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At least 1,711 records · Page 95
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Beat, modulational, and decay instabilities of a circularly polarized Alfven wave

A circulary polarized low-frequency electomagnetic pump wave propoagating along an ambient magnetic field is known to be unstable to the growth of several parallel-propagating parametric instabilties. If ion-cyclotron effects are retained in a two-fluid description, the dispersion relation is a sixth-order polynomial. We present a series of new analytical approximations to this dispersion relation. We emphasize new results for the beat instability that occurs as an interaction of the forward prpagating upper sideband with the backward propagating lower sideband. The nature of the beat instabitlity depends on beta = (v(sub sound)/v(sub A)(exp 2) and on the sense of polarization of the pump wave. The beat and decay instabilities can occur together if the pump is left-handed (i.e., ion resonant) and if beta is less than or approximately 1, but they cannot occur together if the pump is right-handed. For a left-handed pump the beat mode is the only instability if beta is greater than or approximately 1. If the pump is right-handed and beta is greater than or approximately 1, then the beat instability exists only when the pump amplitude exceeds a threshold value, and the beat will be the only instability if the pump amplitude is large enough to stabilize the modulational instability. If the pump is left-handed and beta is less than or approximately 1, then the beat mode is stabilized when the pump amplitude becomes sufficiently large. The beat instability primarily produces a forward propagating transverse wave in the upper sideband. Thus if beta is greater than or approximately 1, the instabilities considered here do not produce the backward propagating waves which are thought to affect turbulence and the evolution of cross helicity in the solar wind. New analytical results are presented also for the decay and modulational instabilites when beta is approximately equal to 1.

Hollweg, Joseph V.↗
NASA NTRS2011

Observations of a High-Latitude Stable Electron Auroral Emission at Approximately 16 MLT During a Large Substorm

During an interval when the interplanetary magnetic field was large and primarily duskward and southward, a stable region of auroral emission was observed on 17 August 2001 by IMAGE at 16 magnetic local time, poleward of the main aurora, for 1 h, from before the onset of a large substorm through the recovery phase. In a region where ions showed the energy dispersion expected for the cusp, strong field \aligned currents and Poynting flux were observed by Polar (at 1.8 RE in the Southern Hemisphere) as it transited field lines mapping to the auroral spot in the Northern Hemisphere. The data are consistent with the hypothesis that the long \lasting electron auroral spot maps to the magnetopause region where reconnection was occurring. Under the assumption of conjugacy between the Northern and Southern hemispheres on these field lines, the Polar data suggest that the electrons on these field lines were accelerated by Alfven waves and/or a quasi \static electric field, primarily at altitudes below a few RE since the in situ Poynting flux (mapped to 100 km) is comparable to the energy flux of the emission while the mapped in situ electron energy flux is much smaller. This event provides the first example of an emission due to electrons accelerated at low altitudes at the foot point of a region of quasi \steady dayside reconnection. Cluster data in the magnetotail indicate that the Poynting flux from the reconnection region during this substorm is large enough to account for the observed nightside aurora.

Cattell, C.↗
DOE OSTI2012

Progress on muon parametric-resonance ionization cooling channel development

Parametric-resonance Ionization Cooling (PIC) is intended as the final 6D cooling stage of a high-luminosity muon collider. To implement PIC, a continuous-field twin-helix magnetic channel was developed. A 6D cooling with stochastic effects off is demonstrated in a GEANT4/G4beamline model of a system where wedge-shaped Be absorbers are placed at the appropriate dispersion points in the twin-helix channel and are followed by short rf cavities. To proceed to cooling simulations with stochastics on, compensation of the beam aberrations from one absorber to another is required. Initial results on aberration compensation using a set of various-order continuous multipole fields are presented. As another avenue to mitigate the aberration effect, we optimize the cooling channel's period length. We observe a parasitic parametric resonance naturally occurring in the channel's horizontal plane due to the periodic beam energy modulation caused by the absorbers and rf. We discuss options for compensating this resonance and/or properly combining it with the induced half-integer parametric resonance needed for PIC.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Effects of Sintering Temperature on Structural, Morphological and Magnetic Properties of Strontium Ferrite Nanoparticles

