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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 1,207 records · Page 67
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

Unconventional magnetism in imbalanced Fermi systems with magnetic dipolar interactions

We study the magnetic structure of the ground state of an itinerant Fermi system of spin-1/2 particles with magnetic dipole-dipole interactions. We show that, quite generally, the spin state of particles depend on its momentum, i.e., spin and orbital degrees of freedom are entangled and taken separately are not 'good' quantum numbers. Specifically, we consider a uniform system with nonzero magnetization at zero temperature. Assuming the magnetization is along z axis, the quantum spin states are k-dependent linear combinations of eigenstates of the sigma{sub z} Pauli matrix. This leads to spin structures in momentum space and to the fact that the Fermi surfaces for 'up' and 'down' spins are not well defined. The system still has a cylindrical axis of symmetry along the magnetization axis. We also show that the self-energy has a universal structure which we determine based on the symmetries of the dipolar interaction and we explicitly calculated it in the Hartree-Fock approximation. We show that the bare magnetic moment of particles is renormalized due to particle-particle interactions and we give order of magnitude estimates of this renormalization effect. We estimate that the above mentioned dipolar effects are small but we discuss possible scenarios where this physics may be realized in future experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1993

Auroral arc thicknesses as predicted by various theories

Twelve electron-acceleration mechanisms and 10 generator mechanisms for auroral arcs are examined, and a characteristic auroral-arc thickness is worked out for each mechanism except one. The arc thicknesses are then mapped down to the ionosphere along the terrestrial magnetic-field lines; near the Earth, a dipole magnetic-field model is used, and farther from the Earth, the mapping includes the effects of magnetic-field-line draping. The 21 theoretical models all predict auroral-arc thicknesses that are at least an order of magnitude wider than the optically observed arcs. As an alternative explanation of the observed narrow auroral arcs, the acceleration of ionospheric electrons to produce airglow in electrical discharge mechanisms appears improbable. Also unsuccessfully explored is the possibility that the observed narrow auroral arcs are caused by interference effects when Alfven waves reflect off the ionosphere. Suggestions are made for future ground-based auroral-arc measurements.

Borovsky, Joseph E.↗
DOE OSTI2013

Final Report: Levitated Dipole Experiment

Since the very first experiments with the LDX, research progress was rapid and significant. Initial experiments were conducted with the high-field superconducting coil suspended by three thin rods. These experiments produced long-pulse, quasi-steady-state microwave discharges, lasting more than 10 s, having peak beta values of 20% [Garnier et al., Physics of Plasmas, 13 (2006) 056111]. High- beta, near steady-state discharges have been maintained in LDX for more than 20 seconds, and this capability made LDX the longest pulse fusion confinement experiment operating in the U.S. fusion program. A significant measure of progress in the LDX research program was the routine investigation of plasma confinement with a magnetically-levitated dipole and the resulting observations of confinement improvement. In both supported and levitated configurations, detailed measurements were made of discharge evolution, plasma dynamics and instability, and the roles of gas fueling, microwave power deposition profiles, and plasma boundary shape. High-temperature plasma was created by multi frequency electron cyclotron resonance heating at 2.45 GHz, 6.4 GHz, 10.5 GHz and 28 GHz allowing control of heating profiles. Depending upon neutral fueling rates, the LDX discharges contain a fraction of energetic electrons, with mean energies above 50 keV. Depending on whether or not the superconducting dipole was levitated or supported, the peak thermal electron temperature was estimated to exceed 500 eV and peak densities to approach 1e18 m -3 . We have found that levitation causes a strong inwards density pinch [Boxer et al., Nature Physics, 6 (2010) 207] and we have observed the central plasma density increase dramatically indicating a significant improvement in the confinement of a thermal plasma species.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Porous silicon nanoparticles as biocompatible contrast agents for magnetic resonance imaging

We propose porous silicon nanoparticles (PSi NPs) with natural oxide coating as biocompatible and bioresorbable contrast agents for magnetic resonant imaging (MRI). A strong shortening of the transversal proton relaxation time (T{sub 2}) was observed for aqueous suspensions of PSi NPs, whereas the longitudinal relaxation time (T{sub 1}) changed moderately. The longitudinal and transversal relaxivities are estimated to be 0.03 and 0.4 l/(g·s), respectively, which are promising for biomedical studies. The proton relaxation is suggested to undergo via the magnetic dipole-dipole interaction with Si dangling bonds on surfaces of PSi NPs. MRI experiments with phantoms have revealed the remarkable contrasting properties of PSi NPs for medical diagnostics.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
NASA NTRS1977

The moon's permanent magnetic field - A cratered-shell model

The possibility is considered that the properties of the larger-scale lunar magnetic field may depend in a rather straightforward manner on the geometrical properties of the distribution of larger craters. The relevant effects of cratering are analyzed in the framework of a model of a spherically symmetric shell magnetized to some specific intensity by a concentric dipolar magnetic field. The remanent dipole moment of the model is determined by calculating the dipole moment of the material removed to form craters and them computing the remanent dipole moment for three mutually orthogonal orientations of the magnetizing dipole. The resulting absolute values for the remanent dipole moment are found to be comparable to the upper limit of 10 to the -19th power G-cu cm estimated from Apollo subsatellite measurements. It is concluded that for a crust with an empirically reasonable mean magnetization, the craters can account for a dipole moment as great as that indicated by the estimated upper limit.

