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All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Tutorial: Performance and reliability in redundant disk arrays

A disk array is a collection of physically small magnetic disks that is packaged as a single unit but operates in parallel. Disk arrays capitalize on the availability of small-diameter disks from a price-competitive market to provide the cost, volume, and capacity of current disk systems but many times their performance. Unfortunately, relative to current disk systems, the larger number of components in disk arrays leads to higher rates of failure. To tolerate failures, redundant disk arrays devote a fraction of their capacity to an encoding of their information. This redundant information enables the contents of a failed disk to be recovered from the contents of non-failed disks. The simplest and least expensive encoding for this redundancy, known as N+1 parity is highlighted. In addition to compensating for the higher failure rates of disk arrays, redundancy allows highly reliable secondary storage systems to be built much more cost-effectively than is now achieved in conventional duplicated disks. Disk arrays that combine redundancy with the parallelism of many small-diameter disks are often called Redundant Arrays of Inexpensive Disks (RAID). This combination promises improvements to both the performance and the reliability of secondary storage. For example, IBM's premier disk product, the IBM 3390, is compared to a redundant disk array constructed of 84 IBM 0661 3 1/2-inch disks. The redundant disk array has comparable or superior values for each of the metrics given and appears likely to cost less. In the first section of this tutorial, I explain how disk arrays exploit the emergence of high performance, small magnetic disks to provide cost-effective disk parallelism that combats the access and transfer gap problems. The flexibility of disk-array configurations benefits manufacturer and consumer alike. In contrast, I describe in this tutorial's second half how parallelism, achieved through increasing numbers of components, causes overall failure rates to rise. Redundant disk arrays overcome this threat to data reliability by ensuring that data remains available during and after component failures.

Gibson, Garth A.↗
NASA NTRS1990

Self-collimated electromagnetic jets from magnetized accretion disks - The even-symmetry case

This paper extends the previous treatment (Lovelace et al., 1987) of the origin of self-collimated EM jets to the case of even field symmetry, where the magnetic flux function Psi(r, z) is an even function of z. A viscous resistive accretion disk is assumed to surround a black hole with a force-free plasma outside of the disk. Inside the disk, the induction equation is solved for Psi(r, z) and the toroidal magnetic field. Outside the disk, previous results are used to study the formation of self-collimated EM jets. In contrast with the odd-symmetry case, for even symmetry the toroidal magnetic field acts to vertically compress the disk; a comparatively large toroidal magnetic field can exist inside the disk; and an appreciable fraction (possibly all) of the available accretion power can go into the jets.

Wang, J. C. L.↗
NASA NTRS1995

The Production of Jets From Magnetized Accretion Disks: Simulation of the Blandford-Payne Mechanism

We have used massively parallel supercomputer simulations to perform an extensive study of a plausible mechanism for producing the jets seen in extragalactic radio sources - acceleration and collimation by coronal magnetic fields in an accretion disk orbiting a central black hole. We find that such disks can propel jets for a wide range of coronal conditions. The terminal jet velocity is a strong function of the magnetic field equatorial component. Acceleration and collimation are produced by a tight azimuthal field coil generated in the corona, rather than by a stiff poloidal field extending to large distances. The jets are pressure-confined when the external medium pressure is high, but magnetically- confined when it is low.

Blandford-Payne Mechanism↗
DOE OSTI2013

Global hydromagnetic simulations of a planet embedded in a dead zone: Gap opening, gas accretion, and formation of a protoplanetary jet

