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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 991 records · Page 55
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

An Extension of the Athena++ Code Framework for Radiation-magnetohydrodynamics in General Relativity Using a Finite-solid-angle Discretization

We extend the general-relativistic magnetohydrodynamics (GRMHD) capabilities of Athena++ to incorporate radiation. The intensity field in each finite-volume cell is discretized in angle, with explicit transport in both space and angle properly accounting for the effects of gravity on null geodesics, and with matter and radiation coupled in a locally implicit fashion. Here we describe the numerical procedure in detail, verifying its correctness with a suite of tests. Motivated in particular by black hole accretion in the high-accretion-rate, thin-disk regime, we demonstrate the application of the method to this problem. With excellent scaling on flagship computing clusters, the port of the algorithm to the GPU-enabled AthenaK code now allows the simulation of many previously intractable radiation-GRMHD systems.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1973

Recent progress in the transition radiation detector techniques

A list of some of the major experimental achievements involving charged particles in the relativistic region are presented. With the emphasis mainly directed to the X-ray region, certain modes of application of the transition radiation for the identification and separation of relativistic charged particles are discussed. Some recent developments in detection techniques and improvements in detector performances are presented. Experiments were also carried out to detect the dynamic radiation, but no evidence of such an effect was observed.

Yuan, L. C. L.↗
NASA NTRS1986

Shape of solitons in classically forbidden states - 'Lorentz expansion'

The shape of extended objects in classically forbidden regions is shown to undergo expansion analogous to Lorentz contraction of a relativistic body of finite velocities. The problem of two interacting Dirac particles moving in one dimension is solved explicitly and the results are generalized to soliton solutions of field theories. An estimate of the effect on tunneling rates is also given, including solitons in (CH)z.

Guinea, F.↗
NASA NTRS1991

HST observations of 3C 66B - A double-stranded optical jet

The central region of the synchrotron jet in 3C 66B imaged with ultrahigh spatial resolution by the Faint Object Camera on the HST is discussed. A close correspondence between the general characteristics of the smoothed optical image and the radio map is observed, implying that the young and old relativistic particles occupy similar regions to within about 130 pc. Focus is placed on the double-stranded filamentary nature of the jet, and it is speculated that this could reflect edge-brightening effects due to enhanced radiation from a skinlike structure surrounding the jet. It is noted that the similarity of optical and radio morphologies points to the fact that localized particle acceleration occurs along the jet; however, the filamentary structure and a low filling factor of radiation agree with a model in which the electrons are accelerated within the nucleus and are transported along the jet in a channel having low magnetic field.

Macchetto, F.↗
DOE OSTI2022

Search for the Chiral Magnetic Effect via Charge-Dependent Azimuthal Correlations Relative to Spectator and Participant Planes in Au + Au Collisions at s N N = 200 GeV

The chiral magnetic effect (CME) refers to charge separation along a strong magnetic field due to imbalanced chirality of quarks in local parity and charge-parity violating domains in quantum chromodynamics. The experimental measurement of the charge separation is made difficult by the presence of a major background from elliptic azimuthal anisotropy. This background and the CME signal have different sensitivities to the spectator and participant planes, and could thus be determined by measurements with respect to these planes. We report such measurements in Au+Au collisions at a nucleon-nucleon center-of-mass energy of 200 GeV at the Relativistic Heavy-Ion Collider. It is found that the charge separation, with the flow background removed, is consistent with zero in peripheral (large impact parameter) collisions. Some indication of finite CME signals is seen with a significance of 1–3 standard deviations in mid-central (intermediate impact parameter) collisions. Furthermore, significant residual background effects may, however, still be present.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

Degeneracy Removal of Spin Bands in Collinear Antiferromagnets with Non–Interconvertible Spin–Structure Motif Pair

Energy bands in antiferromagnets are supposed to be spin degenerate in the absence of spin–orbit coupling (SOC). Recent studies have identified formal symmetry conditions for antiferromagnetic crystals in which this degeneracy can be lifted, spin splitting, even in the vanishing SOC (i.e., non-relativistic) limit. Materials having such symmetries could enable spin-split antiferromagnetic spintronics without the burden of using heavy-atom compounds. However, the symmetry conditions that involve spin and magnetic symmetry are not always effective as practical material selection filters. Furthermore, these symmetry conditions do not readily disclose trends in the magnitude and momentum dependence of the spin-splitting energy. Here, it is shown that the formal symmetry conditions enabling spin-split antiferromagnets can be interpreted in terms of local motif pairs, such as octahedra or tetrahedra, each carrying opposite magnetic moments. Here, collinear antiferromagnets with such a spin-structure motif pair, whose components interconvert by neither translation nor spatial inversion, will show spin splitting. Such a real-space motif-based approach enables an easy way to identify and design materials (illustrated in real example materials) having spin splitting without the need for SOC, and offers insights into the momentum dependence and magnitude of the spin splitting.