In the present investigation, structural, morphological and magnetic properties of SrFe{sub 2}O{sub 4} ferrite nanoparticles prepared by a sol–gel method were studied. The techniques used in this investigation were thermogravimetric analysis/differential thermal analysis (TGA/DTA), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectrometry (FT-IR), vibrating sample magnetometry (VSM) and Curie temperature analysis. The average crystallite sizes of the SrFe{sub 2}O{sub 4} were calculated using the Scherrer formula and found in the range of 11–22 nm, and this can be attributed to the grain growth of the particles. The lattice parameter of SrFe{sub 2}O{sub 4} nanoparticles decreased from 6.64 to 6.51 nm as the sintering temperature was raised from 600 to 900 °C. FT-IR analysis confirmed that bands in the range 430–590 cm{sup − 1} are due to the stretching vibration of oxygen atom and metal ions (M–O) that is Fe–O confirming the formation of spinel ferrite. Surface morphology of the samples has been studied using FESEM. The elemental composition along with their relative ratios was given by EDX and was found to be in agreement with their initial calculated values. The saturation magnetization, magnetic coercivity and remanence increase with an increasing sintering temperature from 600 to 900 °C. The present study reports the Curie temperature with an amount of 551 °C of the synthesized strontium nanoferrites using the Faraday technique.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Study of the structural and magnetic phase-transitions and multiferroic properties in BiFeO3-Ba0.95Ca0.05TiO3 solid solutions

Highlights: • The detailed structural analysis has been done for (1-x) BiFeO{sub 3}-xBa{sub 0.95}Ca{sub 0.05}TiO{sub 3} (0.1 ≤ x ≤ 0.9). • The detailed magnetic behavior has been studied for (1-x) BiFeO{sub 3}-xBa{sub 0.95}Ca{sub 0.05}TiO{sub 3} (0.1 ≤ x ≤ 0.9). • The dielectric constant (ε') and the dielectric loss (ε'') versus frequency curves have been fitted by using Cole-Cole dispersion formula and non-Debye relaxation behavior has been found. • The weak magnetodielectric response (MD) has been found. • The non saturating P-E loops have been found for 0.7 ≤ x ≤ 0.9 samples. - Abstract: We synthesized (1-x) BiFeO{sub 3}-x Ba{sub 0.95}Ca{sub 0.05}TiO{sub 3} solid solutions for 0.1 ≤ x ≤ 0.9, using a solid-state-reaction. The XRD analysis reveals that a composition-driven rhombohedral-to-cubic and a cubic-to-tetragonal phase-transition occur at x ≈ 0.4 and x ≈ 0.7, respectively. The experiments show that BaCaTiO{sub 3}-substitution suppresses the spiral spin-structure, which leads to net magnetization. The samples have been found to exhibit magnetic order for 0.1 ≤ x ≤ 0.7 and paramagnetic behavior for x ≥ 0.8. The electrical conductivity increases with increasing frequency of the applied electric field. The dielectric measurements suggest a non-Debye type relaxation. The samples show magneto-dielectric response (MD) upto x = 0.5, with the highest MD of 11.61% at 10 kHz for x = 0.3. The maximum attainable remanent polarization (P{sub r}) ∼ 5.24 μC/cm{sup 2} was observed for x = 0.7.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Collective charge and spin dynamics in low-dimensional itinerant electron systems with spin-orbit coupling

