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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Determining coronal electron temperatures from observations with UVCS/SOHO

The electron temperature is a fundamental physical parameter of the coronal plasma. Currently, there are no direct measurements of this quantity in the extended corona. Observations with the Ultraviolet Coronagraph Spectrometer (UVCS) aboard the upcoming Solar and Heliospheric Observatory (SOHO) mission can provide the most direct determination of the electron kinetic temperature (or, more precisely, the electron velocity distribution along the line of sight). This measurement is based on the observation of the Thomson-scattered Lyman alpha (Ly-alpha) profile. This observation is made particularly challenging by the fact that the integrated intensity of the electron-scattered Ly-alpha line is about 10(exp 3) times fainter than that of the resonantly-scattered Ly-alpha component. In addition, the former is distributed across 50 A (FWHM), unlike the latter that is concentrated in 1 A. These facts impose stringent requirements on the stray-light rejection properties of the coronagraph/spectrometer, and in particular on the requirements for the grating. We make use of laboratory measurements of the UVCS Ly-alpha grating stray-light, and of simulated electron-scattered Ly-alpha profiles to estimate the expected confidence levels of electron temperature determination. Models of different structures typical of the corona (e.g., streamers, coronal holes) are used for this parameter study.

Fineschi, S.↗
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 NTRS1995

One-dimensional hybrid satellite track model for the Dynamics Explorer 2 (DE 2) satellite

A one-dimensional hybrid satellite track model has been developed to calculate the high-latitude thermospheric/ionospheric structure below the satellite altitude using Dynamics Explorer 2 (DE 2) satellite measurements and theory. This model is based on Emery et al. (1985) satellite track code but also includes elements of Roble et al. (1987b) global mean thermosphere/ionosphere model. A number of parameterizations and data handling techniques are used to input satellite data from several DE 2 instruments into this model. Profiles of neutral atmospheric densities are determined from the Mass Spectrometer Incoherent Scatter 1990 (MSIS-90) model and measured neutral temperatures. Measured electron precipitation spectra are used in an auroral model to calculate particle impact ionization rates below the satellite. These rates are combined with a solar ionization rate profile and used to solve the O(+) diffusion equation, with the measured electron density as an upper boundary condition. The calculated O(+) density distribution, as well as the ionization profiles, are then used in a photochemical equilibrium model to calculate the electron and molecular ion densities. The electron temperature is also calculated by solving the electron energy equation with an upper boundary condition determined by the DE 2 measurement. The model enables calculations of altitude profiles of conductivity and Joule heating rate along and below the satellite track. In a first application of the new model, a study is made of thermospheric and ionospheric structure below the DE 2 satellite for a single orbit which occurred on October 25, 1981. The field-aligned Poynting flux, which is independently obtained for this orbit, is compared with the model predictions of the height-integrated energy conversion rate. Good quantitative agreement between these two estimates has been reached. In addition, measurements taken at the incoherent scatter radar site at Chatanika (65.1 deg N, 147.4 deg W) during a DE 2 overflight are compared with the model calculations. A good agreement was found in lower thermospheric conductivities and Joule heating rate.

Deng, Wei↗
NASA NTRS1981

Large-scale current systems in the dayside Venus ionosphere

The occasional observation of large-scale horizontal magnetic fields within the dayside ionosphere of Venus by the flux gate magnetometer on the Pioneer Venus orbiter suggests the presence of large-scale current systems. Using the measured altitude profiles of the magnetic field and the electron density and temperature, together with the previously reported neutral atmosphere density and composition, it is found that the local ionosphere can be described at these times by a simple steady state model which treats the unobserved quantities, such as the electric field, as parameters. When the model is appropriate, the altitude profiles of the ion and electron velocities and the currents along the satellite trajectory can be inferred. These results elucidate the configurations and sources of the ionospheric current systems which produce the observed large-scale magnetic fields, and in particular illustrate the effect of ion-neutral coupling in the determination of the current system at low altitudes.

Luhmann, J. G.↗
NASA NTRS1979

Physical parameter profiles of selected X-ray flares

Soft X-ray proportional-counter data for nine X-ray flares of class C1 to M1 occurring during the Skylab mission, have been analyzed to yield temperature, volume emission measure, electron density and thermal energy content. Profiles of these quantities are presented, using 1-min averages of 2.5-s resolution data. The event profiles are discussed and comparisons with previous studies made.