36 MATERIALS SCIENCE↗
DOE OSTI2025

The impact of $γN$ and $γ^∗N$ interactions on our understanding of nucleon excitations

We review recent progress in our understanding of the nucleon excitation spectrum. Thanks to dedicated efforts at facilities such as ELSA, MAMI and Jefferson Lab, several new nucleon resonances have been discovered, and evidence for previously elusive states has been significantly improved. Numerous decay channels have been observed for the first time, and resonance properties are being extracted from these data by several groups through coupled-channel analyses of varying complexity. Electroproduction experiments have provided further insights into the internal structure of light baryon resonances – for example, the long-debated Roper resonance N (1440) is observed as a three-quark state with a significant meson-cloud component. While the non-relativistic quark model remains a valuable tool for organizing the spectrum of nucleon and Δ resonances, a variety of theoretical frameworks have emerged to offer deeper understanding, including phenomenological quark models, holographic QCD, functional methods, effective field theories, and lattice QCD. We examine the interplay between these approaches, highlight their respective strengths and explore how they complement each other in shaping our knowledge of light baryon resonances. We address several open questions in baryon spectroscopy, including the nature of the enigmatic Λ (1405), ongoing searches for exotic states such as hybrid baryons and pentaquarks, and the dichotomy between microscopic descriptions of baryons in terms of quarks and gluons versus effective hadronic descriptions based on meson-baryon dynamics.

Baryon resonances↗
DOE OSTI2020

Toroidal modeling of runaway electron loss due to 3-D fields in DIII-D and COMPASS

The 3-D field induced relativistic runaway electron (RE) loss has been simulated for DIII-D, COMPASS and ITER plasmas, utilizing the MARS-F code incorporated with the recently developed and updated RE orbit module (REORBIT). Modeling shows effectively 100% loss of a post-disruption, high-current runaway beam in DIII-D, due to the 1 kG level of magnetic field perturbation produced by a fast growing n = 1 resistive kink instability. The RE loss is shown to be independent of the particle energy or the initial location of particles in the configuration space. Applied resonant magnetic perturbation (RMP) fields from in-vessel coils are not effective for RE beam mitigation in DIII-D, but do produce finite (>10%) RE loss in COMPASS, consistent with experimental observations in above two devices. The major reasons for this difference in RE control by RMP between these two devices are (i) the coil proximity to the RE beam and (ii) the effective coil current scaling versus the machine size and the toroidal magnetic field. About 10% RE loss fraction is also predicted for an ITER 15 MA scenario with pre-disruption plasma, highlighting the role of the plasma response. Up to 30% loss is computed, however, by artificially scaling the equilibrium pressure to zero. This is due to the more resistive plasma response and stronger resulting field line stochasticity. Distributions of the lost REs to the limiting surface show poloidally peaked profile near the high-field-side in both DIII-D and COMPASS, covering about 100o poloidal angle. Higher perturbation field level and/or higher particle energy also result in REs being lost to the low-field-side of the limiting surface of these two devices, increasing the effective wetted area. Finally, for the modeled ITER plasmas, the applied RMP field, with optimal poloidal spectrum that maximizes the edge localized mode control, leads to REs being lost to the lower divertor region of the limiting surface within a narrow poloidal band.

3-D field↗
DOE OSTI2021

Observation of Forward Neutron Multiplicity Dependence of Dimuon Acoplanarity in Ultraperipheral Pb-Pb Collisions at $\sqrt{s_{NN}}$ = 5.02 TeV

The first measurement of the dependence of $γγ → μ^+μ^–$ production on the multiplicity of neutrons emitted very close to the beam direction in ultraperipheral heavy ion collisions is reported. Data for lead-lead interactions at $\sqrt{s_{NN}}$ = 5.02 TeV, with an integrated luminosity of approximately 1.5 nb –1 , are collected using the CMS detector at the LHC. The azimuthal correlations between the two muons in the invariant mass region 8 < $m_{μμ}$ < 60 GeV are extracted for events including 0, 1, or at least 2 neutrons detected in the forward pseudorapidity range |η| > 8.3. The back-to-back correlation structure from leading-order photon-photon scattering is found to be significantly broader for events with a larger number of emitted neutrons from each nucleus, corresponding to interactions with a smaller impact parameter. This observation provides a data-driven demonstration that the average transverse momentum of photons emitted from relativistic heavy ions has an impact parameter dependence. These results provide new constraints on models of photon-induced interactions in ultraperipheral collisions. They also provide a baseline to search for possible final-state effects on lepton pairs caused by traversing a quark-gluon plasma produced in hadronic heavy ion collisions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2025