Weiss, H.↗
DOE OSTI2011

Novel Approach to Linear Accelerator Superconducting Magnet System

Superconducting Linear Accelerators include a superconducting magnet system for particle beam transportation that provides the beam focusing and steering. This system consists of a large number of quadrupole magnets and dipole correctors mounted inside or between cryomodules with SCRF cavities. Each magnet has current leads and powered from its own power supply. The paper proposes a novel approach to magnet powering based on using superconducting persistent current switches. A group of magnets is powered from the same power supply through the common, for the group of cryomodules, electrical bus and pair of current leads. Superconducting switches direct the current to the chosen magnet and close the circuit providing the magnet operation in a persistent current mode. Two persistent current switches were fabricated and tested. In the paper also presented the results of magnetic field simulations, decay time constants analysis, and a way of improving quadrupole magnetic center stability. Such approach substantially reduces the magnet system cost and increases the reliability.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

Phase transitions in dipolar spin-1 Bose gases

We study phase transitions in homogeneous spin-1 Bose gases in the presence of long-range magnetic dipole-dipole interactions (DDI). We concentrate on three-dimensional (3D) geometries and employ momentum shell renormalization group to study the possible instabilities caused by the DDI. At the zero-temperature limit where quantum fluctuations prevail, we find the phase diagram to be unaffected by the DDI. When the thermal fluctuations dominate, polar and ferromagnetic condensates with DDI become unstable and we discuss this crossover in detail. On the other hand, the spin-singlet condensate remains stable in the presence of DDI.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

General Relativistic Radiation MHD Simulations of Supercritical Accretion onto a Magnetized Neutron Star: Modeling of Ultraluminous X-Ray Pulsars

By performing 2.5-dimensional general relativistic radiation magnetohydrodynamic simulations, we demonstrate supercritical accretion onto a non-rotating, magnetized neutron star, where the magnetic field strength of dipole fields is 10{sup 10} G on the star surface. We found the supercritical accretion flow consists of two parts: the accretion columns and the truncated accretion disk. The supercritical accretion disk, which appears far from the neutron star, is truncated at around ≃3 R {sub *} ( R {sub *} = 10{sup 6} cm is the neutron star radius), where the magnetic pressure via the dipole magnetic fields balances with the radiation pressure of the disks. The angular momentum of the disk around the truncation radius is effectively transported inward through magnetic torque by dipole fields, inducing the spin up of a neutron star. The evaluated spin-up rate, ∼−10{sup −11} s s{sup −1}, is consistent with the recent observations of the ultraluminous X-ray pulsars. Within the truncation radius, the gas falls onto a neutron star along the dipole fields, which results in a formation of accretion columns onto the northern and southern hemispheres. The net accretion rate and the luminosity of the column are ≃66 L {sub Edd}/ c {sup 2} and ≲10 L {sub Edd}, where L {sub Edd} is the Eddington luminosity and c is the light speed. Our simulations support a hypothesis whereby the ultraluminous X-ray pulsars are powered by the supercritical accretion onto the magnetized neutron stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Quantum spin ice response to a magnetic field in the dipole-octupole pyrochlore Ce 2 Zr 2 O 7

The pyrochlore magnet Ce 2 Zr 2 O 7 has attracted much attention as a quantum spin ice candidate whose novelty derives in part from the dipolar-octupolar nature of the Ce 3+ pseudospin-1/2 degrees of freedom it possesses. We report heat capacity measurements on single crystal samples of Ce 2 Zr 2 O 7 down to T~0.1K in a magnetic field along the [$1,\bar{1},0$] direction. These measurements show that the broad hump in the zero-field heat capacity moves higher in temperature with increasing field strength and is split into two separate humps by the [$1,\bar{1},0$] magnetic field at ~2T. These separate features are due to the decomposition of the pyrochlore lattice into effectively decoupled chains for fields in this direction: One set of chains (α chains) is polarized by the field while the other (β chains) remains free. This situation is similar to that observed in the classical spin ices Ho 2 Ti 2 O 7 and Dy 2 Ti 2 O 7 , but with the twist that here the strong transverse exchange interactions produce substantial quantum effects. Our theoretical modeling suggests that the β chains are close to a critical state, with nearly-gapless excitations. We also report elastic and inelastic neutron scattering measurements on single crystal Ce 2 Zr 2 O 7 in [$1,\bar{1},0$] and [0,0,1] magnetic fields at temperatures down to T=0.03K. The elastic scattering behaves consistently with the formation of independent chains for a [$1,\bar{1},0$] field, while the [0,0,1] field produces a single field-induced elastic magnetic Bragg peak at (0,2,0) and equivalent wavevectors, indicating a polarized spin ice state for fields above ~3T. For both [$1,\bar{1},0$] and [0,0,1] magnetic fields, our inelastic neutron scattering results show an approximately dispersionless continuum of scattering that increases in both energy and intensity with increasing field strength. By modeling the complete set of experimental data using numerical linked cluster and semiclassical molecular dynamics calculations, we demonstrate the dominantly multipolar nature of the exchange interactions in Ce 2 Zr 2 O 7 and the smallness of the parameter θ, which controls the mixing between dipolar and octupolar degrees of freedom. Finally, these results support previous estimates of the microscopic exchange parameters and place strong constraints on the theoretical description of this prominent spin ice candidate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Observation of magnetic fluctuations and rapid density decay of magnetospheric plasma in Ring Trap 1