We present global hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations with mesh refinement of accreting planets embedded in protoplanetary disks (PPDs). The magnetized disk includes Ohmic resistivity that depends on the overlying mass column, leading to turbulent surface layers and a dead zone near the midplane. The main results are: (1) the accretion flow in the Hill sphere is intrinsically three-dimensional for HD and MHD models. Net inflow toward the planet is dominated by high-latitude flows. A circumplanetary disk (CPD) forms. Its midplane flows outward in a pattern whose details differ between models. (2) The opening of a gap magnetically couples and ignites the dead zone near the planet, leading to stochastic accretion, a quasi-turbulent flow in the Hill sphere, and a CPD whose structure displays high levels of variability. (3) Advection of magnetized gas onto the rotating CPD generates helical fields that launch magnetocentrifugally driven outflows. During one specific epoch, a highly collimated, one-sided jet is observed. (4) The CPD's surface density is ∼30 g cm{sup −2}, small enough for significant ionization and turbulence to develop. (5) The accretion rate onto the planet in the MHD simulation reaches a steady value 8 × 10{sup –3} M {sub ⊕} yr{sup –1} and is similar in the viscous HD runs. Our results suggest that gas accretion onto a forming giant planet within a magnetized PPD with a dead zone allows rapid growth from Saturnian to Jovian masses. As well as being relevant for giant planet formation, these results have important implications for the formation of regular satellites around gas giant planets.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Strain induced vortex-to-uniform polarization transitions in soft-ferroelectric nanoparticles

Domain structures of ferroelectric polarization in patterned nanostructures depend sensitively on an interplay between their geometry and crystallographic anisotropy. In the recently predicted layered perovskite PbSr2Ti2O7 [S. M. Nakhmanson and I. Naumov, Phys. Rev. Lett. 104, 097601 (2010)], the in-plane anisotropy can be controlled by strain to be along in-plane [110]-directions or to vanish, in which case the polarization is free to rotate in the perovskite-layer. Using a microscopic Landau-Ginzburg-Devonshire free energy functional theory, we demonstrate that the domain structure in patterned disks of PbSr2Ti2O7 can be varied between uniform polarization and a vortex structure, analogous to vortices in soft magnetic disks. This opens up the possibility of designing nanostructured layered materials whose dielectric response can be manipulated with small elastic distortions.

Lee, Byounghak↗
NASA NTRS2003

Hydrodynamic Instability and Enhanced Transport in Protoplanetary Nebulae

The nature of turbulence (and the enhanced transport it provides) is a key element to comprehend the dynamics, physics and chemistry of the protoplanetary nebulae and consequently the planet formation process. Early accretion disk models postulated the turbulent transport through the well-known "alpha-viscosity" model, introduced by Shakura and Sunyaev in 1973. Since then, the nature of the turbulence in disks has been a subject of investigation. In 1991, the rediscovery by Balbus and Hawley of Chandrasekhar's linear instability in magnetized disks was a breakthrough in the discipline. Unfortunately, the mechanisms leading to turbulence in non-magnetized disks, such as protoplanetary nebulae, remain poorly understood. We will present results from laboratory experiments along with analytical arguments showing that, despite skepticism in the Astrophysical community, differential rotation may indeed be sufficient to trigger and sustain turbulence. We will also propose an alternative viscosity prescription derived from both experiments and analysis.

Richard, Denis T.↗
NASA NTRS1977

X-ray bursts from magnetized accretion disks

A model for the X-ray bursts of MXB 1730-335 and 3U 1820-30 (identified with the globular cluster NGC 6624) is presented in which a burst is due to the flarelike release of magnetic energy generated and stored as a toroidal field in an accretion disk. To fit the observed parameters, this model requires compact objects of the order of 10 solar masses, presumably black holes.

Wheeler, J. C.↗
DOE OSTI2016

Magneto-thermal disk winds from protoplanetary disks

The global evolution and dispersal of protoplanetary disks (PPDs) are governed by disk angular-momentum transport and mass-loss processes. Recent numerical studies suggest that angular-momentum transport in the inner region of PPDs is largely driven by magnetized disk wind, yet the wind mass-loss rate remains unconstrained. On the other hand, disk mass loss has conventionally been attributed to photoevaporation, where external heating on the disk surface drives a thermal wind. We unify the two scenarios by developing a one-dimensional model of magnetized disk winds with a simple treatment of thermodynamics as a proxy for external heating. The wind properties largely depend on (1) the magnetic field strength at the wind base, characterized by the poloidal Alfvén speed v{sub Ap}, (2) the sound speed c{sub s} near the wind base, and (3) how rapidly poloidal field lines diverge (achieve R{sup −2} scaling). When v{sub Ap}≫c{sub s}, corotation is enforced near the wind base, resulting in centrifugal acceleration. Otherwise, the wind is accelerated mainly by the pressure of the toroidal magnetic field. In both cases, the dominant role played by magnetic forces likely yields wind outflow rates that exceed purely hydrodynamical mechanisms. For typical PPD accretion-rate and wind-launching conditions, we expect v{sub Ap} to be comparable to c{sub s} at the wind base. The resulting wind is heavily loaded, with a total wind mass-loss rate likely reaching a considerable fraction of the wind-driven accretion rate. Implications for modeling global disk evolution and planet formation are also discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Herbig stars' near-infrared excess: An origin in the protostellar disk's magnetically supported atmosphere