Generalized parton distributions and gravitational form factors at large momentum transfer

Within the soft collinear effective theory (SCET), we derive a factorization theorem which resums Sudakov logarithms (α s ln 2 ( –t)) n to all orders in the quark-in-quark generalized parton distribution (GPD) at large momentum transfer t, and perform a consistency check to one-loop. We show that the same Sudakov factor appears in the ‘Feynman’ contribution to the GPDs of the nucleon. Our result enables the resummation of all the large logarithms ln Q 2 and ln 2 t in exclusive processes with two hard scales Λ$^{2}_{QCD}$ ≪ |t| ≪ Q 2 . We also present a SCET power counting analysis of the Feynman contributions to the GPDs and show that the x-dependence of GPDs factorizes at large-t with controlled corrections. This in particular implies that any ratio of GPD moments such as the electromagnetic and gravitational form factors (GFF) is perturbatively calculable in this approximation. Furthermore, we identify a novel order α s power-law t-dependence in the GPD and the D-type GFF that will dominate over the standard order α$^{2}_{s}$ ‘leading twist’ asymptotic contribution in the phenomenologically relevant region of t.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1987

Relativistic beaming and the nuclei of double-lobed quasars

If the simple relativistic beaming model for extragalactic radio sources is at all correct, Doppler boosting may bias us toward choosing those objects pointing nearly at us in flux-density limited samples. In order to minimize any such effects in statistical tests of physical theories of these objects, an attempt is made to define a complete sample of sources with random orientations. For a sample of 26 double-lobed quasars, the dependence of a number of observable features on the angle to the line of sight and on the superluminal velocity is examined, and the distribution of superluminal velocities for the sources is measured. Two new superluminals are reported, and their apparent velocities and nuclear strengths are discussed.

Hough, David H.↗
NASA NTRS1993

Quasar X-ray spectra revisited

A sample of 45 quasars observed by the IPC on the Einstein satellite is used to reexamine the relationship of the soft X-ray energy index with radio properties and the optical Fe II emission. The tendency for radio-loud quasars to have systematically flatter X-ray energy indices than radio-quiet quasars is confirmed with the soft X-ray excess having negligible effect. There is a tendency for the flatness of the X-ray slope to correlate with radio core dominance for radio-loud quasars, suggesting that a component of the X-ray emission is relativistically beamed. For the radio-quiet quasars, the soft X-ray energy indices with a mean of about 1.0 are consistent with the indices found at higher energies, although steeper than those observed for Seyfert 1 galaxies where the reflection model gives a good fit to the data. The correlation of Fe II emission line strength with X-ray energy index is confirmed for radio-quiet quasars using a subset of 18 objects. The radio-loud quasars show no evidence for a correlation. This relation suggests a connection between the ionizing continuum and line emission from the broad emission-line region (BELR) of radio-quiet quasars, but in the opposite sense to that predicted by current photoionization models. The correlations of X-ray slope with radio core dominance and Fe II equivalent width within the radio-loud and radio-quiet subclasses, respectively, imply that the observed wide range of X-ray energy indices is real rather than due to the large measuring uncertainties for individual objects.

Shastri, P.↗
NASA NTRS1994

Physical processes in eclipsing pulsars: Eclipse mechanisms and diagnostics

We investigate how the radio emission of a pulsar interacts with plasma derived from a stellar companion. Various physical mechanisms that can cause radio pulse eclipse are discussed, and predictions are made for the polarization properties of the emergent radio wave. We consider eclipses by a wind from the stellar companion, by a stellar magnetosphere, or by material entrained in the pulsar wind. Eclipses due to refraction require either a relatively high plasma density or a sharp edge to the plasma distribution. The conditions that must prevail for free-free absorption to be effective in eclipsing a radio beam are also outlined. Pulse smearing may be important at higher frequencies; related eclipse mechanisms include pulse spreading due to a rapidly changing electron column, and scattering by Langmuir turbulence. The high brightness temperature radio beam can generate its own plasma turbulence via a number of nonlinear parametric instabilities, such as the instability associated with stimulated Raman scattering. When the plasma turbulence is heavily damped, the radio bean can still undergo induced Compton scattering. Stimulated scattering effects such as these are very sensitive to the presence of narrow-band substructure in the pulsar radio emission. Finally, we consider the possibility that plasma derived from a stellar companion may mix with the relativistic pulsar wind and cause cyclotron absorption at low radio frequencies. Even if the cyclotron optical depth is small, fluctuations in the emergent polarization of the radio beam on the timescale of a few seconds are a very sensitive probe of the spatial structure of the magnetic field in the pulsar wind. The current observational properties of two known eclipsing pulsar systems, PSR 1957+20 and PSR 1744-24A, are used to construct tentative eclipse models. The favored model for PSR 1957+20 is cyclotron or synchrotron absorption by plasma embedded in the pulsar wind combined with pulse smearing at high frequency, and the favored model for PSR 1744-24A is backscattering off plasma turbulence generated by the stimulated Raman scattering parametric instability. Pulsar eclipses promise to provide a good diagnostic of pulsar winds and possible of the pulse emission mechanism.

