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At least 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

The Effect of Combined Magnetic Geometries on Thermally Driven Winds. I. Interaction of Dipolar and Quadrupolar Fields

Cool stars with outer convective envelopes are observed to have magnetic fields with a variety of geometries, which on large scales are dominated by a combination of the lowest-order fields such as the dipole, quadrupole, and octupole modes. Magnetized stellar wind outflows are primarily responsible for the loss of angular momentum from these objects during the main sequence. Previous works have shown the reduced effectiveness of the stellar wind braking mechanism with increasingly complex but singular magnetic field geometries. In this paper, we quantify the impact of mixed dipolar and quadrupolar fields on the spin-down torque using 50 MHD simulations with mixed fields, along with 10 each of the pure geometries. The simulated winds include a wide range of magnetic field strength and reside in the slow-rotator regime. We find that the stellar wind braking torque from our combined geometry cases is well described by a broken power-law behavior, where the torque scaling with field strength can be predicted by the dipole component alone or the quadrupolar scaling utilizing the total field strength. The simulation results can be scaled and apply to all main-sequence cool stars. For solar parameters, the lowest-order component of the field (dipole in this paper) is the most significant in determining the angular momentum loss.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

PERSISTENT CURRENT EFFECT IN 15-16 T NB3SN ACCELERATOR DIPOLES AND ITS CORRECTION

Nb3Sn magnets with operating fields of 15-16 T are considered for the LHC Energy Doubler and a future Very High Energy pp Collider. Due to large coil volume, high critical current density and large superconducting (SC) filament size the persistent current effect is very large in Nb3Sn dipoles al low fields. This paper presents the results of analysis of the persistent current effect in the 15 T Nb3Sn dipole demonstrator being developed at FNAL, and describes different possibilities of its correction including passive SC wires, iron shims and coil geometry.

43 PARTICLE ACCELERATORS↗
DOE OSTI2022

Modification of the Loss Cone for Energetic Particles in the Earth's Inner Magnetosphere

The behavior of energetic particles in the Earth's dipole and stretched magnetic-field models is explored with a focus on the shift of the atmospheric loss cone away from the magnetic-field direction and on non-adiabatic behavior occurring when the particle gyroradius becomes comparable with the gradient scale length of the local magnetic field. It is shown that the equatorial loss cone is aberrated away from the magnetic-field direction (pitch angle of 0°) by the perpendicular drift of a charged particle (which is referred to as the “Mozer transform” described in Mozer (1966, https://doi.org/10.1029/JZ071i011p02701). It is found that the Mozer coordinate transformation in pitch-angle/gyrophase-angle space better organizes the behavior of bounce times, mirror altitudes, drift speeds, and first adiabatic invariant. It also describes the loss-cone shift accurately for a certain range of values of the adiabaticity parameter ϵ, defined by the ratio of the particle gyroradius to the local radius of curvature of the magnetic field. For particles with larger ϵ, the Mozer transform (evaluated with the standard gradient-curvature drift) breaks down and the “central trajectory” theory can be used to calculate the angular shift of the loss cone, which now includes an Earthward shift. When ϵ becomes relatively large, stochastic field line curvature (FLC) scattering occurs: we show the intimate connection between the strength of FLC scattering and the loss-cone shift. Finally, by comparing ion orbits in the dipole and Tsyganenko (Ts89 and Ts04) magnetic fields, we conclude that the loss cone of ring-current ions is significantly modified during geomagnetically active times.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Engineering the Spectrum of Dipole Field-Localized Spin-Wave Modes to Enable Spin-Torque Antidamping

Auto-oscillation of a ferromagnet due to spin-orbit torques in response to a dc current is of wide interest as a flexible mechanism for generating controllable high-frequency magnetic dynamics. However, degeneracies of the spin-wave modes and nonlinear magnon-magnon scattering impede coherent precession. Discretization of the spin-wave modes can reduce this scattering. Furthermore, spatial localization of the spin-wave modes by the strongly inhomogeneous dipole magnetic field of a nearby spherical micromagnet provides variable spatial confinement, thus offering the option of systematic tunability of magnon spectrum for studying multimode interactions. Here we demonstrate that field localization generates a discrete spin-wave spectrum observable as a series of well-resolved localized modes in the presence of imposed spin currents arising from the spin Hall effect in a permalloy-platinum (Py/Pt) microstrip. The observation of linewidth reduction through damping control in this micromagnetically engineered spectrum demonstrates that localized modes can be controlled efficiently, an important step toward continuously tunable spin Hall effect–driven auto-oscillators.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Unusual magnetodielectric effects in La2CuMnO6 induced by a dynamic crossover in dielectric relaxation at TC

