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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 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Kinematic and dynamical origins of mean-p T fluctuations in heavy-ion collisions

Event-by-event fluctuations of the mean transverse momentum (mean-p T ) provide a sensitive probe of collective dynamics beyond single-particle spectra and anisotropic flow. We present a systematic study of mean-p T fluctuation observables using a Bayesian-calibrated multistage hydrodynamic framework, including quantitative comparisons to RHIC measurements and model-based investigations of beam-energy and kinematic-acceptance effects. The experimental definitions employed by the STAR and ALICE Collaborations are implemented explicitly and found to yield consistent results within controlled limits. We study the centrality and beam-energy dependence of the observable, its sensitivity to key soft-sector ingredients, and the impact of the kinematic p T acceptance. By introducing scaled-p T cuts, we demonstrate that a part of the apparent energy dependence arises from kinematic projection effects, while the remaining trends reflect genuine collective dynamics. Our results establish mean-p T fluctuations as a nontrivial and independent validation of calibrated hydrodynamic descriptions of the quark–gluon plasma.

Event-by-event correlations↗
DOE OSTI2022

Λ spin polarization in event-by-event relativistic heavy-ion collisions

We present a systematic study of $\Lambda$ hyperon's polarization observables using event-by-event (3+1)D relativistic hydrodynamics. The effects of initial hot spot size and QGP's specific shear viscosity on the polarization observables are quantified. We examine the effects of the two formulations of the thermal shear tensor on the polarization observables using the same hydrodynamic background. With event-by-event simulations, we make predictions for the Fourier coefficients of $\Lambda$'s longitudinal polarization $P^z$ with respect to the event planes of different orders of anisotropic flow. We propose new correlations among the Fourier coefficients of $P^z$ and charged hadron anisotropic flow coefficients to further test the mapping from fluid velocity gradients to hyperon's polarization. Finally, we present a system size scan with Au+Au, Ru+Ru, and O+O collisions at $\sqrt{s_\mathrm{NN}} = 200$ GeV to study the system size dependence of polarization observables at the Relativistic Heavy-ion Collider.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Rapidity dependence of initial state geometry and momentum correlations in p + Pb collisions

Event geometry and initial state correlations have been invoked as possible explanations of long range azimuthal correlations observed in high multiplicity p+p and p+Pb collisions. We study the rapidity dependence of initial state momentum correlations and event-by-event geometry in √s = 5.02 TeV p+Pb collisions within the 3+1D IP-Glasma model [1], where the longitudinal structure is governed by JIMWLK rapidity evolution of the incoming nuclear gluon distributions. Furthermore, we find that the event geometry is correlated across large rapidity intervals whereas initial state momentum correlations are relatively short range in rapidity. Based on our results, we discuss implications for the relevance of both effects in explaining the origin of collective phenomena in small systems.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Observable signatures of initial state momentum anisotropies in nuclear collisions

In this work, we show that the correlation between the elliptic momentum anisotropy v 2 and the average transverse momentum [ p T ] at fixed multiplicity in small system nuclear collisions carries information on the origin of the observed momentum anisotropy. A calculation using a hybrid IP-Glasma+ music +UrQMD model that includes contributions from final state response to the initial geometry as well as initial state momentum anisotropies of the color glass condensate predicts a characteristic sign change of the correlator ρ ^ ( v 2 2 , [ p T ] ) as a function of charged particle multiplicity in p + Au and d + Au collisions at s = 200 GeV , and p + Pb collisions at s = 5.02 TeV . This sign change is absent in calculations without initial state momentum anisotropies. The model further predicts a qualitative difference between the centrality dependence of ρ ^ ( v 2 2 , [ p T ] ) in Au + Au collisions at s = 200 GeV and Pb + Pb collisions at s = 5.02 TeV , with only the latter showing a sign change in peripheral events. Predictions for O + O collisions at different collision energy show a similar behavior. Experimental observation of these distinct qualitative features of ρ ^ ( v 2 2 , [ p T ] ) in small and large systems would constitute strong evidence for the presence and importance of initial state momentum anisotropies predicted by the color glass condensate effective theory.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

Intermittency and Dissipative Structures Arising from Relativistic Magnetized Turbulence