This research project in theoretical condensed-matter physics aims to study the collective charge and spin excitations of itinerant quasi-two-dimensional electronic systems such as doped semiconductor nanostructures and carbon-based materials, in the presence of impurities and spin-orbit coupling. In previous work under DOE support, we had discovered an interesting interplay between many-body effects and Rashba and Dresselhaus spin-orbit interactions: contrary to many situation in spintronics where spin-orbit interactions lead to loss of spin coherence, collective spin modes are robust against such coherence losses, and behave as macroscopic quantum objects with a collective magnetic moment precessing about an effective spin-orbit magnetic field. Experimental data for GaAs and CdMnTe quantum wells, obtained by our collaborators at the University Pierre et Marie Curie in Paris, confirmed our predictions and suggested that the observed effect is universal. However, a full theoretical explanation of the effect is still missing, and a wealth of new experimental data remains to be analyzed. We therefore plan to address the following three interrelated objectives: 1. Theory of collective modes in chiral systems. We will explore the various ways in which spin-orbit interactions affect collective spin-density and spin-flip excitations in semiconductor quantum wells. The goal will be to develop a universal formalism for chiral collective modes, with a particular emphasis on analytic results for dispersion relations, which will be compared with experimental results from inelastic light scattering. We will calculate the linewidths of spin plasmons and spin-flip waves due to impurity scattering and spin Coulomb drag from first principles. 2.Many-body effects in noncollinear magnetism. We will describe electronic many-body effects using time-dependent density-functional theory (TDDFT) in the linear-response regime. Quantum wells with spin-orbit coupling are noncollinear itinerant magnetic systems at the crossover between two and three dimensions, which poses subtle challenges for standard exchange-correlation (xc) functionals such as the local-density approximation (LDA). We will develop new orbital-dependent xc functionals for noncollinear magnetic systems by generalizing the so-called STLS approach. 3. Beyond single semiconductor quantum wells. We will consider quantum well bilayer or multilayer systems with spin-orbit coupling, where the coupling between the quasi-2D electron systems leads to acoustic and optical modes (in phase and out of phase). How are these modes affected if the two layers have the same or different chirality? Plasmon modes have been widely studied in graphene and other topological materials, and we will study collective spin excitations in graphene and other carbon-based materials in the presence of spin-orbit coupling and magnetic fields. The potential impact of the proposed research will be both on a fundamental and a practical level. A universal theory of chiral collective modes will advance our understanding of the interplay between many-body effects and spin-orbit interactions in a wide variety of materials and systems. A generalized STLS approach for noncollinear magnetism will be of wide interest beyond the systems studied here. This work may also point towards new ways of manipulating and controlling spin waves in low-dimensional itinerant electron systems: this could be used to encode, transport and process information, which would be of interest to the field of magnon spintronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Site directed spin labeling studies of Escherichia coli dihydroorotate dehydrogenase N-terminal extension

Highlights: Black-Right-Pointing-Pointer EcDHODH is a membrane-associated enzyme and a promising target for drug design. Black-Right-Pointing-Pointer Enzyme's N-terminal extension is responsible for membrane association. Black-Right-Pointing-Pointer N-terminal works as a molecular lid regulating access to the protein interior. -- Abstract: Dihydroorotate dehydrogenases (DHODHs) are enzymes that catalyze the fourth step of the de novo synthesis of pyrimidine nucleotides. In this reaction, DHODH converts dihydroorotate to orotate, using a flavine mononucleotide as a cofactor. Since the synthesis of nucleotides has different pathways in mammals as compared to parasites, DHODH has gained much attention as a promising target for drug design. Escherichia coli DHODH (EcDHODH) is a family 2 DHODH that interacts with cell membranes in order to promote catalysis. The membrane association is supposedly made via an extension found in the enzyme's N-terminal. In the present work, we used site directed spin labeling (SDSL) to specifically place a magnetic probe at positions 2, 5, 19, and 21 within the N-terminal and thus monitor, by using Electron Spin Resonance (ESR), dynamics and structural changes in this region in the presence of a membrane model system. Overall, our ESR spectra show that the N-terminal indeed binds to membranes and that it experiences a somewhat high flexibility that could be related to the role of this region as a molecular lid controlling the entrance of the enzyme's active site and thus allowing the enzyme to give access to quinones that are dispersed in the membrane and that are necessary for the catalysis.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2017

Progress on the Development of the Nb 3 Sn 11T Dipole for the High Luminosity Upgrade of LHC

The high-luminosity large hadron collider (LHC) project at CERN entered into the production phase in October 2015 after the completion of the design study phase. In the meantime, the development of the 11 T dipole needed for the upgrade of the collimation system of the machine made significant progress with very good performance of the first two-in-one magnet model of 2-m length made at CERN. The 11 T dipole, which is more powerful than the current main dipoles of LHC, can be made shorter with an equivalent integrated field. This will allow creating space for the installation of additional collimators in specific locations of the dispersion suppressor regions. Following tests carried out during heavy ions runs of LHC in the end of 2015, and a more recent review of the project budget, the installation plan for the 11 T dipole was revised. Consequently, one 11 T dipole full assembly containing two 11 T dipoles of 5.5-m length will be installed on either side of interaction point 7. These two units shall be installed during the long shutdown 2 in years 2019-2020. After a brief reminder on the design features of the magnet, this paper describes the current status of the development activities, in particular the short model programme and the construction of the first full scale prototype at CERN. Finally, critical operations such as the reaction treatment and the coil impregnation are discussed, the quench performance tests results of the two-in-one model are reviewed and finally, the plan toward the production for the long shut down 2 is described.