Young stars with masses 2-8 times solar, the Herbig Ae and Be stars, often show a near-infrared excess too large to explain with a hydrostatically supported circumstellar disk of gas and dust. At the same time, the accretion flow carrying the circumstellar gas to the star is thought to be driven by magnetorotational turbulence, which, according to numerical MHD modeling, yields an extended low-density atmosphere supported by the magnetic fields. We demonstrate that the base of the atmosphere can be optically thick to the starlight and that the parts lying near 1 AU are tall enough to double the fraction of the stellar luminosity reprocessed into the near-infrared. We generate synthetic spectral energy distributions (SEDs) using Monte Carlo radiative transfer calculations with opacities for submicron silicate and carbonaceous grains. The synthetic SEDs closely follow the median Herbig SED constructed recently by Mulders and Dominik and, in particular, match the large near-infrared flux, provided the grains have a mass fraction close to interstellar near the disk's inner rim.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1993

High-performance mass storage system for workstations

Reduced Instruction Set Computer (RISC) workstations and Personnel Computers (PC) are very popular tools for office automation, command and control, scientific analysis, database management, and many other applications. However, when using Input/Output (I/O) intensive applications, the RISC workstations and PC's are often overburdened with the tasks of collecting, staging, storing, and distributing data. Also, by using standard high-performance peripherals and storage devices, the I/O function can still be a common bottleneck process. Therefore, the high-performance mass storage system, developed by Loral AeroSys' Independent Research and Development (IR&D) engineers, can offload a RISC workstation of I/O related functions and provide high-performance I/O functions and external interfaces. The high-performance mass storage system has the capabilities to ingest high-speed real-time data, perform signal or image processing, and stage, archive, and distribute the data. This mass storage system uses a hierarchical storage structure, thus reducing the total data storage cost, while maintaining high-I/O performance. The high-performance mass storage system is a network of low-cost parallel processors and storage devices. The nodes in the network have special I/O functions such as: SCSI controller, Ethernet controller, gateway controller, RS232 controller, IEEE488 controller, and digital/analog converter. The nodes are interconnected through high-speed direct memory access links to form a network. The topology of the network is easily reconfigurable to maximize system throughput for various applications. This high-performance mass storage system takes advantage of a 'busless' architecture for maximum expandability. The mass storage system consists of magnetic disks, a WORM optical disk jukebox, and an 8mm helical scan tape to form a hierarchical storage structure. Commonly used files are kept in the magnetic disk for fast retrieval. The optical disks are used as archive media, and the tapes are used as backup media. The storage system is managed by the IEEE mass storage reference model-based UniTree software package. UniTree software will keep track of all files in the system, will automatically migrate the lesser used files to archive media, and will stage the files when needed by the system. The user can access the files without knowledge of their physical location. The high-performance mass storage system developed by Loral AeroSys will significantly boost the system I/O performance and reduce the overall data storage cost. This storage system provides a highly flexible and cost-effective architecture for a variety of applications (e.g., realtime data acquisition with a signal and image processing requirement, long-term data archiving and distribution, and image analysis and enhancement).