Highlights: • Ferromagnetic short-range correlations above T{sub C} = 72 K and a sign reversal of magnetoresistance around T{sub C}. • Dielectric relaxation in La{sub 2}CuMnO{sub 6}, with a dynamic (Arrhenius to Arrhenius) crossover at T{sub C}. • An unusual trend in the magnetodielectric coupling around T{sub C}. • A large magnetodielectric coupling of 55% at 68 K for 4 kHz and 61% at 105 K for 285.8 kHz under a magnetic field of 5 kOe. • Both the magnetic ordering and the external magnetic field control the electric dipole relaxation in the material. - Abstract: A series of fixed frequency dielectric measurements shows dielectric relaxation in La{sub 2}CuMnO{sub 6}, with a dynamic (Arrhenius to Arrhenius) crossover at T{sub C}. The external magnetic field alters the relaxation parameters in the vicinity of crossover and induces an unusual trend in the magnetodielectric coupling around T{sub C}. A large magnetodielectric coupling of 55% (at 68 K, 4 kHz) and 61% (at 105 K, 285.8 kHz) under a small magnetic field of 5 kOe is discovered. Presence of ferromagnetic short-range correlations above T{sub C} and a sign reversal of magnetoresistance around T{sub C} are observed. Specific heat analysis revealed the presence of ferromagnetic, variable range hopping active charge localized state. The presence of ferromagnetic short-range correlations and the influence of a core-grain dominated magnetoresistance on the Maxwell-Wagner interfacial polarization are responsible for the observed large magnetodielectric effect. Both the magnetic ordering and external magnetic field control the electric dipole relaxation in the material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Visualizing Electric and Magnetic Field Coupling in Au-Nanorod Trimer Structures via Stimulated Electron Energy Gain and Cathodoluminescence Spectroscopy: Implications for Meta-Atom Imaging

In this work, trimer meta-atoms composed of three gold rods in an equilateral triangular geometry were fabricated, and their near-field plasmonic responses were characterized via electron energy loss (EEL), cathodoluminescence (CL), and stimulated electron energy loss/gain (sEEL/sEEG) spectroscopy. The trimer structure hybridizes into a low-energy mode with all three rods coupling in-phase, which produces a circulating current and thus a magnetic field. The next highest-energy mode consists of two rods coupling out-of-phase and produces a net electric dipole. We investigate the near fields of hybridized magnetic and electric modes via EEL and CL and correlate their spectral characteristics and intensity maps. Then, by changing the length of the trimer rods, we tune the magnetic and electric modes to our laser energy and characterize the excited state via sEEL/sEEG spectroscopy. Exploration of the tilt dependence, relative to the optical source, of the two modes reveals that the electric mode sEEG intensity increases more than the expected sin 2 (θ) dependence of the optical electric field coupling (see the Supporting Information for a detailed description). After correcting for the tail of the close-proximity electric mode, we demonstrate sEEG via coupling of the magnetic component of the optical field to the magnetic meta-atoms, which has the expected cos 2 (θ) tilt dependence. This realization opens the possibility to explore the nanoscale excited-state near-field imaging of other magnetic meta-atom structures.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Beam Optics Studies for Larger Beampipe Diameters for the LAMP-PSR

The proposed beampipe diameter increase of the PSR beamlines for the LAMP PSR upgrade is expected decrease beam loss due to tuning errors and misalignment. However, a larger beampipe diameter would require increasing the pole-to-pole gap height within all magnets in the PSR (dipole gap, quadrupole bore, etc.). This would in turn affect the extent and profile of the fringe fields at the ends of the magnets, altering the effective length of the magnets and thus the magnetic optics. In this work, we used the simulation code MAD-X to model the PSR lattice for different beampipe diameters. The results of these simulations will be later used to benchmark particle simulations using the code pyORBIT. The MAD-X simulations are described, and the simulation results are discussed in this technote.