43 PARTICLE ACCELERATORS↗
DOE OSTI2013

On the multistream approach of relativistic Weibel instability. I. Linear analysis and specific illustrations

A one-dimensional multistream formalism is extended for the study of temperature anisotropy driven Weibel-type instabilities in collisionless and relativistic plasma. The formulation is based on a Hamiltonian reduction technique using the invariance of generalized canonical momentum in transverse direction. The Vlasov-Maxwell model is expressed in terms of an ensemble of one-dimensional Vlasov-type equations, coupled together with the Maxwell equations in a self-consistent way. Although the model is fundamentally nonlinear, this first of three companion papers focuses on the linear aspect. Dispersion relations of the Weibel instability are derived in the linear regime for different kinds of polarization of the electromagnetic potential vector. The model allows new unexpected insights on the instability: enhanced growth rates for the Weibel instability are predicted when a dissymmetric distribution is considered in p{sub ⊥}. In the case of a circular polarization, a simplification of the linear analysis can be obtained by the introduction of the “multiring” approach allowing to extend the analytical model of Yoon and Davidson [Phys. Rev. A 35, 2718 (1987)]. Applications of this model are left to the other two papers of the series where specific problems are addressed pertaining to the nonlinear and relativistic dynamics of magnetically trapped particles met in the saturation regime of the Weibel instability.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Magnon spectrum of the helimagnetic insulator Cu 2 OSeO 3

We report that complex low-temperature-ordered states in chiral magnets are typically governed by a competition between multiple magnetic interactions. The chiral-lattice multiferroic Cu 2 OSeO 3 became the first insulating helimagnetic material in which a long-range order of topologically stable spin vortices known as skyrmions was established. Here we employ state-of-the-art inelastic neutron scattering to comprehend the full three-dimensional spin-excitation spectrum of Cu 2 OSeO 3 over a broad range of energies. Distinct types of high- and low-energy dispersive magnon modes separated by an extensive energy gap are observed in excellent agreement with the previously suggested microscopic theory based on a model of entangled Cu 4 tetrahedra. The comparison of our neutron spectroscopy data with model spin-dynamical calculations based on these theoretical proposals enables an accurate quantitative verification of the fundamental magnetic interactions in Cu 2 OSeO 3 that are essential for understanding its abundant low-temperature magnetically ordered phases.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Rotational stabilization of the resistive wall modes in tokamaks with a ferritic wall

The dynamics of the rotating resistive wall modes (RWMs) is analyzed in the presence of a uniform ferromagnetic resistive wall with μ{sup ^}≡μ/μ{sub 0}≤4 (μ is the wall magnetic permeability, and μ{sub 0} is the vacuum one). This mimics a possible arrangement in ITER with ferromagnetic steel in test blanket modules or in future experiments in JT-60SA tokamak [Y. Kamada, P. Barabaschi, S. Ishida, the JT-60SA Team, and JT-60SA Research Plan Contributors, Nucl. Fusion 53, 104010 (2013)]. The earlier studies predict that such a wall must provide a destabilizing influence on the plasma by reducing the beta limit and increasing the growth rates, compared to the reference case with μ{sup ^}=1. This is true for the locked modes, but the presented results show that the mode rotation changes the tendency to the opposite. At μ{sup ^}>1, the rotational stabilization related to the energy sink in the wall becomes even stronger than at μ{sup ^}=1, and this “external” effect develops at lower rotation frequency, estimated as several kHz at realistic conditions. The study is based on the cylindrical dispersion relation valid for arbitrary growth rates and frequencies. This relation is solved numerically, and the solutions are compared with analytical dependences obtained for slow (s/d{sub w}≫1) and fast (s/d{sub w}≪1) “ferromagnetic” rotating RWMs, where s is the skin depth and d{sub w} is the wall thickness. It is found that the standard thin-wall modeling becomes progressively less reliable at larger μ{sup ^}, and the wall should be treated as magnetically thick. The analysis is performed assuming only a linear plasma response to external perturbations without constraints on the plasma current and pressure profiles.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Multi-point, high-speed passive ion velocity distribution diagnostic on the Pegasus Toroidal Experiment