Wilson, R. M.↗
DOE OSTI2025

An experimental study of the existence regions and non-linear interactions of drift wave and Kelvin–Helmholtz instabilities in a linear magnetized plasma

Experimental observations of the intrinsic excitation and non-linear interactions of drift wave (DW) and Kelvin–Helmholtz (KH) instabilities in a linear magnetized plasma column are presented. The experiments are carried out in the inverse mirror plasma experimental device (IMPED)—a cylindrical, magnetized, linear plasma machine designed to study low-frequency waves and instabilities in plasma. A novel feature of IMPED is the ability to control plasma profiles, such as the density n(r)⁠, electron temperature T e (r)⁠, and plasma potential V p (r) by varying the ratio Rm of the magnetic field in the main chamber to that in the source chamber. At high values of Rm, higher-density gradient scale length promotes the drift wave (DW) instability while lower Rm value results in a higher radial electric field, inducing a sheared poloidal flow that enhances the dominance of the Kelvin–Helmholtz (KH) mode. The background and fluctuating plasma parameters are characterized using various configurations of multiple in situ electric probes at different spatial locations to quantify the local gradients that excite the low-frequency primary instabilities. Statistical, spectral, and bispectral analysis of the density and potential signals help identify these modes in terms of wave number, frequency, phase, and amplitude and also delineate the nature of their non-linear interactions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Research and Development to Reduce Impurity Production and Transport of the Impurities to the Target in Linear Plasma Devices Using Helicon Plasma Sources

Linear plasma devices used to test plasma facing materials (PFMs) and components for fusion reactors are often suffering under the production of intrinsic impurities from the plasma source system. Most linear plasma devices use internal electrodes (hollow cathodes, reflex arc, or cascading arc), which typically are the source of the main impurities. The next-generation plasma generators use radio frequency (RF) plasma sources like helicons to avoid internal electrodes. However, high power operation of helicons has proven to result in impurity production due to the high rectified sheath voltages created. Depending on the plasma parameters and magnetic configuration, these impurities can be transported to the target and deposited there to unacceptable high levels. Here, in this contribution, the experimental results from Proto-MPEX are summarized, and the conclusions of the impurity source physics are given. Methods to reduce the impurity production, the impurity transport, and the net deposition on the target are presented. These methods to reduce the impurity production include Faraday screens to reduce the sheath voltage drop, high-Z refractory coatings to reduce the erosion yield, and wall conditioning methods. Methods to reduce the impurity transport include changes in the magnetic configuration as well as electron heating to change axial and radial temperature profiles. Preliminary results on the effectiveness of some of these methods are presented.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1972

Error analysis in the Mariner 6 and 7 occultation experiments

The Mariner 6 and 7 occultation experiments provide sets of Doppler residuals from which physical properties of the Martian atmosphere may be derived, such as the ionospheric electron density distribution and the lower atmosphere temperature profile. The Doppler residuals contain both systematic and random errors. The former are removed during the data reduction, and the latter are analyzed to yield error limits on the derived physical properties of the atmosphere.

Stewart, R. W.↗
NASA NTRS1978

The spiral field inhibition of thermal conduction in two-fluid solar wind models

The paper reports on two-field models which include the inhibition of thermal conduction by the spiraling interplanetary field to determine whether any of the major conclusions obtained by Nerney and Barnes (1977) needs to be modified. Comparisons with straight field line models reveal that for most base conditions, the primary effect of the inhibition of thermal conduction is the bottling-up of heat in the electrons as well as the quite different temperature profiles at a large heliocentric radius. The spiral field solutions show that coronal hole boundary conditions do not correspond to states of high-speed streams as observed at 1 AU. The two-fluid models suggest that the spiral field inhibition of thermal conduction in the equatorial plane will generate higher gas pressures in comparison with flows along the solar rotation axis (between 1 and 10 AU). In particular, massive outflows of stellar winds, such as outflow from T Tauri stars, cannot be driven by thermal conduction. The conclusions of Nerney and Barnes remain essentially unchanged.

Nerney, S.↗
NASA NTRS1984

Structure of the November 12, 1978, quasi-parallel interplanetary shock

The jump in plasma parameters exhibited by the intense interplanetary shock event of Nov. 12, 1978 is analyzed using ISEE 1, 2 and 3 data. Magnetic and electric field measurements indicated that the shock magnetic field profile was similar to the earth bow shock profile. Data on the electron and proton densities, temperatures, bulk velocities and alpha particles showed a steady electron temperature increase across the shock on a 12 earth radii scale. The upstream and downstream flow parameters are found to be within 10 percent of Rankin-Hugoniot jump conditions. The shock moved at 614 km/sec and had three dissipative scales, one a few Larmor radii determined by the magnetic field jump, a second 10 earth radii correlated with the electron equilibrium and the other 30 earth radii connected to the energetic proton foreshock.