Thompson, C.↗
DOE OSTI2023

Revisiting Experimental Signatures of the Ponderomotive Force

The classical theory of single-electron dynamics in focused laser pulses is the foundation of both the relativistic ponderomotive force (RPF), which underlies models of laser-collective-plasma dynamics, and the discovery of novel strong-field radiation dynamics. Despite this bedrock importance, consensus eludes the community as to whether acceleration of single electrons in vacuum has been observed in experimental conditions. We analyze an early experiment on the RPF with respect to several features that were neglected in modeling and that can restore consistency between theory predictions and experimental data. The right or wrong pulse profile function, laser parameters, or initial electron distribution can each make or break the agreement between predictions and data. The laser phase at which the electron’s interaction with the pulse begins has a large effect, explaining why much larger energies are achieved by electrons liberated in the focal region by photoionization from high-Z atoms and by electrons ejected from a plasma mirror. Finally, we compute the difference in a typical electron spectrum arising from fluctuating focal spot size in state-of-the-art ultra-relativistic laser facilities. Our results emphasize the importance of thoroughly characterizing laser parameters in order to achieve quantitatively accurate predictions and the precision required for discovery science.

Hegelich, Bjorn Manuel↗
DOE OSTI2024

Global quark spin correlations in relativistic heavy ion collisions

The observation of the vector meson’s global spin alignment by the STAR Collaboration reveals that strong spin correlations may exist for quarks and antiquarks in relativistic heavy-ion collisions in the normal direction of the reaction plane. We propose a systematic method to describe such correlations in the quark matter. We classify them as local and long range quark spin correlations in the system. We show in particular that the effective quark spin correlations contain the genuine spin correlations originated directly from the dynamical process as well as those induced by averaging over other degrees of freedom. We also show that such correlations can be studied by measuring the vector meson’s spin density matrix and hyperon-hyperon and hyperon-anti-hyperon spin correlations. We present the relationships between these measurable quantities and spin correlations of quarks and antiquarks. Published by the American Physical Society 2024

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2026

Radial oscillations of viscous neutron stars: Zero diffusion case

In this article, the spectrum of radial oscillations of neutron stars is systematically studied within two frameworks of viscous relativistic hydrodynamics: the relativistic Navier-Stokes and Israel-Stewart theories. A correspondence is established between the discrete stellar eigenmodes and the continuous dispersion relation of perturbations around a homogeneous fluid, providing a basis for interpreting our numerical results. We analyze the Newtonian limit and assess the impact of relativistic corrections, such as the gravitational redshifting of microscopic relaxation timescales. We show that bulk viscosity can significantly affect the behavior of both hydrodynamic and nonhydrodynamic fundamental modes, and that, depending on the magnitude of the viscous effects, it is the nonhydrodynamic mode that becomes unstable beyond the turning point in a sequence of equilibrium configurations. These results provide a useful step toward systematic studies of neutron star quasinormal modes in the presence of viscosity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Effect of optical field ionisation on the generation of wake fields by femtosecond laser pulses in an inhomogeneous plasma

We consider the interaction of a high-intensity short laser pulse with an argon gas during optical field ionisation. Modelling in a three-dimensional cylindrical symmetric geometry is performed at various moderately relativistic intensities and positions of the focal plane of the laser beam in the case of both optical gas ionisation and a pre-ionised plasma. The influence of the processes occurring during optical field ionisation of the gas on the generation of wake waves is investigated, and the conditions are found under which intense wake fields are produced in a plasma formed from an inhomogeneous argon gas jet. The possibility of using a gas with a large number of electrons on the outer shell (argon) to excite intense wake waves and accelerate electrons is demonstrated. Despite significant ionisation refraction of the laser pulse on a radially inhomogeneous density profile of plasma electrons formed during optical field ionisation, the region of the parameters of the laser pulse and the gas target is determined at which ionisation refraction leads to the excitation of an intense wake wave. It is found that in a pre-ionised plasma a wake wave is not generated even at the same laser radiation intensity. (paper)

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