43 PARTICLE ACCELERATORS↗
DOE OSTI2017

A New Two-fluid Radiation-hydrodynamical Model for X-Ray Pulsar Accretion Columns

Previous research centered on the hydrodynamics in X-ray pulsar accretion columns has largely focused on the single-fluid model, in which the super-Eddington luminosity inside the column decelerates the flow to rest at the stellar surface. This type of model has been relatively successful in describing the overall properties of the accretion flows, but it does not account for the possible dynamical effect of the gas pressure. On the other hand, the most successful radiative transport models for pulsars generally do not include a rigorous treatment of the dynamical structure of the column, instead assuming an ad hoc velocity profile. In this paper, we explore the structure of X-ray pulsar accretion columns using a new, self-consistent, “two-fluid” model, which incorporates the dynamical effect of the gas and radiation pressures, the dipole variation of the magnetic field, the thermodynamic effect of all of the relevant coupling and cooling processes, and a rigorous set of physical boundary conditions. The model has six free parameters, which we vary in order to approximately fit the phase-averaged spectra in Her X-1, Cen X-3, and LMC X-4. In this paper, we focus on the dynamical results, which shed new light on the surface magnetic field strength, the inclination of the magnetic field axis relative to the rotation axis, the relative importance of gas and radiation pressures, and the radial variation of the ion, electron, and inverse-Compton temperatures. The results obtained for the X-ray spectra are presented in a separate paper.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A novel method to determine magnetic fields in low-density plasma facilitated through accidental degeneracy of quantum states in Fe{sup 9+}

We propose a new method to determine magnetic fields, by using the magnetic-field-induced electric dipole transition 3p{sup 4} 3d {sup 4}D{sub 7/2} →3p{sup 5} {sup 2}P{sub 3/2} in Fe{sup 9+} ions. This ion has a high abundance in astrophysical plasma and is therefore well suited for direct measurements of even rather weak fields in, e.g., solar flares. This transition is induced by an external magnetic field and its rate is proportional to the square of the magnetic field strength. We present theoretical values for what we will label the reduced rate and propose that the critical energy difference between the upper level in this transition and the close-to-degenerate 3p{sup 4} 3d {sup 4}D{sub 5/2} should be measured experimentally since it is required to determine the relative intensity of this magnetic line for different magnetic fields.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Magnetoelectric behavior via a spin state transition

Abstract In magnetoelectric materials, magnetic and dielectric/ferroelectric properties couple to each other. This coupling could enable lower power consumption and new functionalities in devices such as sensors, memories and transducers, since voltages instead of electric currents are sensing and controlling the magnetic state. We explore a different approach to magnetoelectric coupling in which we use the magnetic spin state instead of the more traditional ferro or antiferromagnetic order to couple to electric properties. In our molecular compound, magnetic field induces a spin crossover from the S = 1 to the S = 2 state of Mn 3+ , which in turn generates molecular distortions and electric dipoles. These dipoles couple to the magnetic easy axis, and form different polar, antipolar and paraelectric phases vs magnetic field and temperature. Spin crossover compounds are a large class of materials where the spin state can modify the structure, and here we demonstrate that this is a route to magnetoelectric coupling.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Fringe field maps for symplectic models of general Cartesian dipoles

We present a framework with which to analyze the effects of magnetic fringe fields. The theory defines the fringe field to be the transition between two regions of nearly constant field, and can incorporate constant multipoles in the magnet body. We then analyze Cartesian dipoles and derive symplectic fringe field maps that are applicable to longitudinal and/or transverse gradient dipoles. We verify the fringe maps with tracking, and show how we incorporated the theory into the tracking code ELEGANT. The resulting elements and several supporting scripts are now available for users, and we conclude with several predictions relevant to the APS Upgrade project.

