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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

The radial phase variation of reversed-shear and toroidicity-induced Alfvén eigenmodes in DIII-D

The eigenfunction of an instability contains information about energy flow in the wave. Here, the amplitude and phase of electron cyclotron emission radiometer data from hundreds of DIII-D reversed shear Alfvén eigenmodes (RSAE) and toroidicity-induced Alfvén eigenmodes (TAE) are analyzed along the outboard horizontal midplane. The radial phase profile can be flat, linearly rising or falling, convex or concave; in other words, a wide variety of shapes is observed. For a particular mode, often the radial phase profile remains approximately constant as the mode evolves in time but sometimes it changes rapidly. Many TAEs and some RSAEs have phase profiles that are rather flat where the mode amplitude is largest but rise steadily by ~2π at large major radius. Rapid phase changes are observed when the frequencies of an RSAE and TAE overlap and the modes couple. The phase profile depends weakly on the fast-ion gradient that would appear in the absence of wave-induced transport. Linear and quadratic fits to the phase profiles, together with many plasma parameters, are assembled into RSAE and TAE databases. In both cases, large variability is observed. For RSAEs, the strongest phase dependencies are on electron temperature T e , RSAE mode frequency, and the density of carbon impurities. For TAEs, the strongest dependencies are on beam power and major radius of the mode. In general, the average RSAE radial phase profile is essentially flat but the TAE profile has non-zero slope and curvature.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1980

Composition of the nighttime ionospheric F 1 region near the magnetic equator

The effects of vertical E x B transport on NO(+), O2(+) and O(+) densities in the nighttime equatorial ionospheric F 1 region are investigated. Ion densities are calculated as functions of altitude, latitude and local time by the numerical solution of coupled, time-dependent ion continuity equations, taking into account production, loss by charge exchange and dissociative recombination and transport by diffusion and E x B drift. The results of the calculations are compared with measurements of NO(+), O2(+) and O(+) ion densities obtained at low altitudes by a mass spectrometer on board the Atmospheric Explorer C satellite, and are found to be consistent with the observations, suggesting that in the equatorial region, vertical transport by E x B drift is primarily responsible for producing the observed NO(+), O2(+) and O(+) density profiles. In addition, the reaction of O2(+) with N(4S) is found to be an important sink for O2(+) and a source of NO(+) ions. Implications of the observed and calculated near constancy of electron and ion densities with altitude when NO(+) is the dominant ion on the growth of large-scale irregularities are also considered

Anderson, D. N.↗
DOE OSTI2025

TEM turbulence in simulation and experiment in quasisymmetric and degraded-symmetry configurations of the HSX stellarator

The Helically Symmetric Experiment (HSX) has demonstrated reduced neoclassical transport and flow damping with quasisymmetry, but the difference of turbulent transport between quasisymmetric and degraded-symmetry configurations has not been definitively assessed. Profile analysis presented here shows that anomalous transport at the mid-radius is larger with degraded quasisymmetry when temperature and density profiles are matched. An initial comparison (${T_\mathrm{i}/T_\mathrm{e}} = 1$) between the quasisymmetric and degraded symmetry configurations finds that trapped-electron mode (TEM) turbulence can support larger heat fluxes in the degraded symmetry configuration despite smaller linear growth rates. However, further simulations at the experimental temperature ratio ($T_\mathrm{i}/T_\mathrm{e} = 0.2$) do not reproduce this effect. Experimental measurements of heat transport and density fluctuation amplitude are compared to nonlinear gyrokinetic simulations at the experimental temperature ratio. The heat flux and density fluctuation amplitude in simulation reproduce a substantial dependence on the density gradient, and the simulated heat flux matches measurements within experimental uncertainties, indicating that TEM turbulence is responsible for the anomalous transport at the steep density gradient in HSX.