Abstract Kinetic simulations of relativistic turbulence have significantly advanced our understanding of turbulent particle acceleration. Recent progress has highlighted the need for an updated acceleration theory that can account for particle acceleration within the plasma’s coherent structures. Here, we investigate how intermittency modeling connects statistical fluctuations in turbulence to regions of high-energy dissipation. This connection is established by employing a generalized She–Leveque model to characterize the exponents ζ p for the structure functions S p ∝ l ζ p . The fitting of the scaling exponents provides us with a measure of the codimension of the dissipative structures, for which we subsequently determine the filling fraction. We perform our analysis for a range of magnetizations σ and relative fluctuation amplitudes δ B 0 / B 0 . We find that increasing values of σ and δ B 0 / B 0 allow the turbulent cascade to break sheetlike structures into smaller regions of dissipation that resemble chains of flux ropes. However, as their dissipation measure increases, the dissipative regions become less volume filling. With this work, we aim to inform future turbulent acceleration theories that incorporate particle energization from interactions with coherent structures within relativistic turbulence.

Astronomy & Astrophysics↗
NASA NTRS1975

Effect of small ionospheric irregularities on radio wave absorption

The ionospheric absorption of a radio wave caused by small-scale irregularities with a gaussian autocorrelation function is calculated for various values of the linear scale height, the radio frequency, the scale size of the irregularities, and the mean-square fractional electron density fluctuations. The absorption is due to scattering of the radio wave into plasma oscillations by the irregularities. It is concluded that the absorption due to such irregularities with a mean-square fractional electron density deviation greater than about 0.000001 exceeds the normal collisional height-integrated absorption. Absorption of this type could play a significant part in heating experiments or in an ionosphere containing naturally occurring irregularities.

Chen, H. C.↗
NASA NTRS1977

The multiple applications of electrons in space

An electron source such as a simple cathode is a cheap and light device which can serve several technological and scientific purposes in space: (1) electrostatic charging of a spacecraft can be limited by releasing electrons accumulated on the conductive elements of their surface; (2) the erosion of conductive coatings and the ability of conductive paints to withstand the space environment can be evaluated by monitoring the flow of charged particles impinging on their surface; (3) measuring the current collected by the spacecraft surface as a function of its potential with respect to an emitter is a very sensitive diagnostic technique which can yield a number of plasma parameters, such as density and temperature; (4) it is possible to convert the thermal motion of space plasmas into electrical energy by collecting energetic electrons and returning them to the medium as cold particles; (5) a wave in a plasma is characterized by a conduction current density which gives rise to fluctuations of the current flowing to the surface.

Grard, R. J. L.↗
NASA NTRS1979

Ion confinement and transport in a toroidal plasma with externally imposed radial electric fields

Strong electric fields were imposed along the minor radius of the toroidal plasma by biasing it with electrodes maintained at kilovolt potentials. Coherent, low-frequency disturbances characteristic of various magnetohydrodynamic instabilities were absent in the high-density, well-confined regime. High, direct-current radial electric fields with magnitudes up to 135 volts per centimeter penetrated inward to at least one-half the plasma radius. When the electric field pointed radially toward, the ion transport was inward against a strong local density gradient; and the plasma density and confinement time were significantly enhanced. The radial transport along the electric field appeared to be consistent with fluctuation-induced transport. With negative electrode polarity the particle confinement was consistent with a balance of two processes: a radial infusion of ions, in those sectors of the plasma not containing electrodes, that resulted from the radially inward fields; and ion losses to the electrodes, each of the which acted as a sink and drew ions out of the plasma. A simple model of particle confinement was proposed in which the particle confinement time is proportional to the plasma volume. The scaling predicted by this model was consistent with experimental measurements.

Roth, J. R.↗
NASA NTRS1984

New electromagnetic mode in a non-Maxwellian high-beta plasma

An electromagnetic (EM) mode outside of the electron cyclotron frequency in a dense plasma discharge is reported. The experimental plasma was generated in a weak, uniform magnetic field and natural magnetic fluctuations were monitored and examined for cross correlations. Wave dispersion, propagation direction polarization and the electron velocity distribution were also derived. The fluctuations observed were neither cyclotron harmonic waves nor whistlers and consisted of circularly polarized waves propagating along field lines in 3-6 cm diam flux tubes. The mode was carried away from the cathode by streaming energetic electrons. The results may be pertinent in studies of EM modes in auroral arcs or magnetic fluctuations in tokamaks with runaway electrons.