43 PARTICLE ACCELERATORS↗
DOE OSTI2019

First Field Quality Measurements of a 15 T Nb3Sn Dipole Demonstrator

Outline: Data collection; Magnetic Measurement System; Measurement sensitivity; Centering Corrections; Measurement discussion; Transfer Function (magnitude of the field); Loop (Dynamic effects, eddy current); Z-scan (behavior along magnet length); Harmonics from Stair Step (geometric harmonics); Comparison with Simulation; Decay and Snapback; Summary

43 PARTICLE ACCELERATORS↗
DOE OSTI2024

Modeling of Bi-2212 strand and Rutherford cable in a dipole coil insert

The U.S. Magnet Development Program (US-MDP) explores high-field accelerator magnets compatible with op- erational conditions beyond the limits of Nb3Sn technology. The ongoing R&D High-Temperature Superconductors (HTS) suggests using Bi2Sr2CaCu2O8−x (Bi-2212) as superconducting element. Bi-2212 Rutherford cables maintain a high critical current (IC ) when exposed to a large external magnetic field. However, Bi-2122 exhibits an oversensitive stress-strain response when subject to large Lorentz forces. This paper reports on the magnetic and mechanical analysis of the Bi-2212 cosine-theta insert being developed at Fermilab for a hybrid magnet composed of 2 external layers of Nb3Sn and 2 internal layers of Bi-2212. We performed a FEM analysis of the insert to estimate the HTS stress state in the coil s strands under magnetic and mechanical loads.

D'Agliano, A.↗
DOE OSTI2014

Thermonuclear supernovae: probing magnetic fields by positrons and late-time IR line profiles

We show the importance of γ and positron transport for the formation of late-time spectra in Type Ia supernovae (SNe Ia). The goal is to study the imprint of magnetic fields (B) on late-time IR line profiles, particularly the [Fe II] feature at 1.644 μm, which becomes prominent two to three months after the explosion. As a benchmark, we use the explosion of a Chandrasekhar mass (M {sub Ch}) white dwarf (WD) and, specifically, a delayed detonation model that can reproduce the light curves and spectra for a Branch-normal SN Ia. We assume WDs with initial magnetic surface fields between 1 and 10{sup 9} G. We discuss large-scale dipole and small-scale magnetic fields. We show that positron transport effects must be taken into account for the interpretation of emission features starting at about one to two years after maximum light, depending on the size of B. The [Fe II] line profile and its evolution with time can be understood in terms of the overall energy input by radioactive decay and the transition from a γ-ray to a positron-dominated regime. We find that the [Fe II] line at 1.644 μm can be used to analyze the overall chemical and density structure of the exploding WD up to day 200 without considering B. At later times, positron transport and magnetic field effects become important. After about day 300, the line profile allows one to probe the size of the B-field. The profile becomes sensitive to the morphology of B at about day 500. In the presence of a large-scale dipole field, a broad line is produced in M {sub Ch} mass explosions that may appear flat-topped or rounded depending on the inclination at which the SN is observed. Small or no directional dependence of the spectra is found for small-scale B. We note that narrow-line profiles require central {sup 56}Ni as shown in our previous studies. Persistent broad-line, flat-topped profiles require high-density burning, which is the signature of a WD close to M {sub Ch}. Good time coverage is required to separate the effects of optical depth, the size and morphology of B, and the aspect angle of the observer. The spectra require a resolution of about 500 km s{sup –1} and a signal-to-noise ratio of about 20%. Two other strong near-IR spectral features at about 1.5 and 1.8 μm are used to demonstrate the importance of line blending, which may invalidate a kinematic interpretation of emission lines. Flat-topped line profiles between 300 and 400 days have been observed and reported in the literature. They lend support for M {sub Ch} mass explosions in at least some cases and require magnetic fields equal to or in excess of 10{sup 6} G. We briefly discuss the effects of the size and morphology of B on light curves, as well as limitations. We argue that line profiles are a more direct measurement of B than light curves because they measure both the distribution of {sup 56}Ni and the redistribution of the energy input by positrons rather than the total energy input. Finally, we discuss possible mechanisms for the formation of high B-fields and the limitations of our analysis.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Goos-Hänchen effect and bending of spin wave beams in thin magnetic films

For magnon spintronic applications, the detailed knowledge of spin wave (SW) beam dispersion, transmission (reflection) of SWs passing through (reflected from) interfaces, or borders or the scattering of SWs by inhomogeneities is crucial. These wave properties are decisive factors on the usefulness of a particular device. Here, we demonstrate, using micromagnetic simulations supported by an analytical model, that the Goos-Hänchen (GH) shift exists for SW reflecting from thin film edge and that with the effect becomes observable. We show that this effect will exist for a broad range of frequencies in the dipole-exchange range, with the magnetization degree of pinning at the film edge as the crucial parameter, whatever its nature. Moreover, we have also found that the GH effect can be accompanied or even dominating by a bending of the SW beam due to the inhomogeneity of the internal magnetic field. This inhomogeneity, created by demagnetizing field taking place at the film edge, causes gradual change of SWs refractive index. The refraction of the SW beams by the non-uniformity of the magnetic field enables the exploration of graded index magnonics and metamaterial properties for the transmission and processing of information at nanoscale.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