HSX↗
NASA NTRS1992

The production and escape of nitrogen atoms on Mars

The lack of agreement between our previously computed values and those measured by Viking of the N-15:N-14 isotope enhancement ratio has led us to reevaluate our model of the Martian ionosphere. In previous models, we were unable to reproduce the ion profiles measured by the RPA on Viking using electron temperatures that were higher that the ion temperatures. When we increased the electron temperatures to 2500-3000 K and with a zero flux upper boundary condition, the ion densities at high altitudes exceeded the measured values by a large factor. We found that we can better fit the observed profiles if we impose a loss process at the upper boundary of our model. If the horizontal fluxes of ions do not constitute a net loss of ions, then the escape of N due to dissociative recombination is also inhibited and better agreement with the measured isotope ratio is found. The production of escaping nitrogen atoms is closely related to the production of thermospheric odd nitrogen; therefore, the densities of NO measured by Viking provide a convenient check on our nitrogen escape model. Our standard model NO densities are less that the measured values by a factor of 2-3, as are those of previous models. We find that reasonable agreement can be obtained by assuming that the rate coefficient for loss of odd nitrogen in the reaction of N with NO is smaller at temperatures that prevail in the lower Martian thermosphere than the standard value, which applies to temperatures of 200-400 K. Other aspects of this investigation are presented.

Fox, J. L.↗
NASA NTRS1977

Observations of the mid-latitude lower ionosphere in winter

Rocket observations of the lower ionosphere in the winter of 1971 at two locations show differences of electron density which are attributed to enhancements of nitric oxide and energetic electron fluxes precipitated into the mesosphere during the poststorm phase of a geomagnetic storm. Electron density distributions were observed above Wallops Island, Virginia, and Keweenaw, Michigan, larger values occurring at Keweenaw. Energetic electron fluxes were greater at Keweenaw (L = 3.9) than at Wallops Island (L = 2.5). While particle ionization was the dominant factor in establishing the electron density during one measurement at Keweenaw, particles were not present two days earlier, even though the electron density distribution was significantly larger than that observed at Wallops Island on both occasions. An accompanying ion composition profile measured at Keweenaw during the earlier flight showed NO(+) to be the dominant ion to 76 km, where the concentration of hydrated ions H3O(+).(H2O)n, exceeded that of NO(+).

Aikin, A. C.↗
NASA NTRS1981

IUE high-dispersion spectrum of R Aquarii

Emission-line spectra of several ions were observed in R Aqr in the wavelength range 1900-3000 A. C II diagnostics gives an electron density of about 400,000/cu cm and an electron temperature of about 10,000 K. The Mg II doublet profiles are grossly asymmetric, of a P Cygni type with terminal ejection velocity of at least 200 km/s. This is like Mg II behavior in Omicron Cet, Zeta Aur, and 32 Cyg, but unlike smaller UV shifts of the Mg II self-absorptions reported so far in other red giants and supergiants.

Johnson, H. M.↗
NASA NTRS1986

Analysis of coronal H I Lyman-alpha measurements in a polar region of the sun observed in 1979

Measurements of the intensities and spectral line profiles of resonantly scattered hydrogen Ly-alpha radiation have been used to determine hydrogen kinetic temperatures and electron densities between r = 1.5 and 2.2 solar radii in a polar region of the corona observed in 1979 near solar maximum. The mean temperature, 1.8 x 10 to the 6th K, in this region is significantly higher, by about 60 percent, than that obtained in a similar region observed in a 1980 rocket flight. The densities in these two polar regions are similar and are a factor of about 4 larger than in polar coronal holes observed at solar minimum. The flow velocities in both regions are most likely subsonic for r less than about 4 solar radii. The results reported here support the hypothesis that polar coronal holes observed at different times during the solar cycle can have different temperatures, densities, and possibly flow velocities.