A passive ion temperature polychromator has been deployed on Pegasus to study power balance and non-thermal ion distributions that arise during point source helicity injection. Spectra are recorded from a 1 m F/8.6 Czerny-Turner polychromator whose output is recorded by an intensified high-speed camera. The use of high orders allows for a dispersion of 0.02 A/mm in 4th order and a bandpass of 0.14 A ({approx}13 km/s) at 3131 A in 4th order with 100 {mu}m entrance slit. The instrument temperature of the spectrometer is 15 eV. Light from the output of an image intensifier in the spectrometer focal plane is coupled to a high-speed CMOS camera. The system can accommodate up to 20 spatial points recorded at 0.5 ms time resolution. During helicity injection, stochastic magnetic fields keep T{sub e} low ({approx}100 eV) and thus low ionization impurities penetrate to the core. Under these conditions, high core ion temperatures are measured (T{sub i} Almost-Equal-To 1.2 keV, T{sub e} Almost-Equal-To 0.1 keV) using spectral lines from carbon III, nitrogen III, and boron IV.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

Comparison of kinetic and extended magnetohydrodynamics computational models for the linear ion temperature gradient instability in slab geometry

We perform linear stability studies of the ion temperature gradient (ITG) instability in unsheared slab geometry using kinetic and extended magnetohydrodynamics (MHD) models, in the regime k{sub ∥}/k{sub ⊥}≪1. The ITG is a parallel (to B) sound wave that may be destabilized by finite ion Larmor radius (FLR) effects in the presence of a gradient in the equilibrium ion temperature. The ITG is stable in both ideal and resistive MHD; for a given temperature scale length L{sub Ti0}, instability requires that either k{sub ⊥}ρ{sub i} or ρ{sub i}/L{sub Ti0} be sufficiently large. Kinetic models capture FLR effects to all orders in either parameter. In the extended MHD model, these effects are captured only to lowest order by means of the Braginskii ion gyro-viscous stress tensor and the ion diamagnetic heat flux. We present the linear electrostatic dispersion relations for the ITG for both kinetic Vlasov and extended MHD (two-fluid) models in the local approximation. In the low frequency fluid regime, these reduce to the same cubic equation for the complex eigenvalue ω=ω{sub r}+iγ. An explicit solution is derived for the growth rate and real frequency in this regime. These are found to depend on a single non-dimensional parameter. We also compute the eigenvalues and the eigenfunctions with the extended MHD code NIMROD, and a hybrid kinetic δf code that assumes six-dimensional Vlasov ions and isothermal fluid electrons, as functions of k{sub ⊥}ρ{sub i} and ρ{sub i}/L{sub Ti0} using a spatially dependent equilibrium. These solutions are compared with each other, and with the predictions of the local kinetic and fluid dispersion relations. Kinetic and fluid calculations agree well at and near the marginal stability point, but diverge as k{sub ⊥}ρ{sub i} or ρ{sub i}/L{sub Ti0} increases. There is good qualitative agreement between the models for the shape of the unstable global eigenfunction for L{sub Ti0}/ρ{sub i}=30 and 20. The results quantify how far fluid calculations can be extended accurately into the kinetic regime. We conclude that for the linear ITG problem in slab geometry with unsheared magnetic field when k{sub ∥}/k{sub ⊥}≪1, the extended MHD model may be a reliable physical model for this problem when ρ{sub i}/L{sub Ti0}<10{sup −2} and k{sub ⊥}ρ{sub i}<0.2.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

A RELATION BETWEEN THE WARM NEUTRAL AND IONIZED MEDIA OBSERVED IN THE CANADIAN GALACTIC PLANE SURVEY

We report on a comparison between 21 cm rotation measure (RM) and the optically thin atomic hydrogen column density (N{sub H{sub I}}({tau} {yields} 0)) measured toward unresolved extragalactic sources in the Galactic plane of the northern sky. H I column densities integrated to the Galactic edge are measured immediately surrounding each of nearly 2000 sources in 1 arcmin 21 cm line data, and are compared to RMs observed from polarized emission of each source. RM data are binned in column density bins 4 Multiplication-Sign 10{sup 20} cm{sup -2} wide, and one observes a strong relationship between the number of hydrogen atoms in a 1 cm{sup 2} column through the plane and the mean RM along the same line of sight and path length. The relationship is linear over one order of magnitude (from 0.8 to 14 Multiplication-Sign 10{sup 21} atoms cm{sup -2}) of column densities, with a constant RM/N{sub H{sub I}}{approx} -23.2 {+-} 2.3 rad m{sup -2}/10{sup 21} atoms cm{sup -2}, and a positive RM of 45.0 {+-} 13.8 rad m{sup -2} in the presence of no atomic hydrogen. This slope is used to calculate a mean volume-averaged magnetic field in the second quadrant of (B{sub Parallel-To }) {approx}1.0 {+-} 0.1 {mu}G directed away from the Sun, assuming an ionization fraction of 8% (consistent with the warm-neutral medium; WNM). The remarkable consistency between this field and (B) = 1.2 {mu}G found with the same RM sources and a Galactic model of dispersion measures (DMs) suggests that electrons in the partially ionized WNM are mainly responsible for pulsar DMs, and thus the partially ionized WNM is the dominant form of the magneto-ionic interstellar medium.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Universal spin wave damping in magnetic Weyl semimetals

