetic flux concentration and zonal flows in magnetorotational instability turbulence

Accretion disks are likely threaded by external vertical magnetic flux, which enhances the level of turbulence via the magnetorotational instability (MRI). Using shearing-box simulations, we find that such external magnetic flux also strongly enhances the amplitude of banded radial density variations known as zonal flows. Moreover, we report that vertical magnetic flux is strongly concentrated toward low-density regions of the zonal flow. Mean vertical magnetic field can be more than doubled in low-density regions, and reduced to nearly zero in high-density regions in some cases. In ideal MHD, the scale on which magnetic flux concentrates can reach a few disk scale heights. In the non-ideal MHD regime with strong ambipolar diffusion, magnetic flux is concentrated into thin axisymmetric shells at some enhanced level, whose size is typically less than half a scale height. We show that magnetic flux concentration is closely related to the fact that the turbulent diffusivity of the MRI turbulence is anisotropic. In addition to a conventional Ohmic-like turbulent resistivity, we find that there is a correlation between the vertical velocity and horizontal magnetic field fluctuations that produces a mean electric field that acts to anti-diffuse the vertical magnetic flux. The anisotropic turbulent diffusivity has analogies to the Hall effect, and may have important implications for magnetic flux transport in accretion disks. The physical origin of magnetic flux concentration may be related to the development of channel flows followed by magnetic reconnection, which acts to decrease the mass-to-flux ratio in localized regions. The association of enhanced zonal flows with magnetic flux concentration may lead to global pressure bumps in protoplanetary disks that helps trap dust particles and facilitates planet formation.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1992

The synchrotron bays of the Crab Nebula - A magnetic structure associated with a presupernova circumstellar disk

We investigate the nature of two large conical indentations in the Crab Nebula's synchrotron emission, commonly known as the east and west 'bays'. Optical images spanning nearly nine decades (1899-1988) reveal the bays to be long-lasting remnant features. Proper motions of the bays away from the remnant's center of expansion are consistent with their locations near the SN progenitor site in A.D. 1054. Based on their morphologies and polarization properties, the bays appear to be portions of a torus encircling the remnant's center of expansion. The nebular magnetic field wrapped around this torus blocks the pulsar's relativistic particles, resulting in the observed lack of synchrotron emission. We propose that this torus is the result of a presupernova disklike ejection not less than 0.1 solar mass by the Crab progenitor, similar to the ring around SN 1987 A. A 0.5 solar mass disk of He-rich filaments along the equatorial plane is probably also related to presupernova mass loss, which might explain the wide range of abundances seen among filaments.

Fesen, Robert A.↗
NASA NTRS1989

Buoyancy-limited magnetic viscosity in quasi-stellar object accretion disk models

Alpha models of astrophysical accretion disks assume an ad hoc viscous stress which scales locally as the total pressure. It is shown here that, if turbulent magnetic Maxwell stresses are the source of this viscosity in quasi-stellar object accretion disk models, then the stress cannot follow the assumed alpha-law in the radiation pressure-dominated inner region of the disk.

Sakimoto, Philip J.↗
DOE OSTI2022

A Nonlocal Magneto-curvature Instability in a Differentially Rotating Disk

A global mode is shown to be unstable to nonaxisymmetric perturbations in a differentially rotating Keplerian disk containing either vertical or azimuthal magnetic fields. In an unstratified cylindrical disk model, using both global eigenvalue stability analysis and linear global initial-value simulations, it is demonstrated that this instability dominates at strong magnetic fields where local standard magnetorotational instability (MRI) becomes stable. Unlike the standard MRI mode, which is concentrated in the high flow shear region, these distinct global modes (with low azimuthal mode numbers) are extended in the global domain and are Alfvén-continuum-driven unstable modes. As its mode structure and relative dominance over MRI are inherently determined by the global spatial curvature as well as the flow shear in the presence of a magnetic field, we call it the magneto-curvature (magneto-spatial-curvature) instability. Consistent with the linear analysis, as the field strength is increased in the nonlinear simulations, a transition from MRI-driven turbulence to a state dominated by global nonaxisymmetric modes is obtained. This global instability could therefore be a source of nonlinear transport in accretion disks at a higher magnetic field than predicted by local models.

79 ASTRONOMY AND ASTROPHYSICS↗