Urrutia, J. M.↗
NASA NTRS2004

Complexity Induced Anisotropic Bimodal Intermittent Turbulence in Space Plasmas

The "physics of complexity" in space plasmas is the central theme of this exposition. It is demonstrated that the sporadic and localized interactions of magnetic coherent structures arising from the plasma resonances can be the source for the coexistence of nonpropagating spatiotemporal fluctuations and propagating modes. Non-Gaussian probability distribution functions of the intermittent fluctuations from direct numerical simulations are obtained and discussed. Power spectra and local intermittency measures using the wavelet analyses are presented to display the spottiness of the small-scale turbulent fluctuations and the non-uniformity of coarse-grained dissipation that can lead to magnetic topological reconfigurations. The technique of the dynamic renormalization group is applied to the study of the scaling properties of such type of multiscale fluctuations. Charged particle interactions with both the propagating and nonpropagating portions of the intermittent turbulence are also described.

Chang, Tom↗
NASA NTRS2005

Flux Transfer Event in the Subsolar Region and Near the Cusp: Simultaneous Polar and Cluster Observations

The phenomenon called flux transfer events (FTEs) is widely accepted as the manifestation of time-dependent reconnection. In this paper, we present an observational evidence of a flux transfer event observed simultaneously at low-latitude by Polar and high-latitude by Cluster. This event occurred on March 21, 2002, when both Cluster and Polar were located near the local noon but with large latitudinal distance. Cluster was moving outbound from polar cusp to the magnetosheath, and Polar was in the magnetosheath near the equatorial magnetopause. The observations show that a flux transfer event was formed between the equator and the northern cusp. Polar and Cluster observed the FTE's two open flux tubes: Polar saw the southward moving flux tube near the equator; and Cluster the , northward moving flux tube at high latitude. Unlike low-latitude FTEs, the high-latitude FTE did not exhibit the characteristic bi-polar BN signature. But the plasma data clearly showed its open flux tube configuration. Enhanced electric field fluctuations were observed within the FTE core, both at low- and high-attitudes. This event provides us a unique opportunity to understand high-latitude FTE signatures and the nature of time-varying reconnection.

Le, G.↗
DOE OSTI2025

Visualization techniques for the gyrokinetic tokamak simulation code

Gyrokinetic simulations of plasma microturbulence in tokamaks are challenging to visualize because the compute grid follows the magnetic field lines that spiral around the torus. We have overcome this challenge by developing three new approaches that improve visualization of gyrokinetics. Our techniques work directly with the topology of magnetic flux surfaces where the simulation stores variables in concentric rings on poloidal planes (vertical cross sections of the torus). Our visualization preview step triangulates each consecutive pair of rings to display the data on a poloidal plane. The second visualization technique follows spiral field lines around the torus and constructs polygons to visualize a flux surface. Third, the poloidal triangles are connected between planes to form prisms that compose a 3-D model of the entire torus. The visualization workflow produces detailed geometry that matches the high resolution, irregular compute grid for every time step. The surface and solid models are displayed in scientific visualization programs to effectively explore and communicate the results, including fluctuation of electron density, ion temperature, and electrostatic potential. Highly detailed renderings verify plasma behavior along magnetic field lines over time.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1985

The effect of hydrodynamic turbulence on Langmuir waves in a slightly ionized plasma

In the lower altitude regions of the ionosphere, the gas present is only slightly ionized, and it behaves as a fluid. It is expected that a turbulent wake will form behind a spacecraft. Information is to be obtained regarding the effect of this turbulence on plasma waves measured by the spacecraft. If the turbulence is two-dimensional (2-D) and carries enstrophy, a fluctuating magnetic field and electric fields are produced. The fields induce fluctuations in the electron population which correlate with the magnetic and electric fluctuations which produced them. This correlation causes diffusion which changes the electron distribution and heats the electrons. As a result, the Langmuir frequency increases, and the Landau damping of Langmuir waves in the plasma decreases. Details regarding these processes are discussed, taking into account the results of a mathematical analysis.