Withbroe, G. L.↗
NASA NTRS2007

TOPLA: A New Empirical Representation of the F-Region Topside and Plasmasphere for the International Reference Ionosphere

The goal of this LWS tools effort is the development of a new data-based F-region TOpside and PLAsmasphere (TOPLA) model for the electron density (Ne) and temperature (Te) for inclusion in the International Reference Ionosphere (IRI) model using newly available satellite data and models for these regions. The IRI model is the de facto international standard for specification of ionospheric parameters and is currently being considered as an ISO Technical Specification for the ionosphere. Our effort is directed towards improving the topside part of the model and extending it into the plasmasphere. Specifically we are planning to overcome the following shortcomings of the current IRI topside model: (I) overestimation of densities above 700 km by a factor of 2 and more, (3) unrealistically steep density profiles at high latitudes during very high solar activities, (4) no solar cycle variations and no semi-annual variations for the electron temperature, (5) discontinuities or unphysical gradients when merging with plasmaspheric models. We will report on first accomplishments and on the current status of the project.

Bilitza, D.↗
NASA NTRS1979

The energetics of the ionosphere of Venus - A preliminary model based on Pioneer Venus observations

A model of the energy balance of the dayside ionosphere of Venus is presented. The coupled energy equations for electrons and ions are solved numerically and the calculated temperatures are compared with temperatures measured by instruments on the Pioneer Venus Orbiter. Neutral and ion density profiles consistent with those measured by various Pioneer Venus experiments are used in the model. It was found that using standard EUV sources and thermal conductivities the calculations produce temperatures that are much lower than the measured ones. Consequently, further calculations were performed in which (1) the thermal conductivities were inhibited by means of an almost horizontal magnetic field and/or (2) heat inflow into both the electron and ion gas at the top of the ionosphere was assumed due to the solar wind interaction. It was found that the calculated and measured temperatures were in reasonably good agreement if the thermal conductivities were inhibited by a near horizontal field of about 10 gammas and if the electron and ion heat fluxes were taken to be 4 x 10 to the 9th eV/sq cm/sec and 10 to the 7th eV/sq cm/sec, respectively.

Cravens, T. E.↗
NASA NTRS1982

Numerical calculations relevant to the initial expansion of the polar wind

The results of a numerical simulation of the expansion of an H(+)-0(+) electron plasma into a vacuum are reported. Variations were introduced in the ionic density ratios, the scale lengths for the density gradient at the plasma-vacuum interface, and the initial electron-ion temperature ratio. Conditions of a gravitationally bound O(+) plasma were also considered. It was found that if H(+) is a major ion, the H(+) density profile in the expansion region is always concave, with a phase-space divided into three regions, i.e., an undisturbed plasma, a rarefaction region, and an expansion region in which ions are accelerated. When the H(+) is a minor ion, the O(+) ions accelerate the H(+) ions, the H(+) phase-space is divided into five regions, and the increased electron temperatures produce enhanced electrostatic potentials and H(+) drifts in the expansion region.

Singh, N.↗
DOE OSTI2026

Neural-Network-Enhanced COTSIM: Advancing Predictive Capabilities for Fast DIII-D Simulations

Sustaining fusion reactions in tokamaks requires heating plasma to thermonuclear temperatures while maintaining confinement and stability. Neutral beam injection (NBI) provides heating, current drive, torque, and fueling, while electron cyclotron (EC) waves are widely used for heating and current drive; together, these actuators shape the plasma current, temperature, and density profiles. The control-oriented tokamak simulator (COTSIM), a predictive, control-oriented code, has been enhanced with neural-network surrogates for transport and sources. Turbulent transport is predicted by MMMnet—a neural-network version of the updated multimode model (MMM 9.0.10)—with significantly reduced computation time relative to MMM; neoclassical transport follows the Chang–Hinton model. NUBEAMnet, a surrogate of the Monte Carlo NUBEAM module, predicts beam-driven heating, current, and torque. EC heating and current drive use a control-oriented, empirically scaled source model; plasma resistivity follows the Spitzer formulation; bootstrap current uses the Sauter model. Equilibrium is computed using both prescribed and fixed-boundary solvers (FBSs), and the pedestal structure is modeled with an empirical pedestal model. For a representative DIII-D discharge, COTSIM predicts electron and ion temperature and safety-factor profiles in close agreement with TRANSP predictive and interpretive simulations while extending predictions through the pedestal region to the plasma edge (versus 80% of the minor radius in TRANSP). Furthermore, the equivalent COTSIM simulation runs in under 3 min compared to about 2 h for TRANSP, enabling rapid scenario planning, optimization of tokamak operation, and between-pulse control design.