The Ring Trap 1 device, a magnetospheric configuration generated by a levitated dipole field magnet, has created high-{beta} (local {beta} {approx} 70%) plasma by using electron cyclotron resonance heating (ECH). When a large population of energetic electrons is generated at low neutral gas pressure operation, high frequency magnetic fluctuations are observed. When the fluctuations are strongly excited, rapid loss of plasma was simultaneously observed especially in a quiet decay phase after the ECH microwave power is turned off. Although the plasma is confined in a strongly inhomogeneous dipole field configuration, the frequency spectra of the fluctuations have sharp frequency peaks, implying spatially localized sources of the fluctuations. The fluctuations are stabilized by decreasing the hot electron component below approximately 40%, realizing stable high-{beta} confinement.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

The one-dimensional camelback potential in the parallel dipole line trap: Stability conditions and finite size effect

We recently demonstrated a magnetic parallel dipole line (PDL) system that serves as a unique diamagnetic trap with a fascinating one-dimensional camelback potential along its longitudinal axis. The system can be realized with a pair of transversely magnetized cylindrical magnets and a cylindrical graphite rod as the trapped object. In this paper, we present more detailed experimental and theoretical studies of the finite size effect of the rod and its impact on the stability and oscillation dynamics of the trap. We show that the camelback potential effect only occurs when the length of the PDL system is beyond certain critical length (L C ). The length of the trapped rod determines the “effective camelback potential” and is subject to maximum and minimum values for the trap to be stable. Both length and radius of the rod determine the damping dynamics or the quality factor of the oscillator. Finally, these characteristics are important for designing the PDL trap system for various sensing applications, for example, we demonstrated a PDL trap gas viscometer system through measurement of the oscillation damping time constant.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1971

Solar magnetic fields - Extended.

Spacecraft observations of the interplanetary magnetic field have revealed that almost always each solar rotation can be divided into sectors, within each of which the field has a predominant polarity toward the sun or away from the sun. Comparisons of this interplanetary magnetic sector pattern with observations of the photospheric magnetic field have revealed a similar solar magnetic pattern. The boundaries between solar magnetic sectors are approximately in the north-south direction over a wide range of latitudes on both sides of the equator. This solar magnetic sector structure can be described as a rotating dipole whose magnetic axis makes an angle of approximately 90 deg with the axis of rotation. Possible similarities between this solar-sector magnetism and the models derived from observations of stellar magnetism are discussed.

Wilcox, J. M.↗
DOE OSTI2014

THE LIGHT CURVE AND INTERNAL MAGNETIC FIELD OF THE MODE-SWITCHING PULSAR PSR B0943+10

A number of radio pulsars exhibit intriguing mode-switching behavior. Recent observations of PSR B0943+10 revealed correlated radio and X-ray mode switches, providing a new avenue for understanding this class of objects. The large X-ray pulse fraction observed during the radio-quiet phase (Q-mode) was previously interpreted as a result of changing obscuration of X-rays by dense magnetosphere plasma. We show that the large X-ray pulse fraction can be explained by including the beaming effect of a magnetic atmosphere, while remaining consistent with the dipole field geometry constrained by radio observations. We also explore a more extreme magnetic field configuration, where a magnetic dipole displaced from the center of the star produces two magnetic polar caps of different sizes and magnetic field strengths. These models are currently consistent with data in radio and X-rays and can be tested or constrained by future X-ray observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

The Interaction Region of PEP-II

The PEP-II e{sup +}e{sup -} collider turned off last year after a very successful 9 years of running. The accelerator achieved a peak luminosity of 12 x 10{sup 33} cm{sup -2} s{sup -1}, 4 times over the design of 3 x 10{sup 33} cm{sup -2} s{sup -1}. The peak beam currents were quite high; over 3 A for the positron beam and over 2 A for the electron beam. The beams were stored in separate storage rings. The PEP-II design called for a head-on collision at the interaction point (IP). This was possible because of the asymmetric beam energies (9 on 3.1 GeV) and was achieved using permanent magnet (PM) dipoles. I describe the actual interaction region (IR) layout including some of the design constraints that led to the final design and discuss operation issues related to the IR design.

43 PARTICLE ACCELERATORS↗