Parish, J. L.↗
NASA NTRS1978

ISEE plasma observations near the subsolar magnetopause

High-resolution plasma observations are analyzed which were performed during four successive inbound passes of the ISEE 1 and 2 spacecraft. A total of nine magnetopause crossings were made near the subsolar point under widely differing orientations of the interplanetary magnetic field. It is found that (1) large fluctuations that often appear to be temporal in nature characterize the magnetosheath flow near the magnetopause; (2) the plasma density and pressure between about 0.1 and 0.3 earth radius outside the magnetopause often begin to decrease gradually as the magnetopause is approached, in conjunction with an increase in magnetic-field strength; (3) the magnetopause, in cases where it can be well resolved, exhibits fluctuations in density, pressure, and bulk velocity about average magnetosheath values; and (5) the only thick (low-latitude) boundary layer observed was characterized by sharp changes at its inner and outer edges.

Paschmann, G.↗
NASA NTRS2011

Dayside Magnetopause Transients Correlated with Changes of the Magnetosheath Magnetic Field Orientation

The paper analyses one long-term pass (26 August 2007) of the THEMIS spacecraft across the dayside low-latitude magnetopause. THEMIS B, serving partly as a magnetosheath monitor, observed several changes of the magnetic field that were accompanied by dynamic changes of the magnetopause location and/or the structure of magnetopause layers observed by THEMIS C, D, and E, whereas THEMIS A scanned the inner magnetosphere. We discuss the plasma and the magnetic field data with motivation to identify sources of observed quasiperiodic plasma transients. Such events at the magnetopause are usually attributed to pressure pulses coming from the solar wind, foreshock fluctuations, flux transfer events or surface waves. The presented transient events differ in nature (the magnetopause surface deformation, the low-latitude boundary layer thickening, the crossing of the reconnection site), but we found that all of them are associated with changes of the magnetosheath magnetic field orientation and with enhancements or depressions of the plasma density. Since these features are not observed in the data of upstream monitors, the study emphasizes the role of magnetosheath fluctuations in the solar wind-magnetosphere coupling.

Tkachenko, O.↗
NASA NTRS1985

Plasma heating at collisionless shocks due to the kinetic cross-field streaming instability

Heating at collisionless shocks due to the kinetic cross-field streaming instability, which is the finite beta (ratio of plasma to magnetic pressure) extension of the modified two stream instability, is studied. Heating rates are derived from quasi-linear theory and compared with results from particle simulations to show that electron heating relative to ion heating and heating parallel to the magnetic field relative to perpendicular heating for both the electrons and ions increase with beta. The simulations suggest that electron dynamics determine the saturation level of the instability, which is manifested by the formation of a flattop electron distribution parallel to the magnetic field. As a result, both the saturation levels of the fluctuations and the heating rates decrease sharply with beta. Applications of these results to plasma heating in simulations of shocks and the earth's bow shock are described.

Winske, D.↗
DOE OSTI2024

Numerical thermalization in 2D PIC simulations: Practical estimates for low-temperature plasma simulations

The process of numerical thermalization in particle-in-cell (PIC) simulations has been studied extensively. It is analogous to Coulomb collisions in real plasmas, causing particle velocity distributions (VDFs) to evolve toward a Maxwellian as macroparticles experience polarization drag and resonantly interact with the fluctuation spectrum. This paper presents a practical tutorial on the effects of numerical thermalization in 2D PIC applications. Scenarios of interest include simulations, which must be run for many thousands of plasma periods and contain a population of cold electrons that leave the simulation space very slowly. This is particularly relevant to many low-temperature plasma discharges and materials processing applications. We present numerical drag and diffusion coefficients and their associated timescales for a variety of grid resolutions, discussing the circumstances under which the electron VDF is modified by numerical thermalization. Though the effects described here have been known for many decades, direct comparison of analytically derived, velocity-dependent numerical relaxation timescales to those of other relevant processes has not often been applied in practice due to complications that arise in calculating thermalization rates in 1D simulations. Using these comparisons, we estimate the impact of numerical thermalization in several examples of low-temperature plasma applications including capacitively coupled plasma discharges, inductively coupled plasma discharges, beam plasmas, and hollow cathode discharges. Finally, we discuss possible strategies for mitigating numerical relaxation effects in 2D PIC simulations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