We analyze the decay of spin waves into Stoner excitations in magnetic Weyl semimetals. The lifetime of a mode is found to have a universal dependence on its frequency and momentum, and on a few parameters that characterize the relativistic Weyl spectrum. At the same time, uniform Gilbert damping by Weyl electrons is absent. Here, the decay rate of spin waves is calculated perturbatively using the s-d model of itinerant Weyl or Dirac electrons coupled to local moments. We show that many details of the Weyl spectrum, such as the momentum-space locations, dispersions, and sizes of the Weyl Fermi pockets, can be deduced indirectly by probing the spin waves of local moments using inelastic neutron scattering.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Weak ferromagnetism and temperature dependent dielectric properties of Zn{sub 0.9}Ni{sub 0.1}O diluted magnetic semiconductor

Highlights: • Single phase wurtzite structure was confirmed from XRD analysis. • Weak ferromagnetic behaviour at room temperature. • Pure semiconducting properties confirmed from temperature dependent conductivity. • Smaller dielectric properties at higher frequency. • Possible potential application in high frequency spintronic devices. - Abstract: In this study the room temperature ferromagnetic behaviour and dielectric properties of ZnO based diluted magnetic semiconductor (DMS) have been investigated using nominal chemical composition Zn{sub 0.9}Ni{sub 0.1}O. The X-ray diffraction analysis confirmed formation of single phase hexagonal wurtzite structure. An increase in grain size with increasing sintering temperature was observed from scanning electron microscopy. Field dependent DC magnetization values indicated dominant paramagnetic ordering along with a slight ferromagnetic behaviour at room temperature. Frequency dependent complex initial permeability showed some positive values around 12 at room temperature. In dielectric measurement, an increasing trend of complex permittivity, loss tangent and ac conductivity with increasing temperature were observed. The temperature dependent dispersion curves of dielectric properties revealed clear relaxation at higher temperature. Frequency dependent ac conductivity was found to increase with frequency whereas complex permittivity and loss tangent showed an opposite trend.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Lensing of Fast Radio Bursts by Plasma Structures in Host Galaxies

The amplitudes of fast radio bursts (FRBs) can be strongly modulated by plasma lenses in their host galaxies, including that of the repeating FRB 121102 at ∼1 Gpc luminosity distance. Caustics require the lens’ dispersion measure depth (DM{sub ℓ}), scale size (a), and distance from the source (d{sub sl}) to satisfy DM{sub ℓ}d{sub sl}/a{sup 2}≳0.65 pc{sup 2} au{sup −2} cm{sup −3}. Caustics produce strong magnifications (≲10{sup 2}) on short timescales (≲ hours to days) that appear as narrow spectral peaks (0.1–1 GHz). They also suppress the flux density in longer-duration (∼months) troughs. Multiply imaged bursts will arrive differentially by <1 μs to tens of ms with different apparent dispersion measures, δDM∼1 pc cm{sup −3}. When differing by less than the burst width, interference effects in dynamic spectra will be seen. Larger arrival time perturbations may mask any underlying periodicity with period ≲1 s. Strong lensing requires sources smaller than (Fresnelscale){sup 2}/a, which includes compact objects such as neutron star magnetospheres but excludes active galactic nuclei. We discuss constraints on densities, magnetic fields, and locations of plasma lenses related to the conditions needed for lensing to occur. Much of the phenomenology of the repeating FRB source FRB 121102 can be accounted for in this picture, which can be tested by obtaining wideband spectra of bursts (from <1 to 10 GHz and possibly higher) that will also help characterize the plasma environment near FRB sources. A rich variety of phenomena is expected from an ensemble of lenses near an FRB source.

79 ASTRONOMY AND ASTROPHYSICS↗