Chiang, T.↗
DOE OSTI2013

EFFECTS OF LOCAL DISSIPATION PROFILES ON MAGNETIZED ACCRETION DISK SPECTRA

We present spectral calculations of non-LTE accretion disk models appropriate for high-luminosity stellar mass black hole X-ray binary systems. We first use a dissipation profile based on scaling the results of shearing box simulations of Hirose et al. to a range of annuli parameters. We simultaneously scale the effective temperature, orbital frequency, and surface density with luminosity and radius according to the standard {alpha}-model. This naturally brings increased dissipation to the disk surface layers (around the photospheres) at small radii and high luminosities. We find that the local spectrum transitions directly from a modified blackbody to a saturated Compton scattering spectrum as we increase the effective temperature and orbital frequency while decreasing midplane surface density. Next, we construct annuli models based on the parameters of a L/L{sub Edd} = 0.8 disk orbiting a 6.62 solar mass black hole using two modified dissipation profiles that explicitly put more dissipation per unit mass near the disk surface. The new dissipation profiles are qualitatively similar to the one found by Hirose et al., but produce strong near power-law spectral tails. Our models also include physically motivated magnetic acceleration support based once again on scaling the Hirose et al. results. We present three full-disk spectra, each based on one of the dissipation prescriptions. Our most aggressive dissipation profile results in a disk spectrum that is in approximate quantitative agreement with certain observations of the steep power-law spectral states from some black hole X-ray binaries.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2005

How to Distinguish Neutron Star and Black Hole X-ray Binaries? Spectral Index and Quasi-Periodic Oscillation Frequency Correlation

Recent studies have revealed strong correlations between 1-10 Hz frequencies of quasiperiodic oscillations (QPOs) and the spectral power law index of several Black Hole (BH) candidate sources when seen in the low/hard state, the steep power-law (soft) state, and in transition between these states. In the soft state these index-QPO frequency correlations show a saturation of the photon index GAMMA approximately equal to 2.7 at high values of the low frequency nu(sub L). This saturation effect was previously identified as a black hole signature. In this paper we argue that this saturation does not occur, at least for one neutron star (NS) source 4U 1728-34, for which the index GAMMA monotonically increases with nu(sub L) to the values of 6 and higher. We base this conclusion on our analysis of approximately 1.5 Msec of RXTE archival data for 4U 1728-34. We reveal the spectral evolution of the Comptonized blackbody spectra when the source transitions from the hard to soft states. The hard state spectrum is a typical thermal Comptonization spectrum of the soft photons which originate in the disk and the NS outer photospheric layers. The hard state photon index is GAMMA approximately 2. The soft state spectrum consists of two blackbody components which are only slightly Comptonized. Thus we can claim (as expected from theory) that in NS sources thermal equilibrium is established for the soft state. To the contrary in BH sources, the equilibrium is never established due to the presence of the BH horizon. The emergent BH spectrum, even in the high/soft state, has a power law component. We also identify the low QPO frequency nu(sub L) as a fundamental frequency of the quasi-spherical component of the transition layer (presumably related to the corona and the NS and disk magnetic closed field lines). The lower frequency nu(sub SL) is identified as the frequency of oscillations of a quasi-cylindrical configuration of the TL (presumably related to the NS and disk magnetic open field lines). We also show that the presence of Fe K(sub alpha), emission-line strengths, QPOs, and the link between them does not depend on radio flux in 4U 1728-34.

Titarchuk, Lev↗
DOE OSTI2015

Magnetization dynamics of coupled ferromagnetic disks

The magnetization configuration in two stacked micron-size ferromagnetic disks can assume different equilibrium states depending on the interfacial coupling between the disks. In this paper, I examine the magnetization dynamics in response to an out-of-plane field pulse for different equilibrium states. For antiferromagnetic coupling, the response spectrum generally consists of a lower-frequency part and a higher-frequency part. The former is related to the response of the core region, which has a significant in-plane response coupled to the out-of-plane one; the latter is related to spin waves generated at the edges of the disk. Finally, for a meron structure the response in the two disks to an out-of-plane pulse is also asymmetric.