Control-oriented tokamak simulator (COTSIM)↗
NASA NTRS1976

Metastable 2D atomic nitrogen in the mid-latitude nocturnal ionosphere

The only source at night at midlatitudes of N(2D) in the F region is the dissociative recombination of the positive NO ion, and the only important sinks are quenching by atomic oxygen and electrons. Ground-based measurements of the 5200 angstrom emission line resulting from the transition from N(2D) to N(4S) combined with satellite observations of neutral and ion densities and temperatures and the shape of the 5200 angstrom profile are used to relate the rate of quenching by atomic oxygen and the efficiency of production of N(2D) to the rate of quenching by electrons. A rate of quenching by atomic oxygen of 1.5-2.5 x 10 to the minus 12th cu cm/s and an efficiency of (0.8-1.0) plus or minus 30% for the production of N(2D) are obtained.

Torr, M. R.↗
DOE OSTI2021

Plasma heating and current sheet structure in anti-parallel magnetic reconnection

A theoretical model and an analytic theory of current sheet structure are presented for understanding anti-parallel driven magnetic reconnection in 2-1/2 dimension in collisionless plasmas. The theoretical model provides formulation to compute the current sheet y-profiles by specifying the profiles of electron and ion flow velocities V ex (x,y) and V ix (x,y). The current sheet solutions depend on the plasma density n in , merging magnetic field B 0 , ion velocity v i , and electron velocity ve in the upstream and the S evz =V ez /V dz parameter where V ez is the electron velocity accelerated by the reconnection electric field E z in the electron orbit meandering region, V dz ≃cE y /B x is the $\overrightarrow{E}$ × $\overrightarrow{B}$ drift velocity as electrons enter the orbit meandering region, B x is the merging magnetic field, and E y is the electrostatic electric field. With simplifying assumptions on the y-profiles of V ex and V ix , we have also developed an analytic theory of the current sheet structure. Analytic expressions for the anomalous resistivity, the electrostatic potential drop, and the maximum E y amplitude E max are obtained. The analytic results agree reasonably well with both the particle-in-cell simulation results and the numerical solutions of the theoretical model. The ions energy gain due to the potential drop is $∝B^2_0/n_{in}$. The electron energy gain is $∝(B^2_0 /8πn_{in})S_{evz}$. The $B^2_0/n_{in}$ scaling of the average ion and electron energy gains are consistent with laboratory experiments and space plasma observations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Precise Fermi level engineering in a topological Weyl semimetal via fast ion implantation

The precise controllability of the Fermi level is a critical aspect of quantum materials. For topological Weyl semimetals, there is a pressing need to fine-tune the Fermi level to the Weyl nodes and unlock exotic electronic and optoelectronic effects associated with the divergent Berry curvature. However, in contrast to two-dimensional materials, where the Fermi level can be controlled through various techniques, the situation for bulk crystals beyond laborious chemical doping poses significant challenges. Here, we report the milli-electron-volt (meV) level ultra-fine-tuning of the Fermi level of bulk topological Weyl semimetal tantalum phosphide using accelerator-based high-energy hydrogen implantation and theory-driven planning. By calculating the desired carrier density and controlling the accelerator profiles, the Fermi level can be experimentally fine-tuned from 5 meV below, to 3.8 meV below, to 3.2 meV above the Weyl nodes. High-resolution transmission electron microscopy reveals the crystalline structure is largely maintained under irradiation, while electrical transport indicates that Weyl nodes are preserved and carrier mobility is also largely retained. Our work demonstrates the viability of this generic approach to tune the Fermi level in semimetal systems and could serve to achieve property fine-tuning for other bulk quantum materials with ultrahigh precision.