TIDALLY DRIVEN DYNAMOS IN A ROTATING SPHERE

Large-scale planetary or stellar magnetic fields generated by a dynamo effect are mostly attributed to flows forced by buoyancy forces in electrically conducting fluid layers. However, these large-scale fields may also be controlled by tides, as previously suggested for the star τ-boo, Mars, or the early Moon. By simulating a small local patch of a rotating fluid, Barker and Lithwick have recently shown that tides can drive small-scale dynamos by exciting a hydrodynamic instability, the so-called elliptical (or tidal) instability. By performing global magnetohydrodynamic simulations of a rotating spherical fluid body, we investigate if this instability can also drive the observed large-scale magnetic fields. We are thus interested in the dynamo threshold and the generated magnetic field in order to test if such a mechanism is relevant for planets and stars. Rather than solving the problem in a geometry deformed by tides, we consider a spherical fluid body and add a body force to mimic the tidal deformation in the bulk of the fluid. This allows us to use an efficient spectral code to solve the magnetohydrodynamic problem. We first compare the hydrodynamic results with theoretical asymptotic results and numerical results obtained in a truly deformed ellipsoid, which confirms the presence of elliptical instability. We then perform magnetohydrodynamic simulations and investigate the dynamo capability of the flow. Kinematic and self-consistent dynamos are finally simulated, showing that the elliptical instability is capable of generating a dipole-dominated large-scale magnetic field in global simulations of a fluid rotating sphere.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Effects of the scatter in sunspot group tilt angles on the large-scale magnetic field at the solar surface

The tilt angles of sunspot groups represent the poloidal field source in Babcock-Leighton-type models of the solar dynamo and are crucial for the build-up and reversals of the polar fields in surface flux transport (SFT) simulations. The evolution of the polar field is a consequence of Hale's polarity rules, together with the tilt angle distribution which has a systematic component (Joy's law) and a random component (tilt-angle scatter). We determine the scatter using the observed tilt angle data and study the effects of this scatter on the evolution of the solar surface field using SFT simulations with flux input based upon the recorded sunspot groups. The tilt angle scatter is described in our simulations by a random component according to the observed distributions for different ranges of sunspot group size (total umbral area). By performing simulations with a number of different realizations of the scatter we study the effect of the tilt angle scatter on the global magnetic field, especially on the evolution of the axial dipole moment. The average axial dipole moment at the end of cycle 17 (a medium-amplitude cycle) from our simulations was 2.73 G. The tilt angle scatter leads to an uncertainty of 0.78 G (standard deviation). We also considered cycle 14 (a weak cycle) and cycle 19 (a strong cycle) and show that the standard deviation of the axial dipole moment is similar for all three cycles. The uncertainty mainly results from the big sunspot groups which emerge near the equator. In the framework of Babcock-Leighton dynamo models, the tilt angle scatter therefore constitutes a significant random factor in the cycle-to-cycle amplitude variability, which strongly limits the predictability of solar activity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

The B1APF Large Aperture Interaction Region Dipole

Brookhaven National Laboratory is hosting the Electron Ion Collider (EIC). B1APF dipole is the last magnet near the interaction point #6. The longitudinal geometrical space available for this magnet is 1.5 m. The diameter of the coil mandrel is 370 mm. The 3-dimensional coil ends design is particularly challenging because of this large aspect ratio. The coil operating current and operating temperature are 12600 A and 1.9 K respectively. It produces an integrated dipole field of 4.05 Tm. The coil is wound with a NbTi Rutherford cable which is 15.1 mm wide and 1.8 mm thick. The parameters for the 3D end design optimization are obtained after a series of winding tests on a real mandrel with the given cable. In conclusion, this study discusses the latest design based on 1) sensitivity studies on variations in wedge thicknesses and variations in B-H data, and 2) experimental data from the winding trials, analysis and its implementation in the 3D design of this magnet.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