36 MATERIALS SCIENCE↗
DOE OSTI2016

High-resolution fully vectorial scanning Kerr magnetometer

We report on the development of a high-resolution scanning magnetometer, which fully exploits the vectorial nature of the magneto-optical Kerr effect. The three-dimensional nature of magnetization is at the basis of many micromagnetic phenomena and from these data, we can fully characterize magnetization processes of nanostructures in static and dynamic regimes. Our scanning Kerr magnetometer uses a high numerical aperture microscope objective where the incident light beam can be deterministically deviated from the objective symmetry axis, therefore, both in-plane (via the longitudinal Kerr effect) and out-of-plane (via the polar Kerr effect) components of the magnetization vector may be detected. These components are then separated by exploiting the symmetries of the polar and longitudinal Kerr effects. From four consecutive measurements, we are able to directly obtain the three orthogonal components of the magnetization vector with a resolution of 600 nm. Performance of the apparatus is demonstrated by a measurement of 3D magnetization vector maps showing out-of-plane domains and in-plane domain walls in an yttrium-iron-garnet film and on a study of magnetization reversal in a 4-μm-wide magnetic disk.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Vertical equilibrium, energetics, and star formation rates in magnetized galactic disks regulated by momentum feedback from supernovae

Recent hydrodynamic (HD) simulations have shown that galactic disks evolve to reach well-defined statistical equilibrium states. The star formation rate (SFR) self-regulates until energy injection by star formation feedback balances dissipation and cooling in the interstellar medium (ISM), and provides vertical pressure support to balance gravity. In this paper, we extend our previous models to allow for a range of initial magnetic field strengths and configurations, utilizing three-dimensional, magnetohydrodynamic (MHD) simulations. We show that a quasi-steady equilibrium state is established as rapidly for MHD as for HD models unless the initial magnetic field is very strong or very weak, which requires more time to reach saturation. Remarkably, models with initial magnetic energy varying by two orders of magnitude approach the same asymptotic state. In the fully saturated state of the fiducial model, the integrated energy proportions E{sub turb}:E{sub th}:δE{sub mag}: E-bar {sub mag} are 0.35:0.39:0.15:0.11, while the proportions of midplane support P{sub turb}:P{sub th}:δΠ{sub mag}: Π-bar {sub mag} are 0.49:0.18:0.18:0.15. Vertical profiles of total effective pressure satisfy vertical dynamical equilibrium with the total gas weight at all heights. We measure the “feedback yields” η{sub c}≡P{sub c}/Σ{sub SFR} (in suitable units) for each pressure component, finding that η{sub turb}∼3.5−4 and η{sub th}∼1.1−1.4 are the same for MHD as in previous HD simulations, and δη{sub mag}∼1.3−1.5. These yields can be used to predict the equilibrium SFR for a local region in a galaxy based on its observed gas and stellar surface densities and velocity dispersions. As the ISM weight (or dynamical equilibrium pressure) is fixed, an increase in η from turbulent magnetic fields reduces the predicted Σ{sub SFR} by ∼20−30% relative to the HD case.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1996

Cygnus X-1: A Case for a Magnetic Accretion Disk?

With the advent of Rossi X-ray Timing Explorer (RXTE), which is capable of broad spectral coverage and fast timing, as well as other instruments which are increasingly being used in multi-wavelength campaigns (via both space-based and ground-based observations), we must demand more of our theoretical models. No current model mimics all facets of a system as complex as an x-ray binary. However, a modern theory should qualitatively reproduce - or at the very least not fundamentally disagree with - all of Cygnus X-l's most basic average properties: energy spectrum (viewed within a broader framework of black hole candidate spectral behavior), power spectrum (PSD), and time delays and coherence between variability in different energy bands. Below we discuss each of these basic properties in turn, and we assess the health of one of the currently popular theories: Comptonization of photons from a cold disk. We find that the data pose substantial challenges for this theory, as well as all other in currently discussed models.

Nowak, Michael A.↗
NASA NTRS1996

Jet Propulsion Laboratories in Space: The Production of Astrophysical Jets by Magnetic Accretion Disks, and a Super-Eddington Wind Model for the Early Stages of GROJ1455-40

Measurements of the properties of gravitational lenses have the power to tell us what sort of universe we live in. The brightest known radio Einstein ring/gravitational lens PKS 1830-211, whilst obscured by our galaxy at optical wavelengths, has recently provided a lensing galaxy redshift of 0.89 through the detection of a new absorption feature, most likely due to neutral hydrogen in a second redshift system at z=0.19.

PKS 1830-211↗