36 MATERIALS SCIENCE↗
NASA NTRS1972

Electrostatic-probe measurements of plasma parameters for two reentry flight experiments at 25000 feet per second

Unique plasma diagnostic measurements at high altitudes from two geometrically similar blunt body reentry spacecraft using electrostatic probe rakes are presented. The probes measured the positive ion density profiles (shape and magnitude) during the two flights. The probe measurements were made at eight discrete points (1 cm to 7 cm) from the vehicle surface in the aft flow field of the spacecraft over the altitude range of 85.3 to 53.3 km (280,000 to 175,000 ft) with measured densities of 10 to the 8th power to 10 to the 12th power electrons/cu cm, respectively. Maximum reentry velocity for each spacecraft was approximately 7620 meters/second (25,000 ft/sec). In the first flight experiment, water was periodically injected into a flow field which was contaminated by ablation products from the spacecraft nose region. The nonablative nose of the second spacecraft thereby minimized flow field contamination. Comparisons of the probe measured density profiles with theoretical calculations are presented with discussion as to the probable cause of significant disagreement. Also discussed are the correlation of probe measurements with vehicle angle of attack motions and the good high altitude agreement between electron densities inferred from the probe measurements, VHF antenna measurements, and microwave reflectometer diagnostic measurements.

Jones, W. L., Jr.↗
NASA NTRS1972

The plasmapause period of magnetic recovery. Combined study of OGO 4, OGO 5 data and of grounded whistler reception

The equatorial structure and dynamics of the plasmasphere during the period of magnetic recovery, lasting from the 13 to 23 of September 1968, are studied. The H(+) ions density profiles measured in the night and afternoon sectors by the excentered orbital satellite OGO 5 and L sub p positions of the plasmapause deduced from the VLF records of the polar orbital satellite OGO 4, are included. Electron densities are calculated from the whistlers received at Kerguelen (L approximately 3, 7) and Byrd (L approximately 7), ground stations 150 degrees of longitude apart.

Corcuff, P.↗
DOE OSTI2022

Direct Measurements of DT Fuel Preheat from Hot Electrons in Direct-Drive Inertial Confinement Fusion

Hot electrons generated by laser-plasma instabilities degrade the performance of laser-fusion implosions by preheating the DT fuel and reducing core compression. The hot-electron energy deposition in the DT fuel has been directly measured for the first time by comparing the hard x-ray signals between DT-layered and mass-equivalent ablator-only implosions. The electron energy deposition profile in the fuel is inferred through dedicated experiments using Cu-doped payloads of varying thickness. The measured preheat energy accurately explains the areal-density degradation observed in many OMEGA implosions. This technique can be used to assess the viability of the direct-drive approach to laser fusion with respect to the scaling of hot-electron preheat with laser energy.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Direct Measurements of DT Fuel Preheat from Hot Electrons in Direct-Drive Inertial Confinement Fusion

Hot electrons generated by laser-plasma instabilities degrade the performance of laser-fusion implosions by preheating the DT fuel and reducing core compression. The hot-electron energy deposition in the DT fuel has been directly measured for the first time by comparing the hard x-ray signals between DT-layered and mass-equivalent ablator-only implosions. The electron energy deposition profile in the fuel is inferred through dedicated experiments using Cu-doped payloads of varying thickness. The measured preheat energy accurately explains the areal-density degradation observed in many OMEGA implosions. Furthermore, this technique can be used to assess the viability of the direct-drive approach to laser fusion with respect to the scaling of hot-electron preheat with laser energy.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