Kennel, C. F.↗
NASA NTRS1996

Spectroscopic Diagnostics and an Arc Jet Heated Air Plasma

Spectral radiation measurements were made in the range of 200 to 900 nm across a section of the plenum of an arc jet wind tunnel using a series of optical fibers. The spectra contained line radiation from Oxygen and Nitrogen atoms and molecular radiation from N2(+), N2, and NO. Abel inversion technique is used to obtain radial distribution of the spectra. The analysis yielded radial profiles of the electronic excitation, vibrational and rotational temperatures of the flow field. Spectral fitting yielded branching ratios for different vibrational and rotational bands. Relatively mild flow conditions, i.e. enthalpy and mass flow rate, were used for prolonged measurements of up to and over two hours to establish the best experimental methods of temperature determinations. Signal to noise was improved by at least an order of magnitude enabling the molecular vibrational band heads of N2(+) (first negative system), N2 (second positive system), and NO (beta, gamma, delta, and epsilon systems) to be resolved in the lower ultraviolet wavelength regions. The increased signal to noise ratio also enabled partial resolution of the rotational lines of N2(+) and N2 in certain regions of minimal overlap. Comparison of the spectra with theoretical models such as the NEQAIR2 code are presented and show potential for fitting the spectra when reliable calibration is performed for the complete wavelength range.

Mack, Larry Howard, Jr.↗
DOE OSTI2021

Gyrokinetic simulation of turbulence and transport in the SPARC tokamak

The turbulence and transport expected in the SPARC tokamak Primary Reference Discharge (PRD) [P. Rodriguez-Fernandez et al., J. Plasma Phys. 86, 865860503 (2020)] have been investigated with the gyrokinetic code CGYRO [J. Candy et al., J. Comput. Phys. 324, 73–93 (2016)]. Linear and nonlinear simulations that focus on ion (kθρs<1.0) and electron-scale (kθρs>1.0) turbulence were used to probe the nature of the turbulence and the resulting transport in the fusion core. It is found that in the SPARC PRD, ion temperature gradient (ITG) turbulence is expected to dominate transport over most of the profile with some potential trapped electron mode impact in the near edge. Stiff turbulence is observed over a part of the plasma core such that SPARC's ion temperature profile will likely be pinned to just above the critical gradient for ITG. The role of electromagnetic turbulence, rotation, and electron-scale turbulence was investigated to provide some insight into the physics required to accurately predict SPARC performance via gyrokinetics. Additionally, predictions of impurity peaking for potential low- and high-Z SPARC first-wall materials are probed using ion-scale simulation. The dominance of low-k turbulence in SPARC provides a potential opportunity for more tractable prediction of plasma profiles using nonlinear gyrokinetics. This work is the first step toward full gyrokinetic profile prediction of SPARC kinetic profiles and the resulting fusion power and plasma gain.

Howard, N. T. (ORCID:0000000287876309)↗
NASA NTRS1984

The thermosphere of Titan

The vertical structure of Titan's thermosphere is calculated down to the mesopause as a function of local time based on Voyager 1 occultation data. The thermal time scales that characterize the diurnal behavior of the thermosphere are discussed, the input model atmosphere used to calculate the temperature profile is presented, and the dominant heating and cooling mechanisms in the thermosphere are examined. The temperature profiles obtained by integrating the heat transfer equation with and without electron heating are presented and discussed. The implications that derived exospheric temperatures have for the neutral hydrogen torus are investigated. The diurnal exospheric temperature is unlikely to exceed 225 K, averages between 187 and 197 K, and has a variational amplitude of 28 K or less. The vertical extent of the hydrogen cloud is too large to be explained in terms of simple thermal escape of hydrogen from the exosphere.

Friedson, A. J.↗
NASA NTRS1998

Radio Occultation Studies of Venus' Atmosphere with Magellan

We have been conducting a systematic study of the middle and lower atmosphere of Venus through analysis of 20 radio occultation experiments conducted with the Magellan spacecraft between October, 1991, and August, 1994. These studies have revealed a rich but sparsely sampled trove of information regarding the structure, composition and dynamics of the Venus atmosphere. The five sets of experiments sampled a variety of latitudes. Basic results include vertical profiles of: (1) electron density in the ionosphere, (2) pressure, temperature, density, and static stability in the neutral atmosphere (from 33 km to 98 km), and (3) sulfuric acid vapor (H2SO4) abundance below the main cloud deck. Further analysis of the temperature profiles led to the discovery of small vertical-scale gravity waves in the neutral atmosphere. The retrieved profiles show intriguing zonal variations that might be due to planetary- scale waves. During the performance period of this grant, we have concentrated on reanalyzing the 15 experiments conducted in 1994 using improved trajectory files for Magellan provided by JPL, obtaining more reliable results, and on conducting an error analysis of the derived profiles. In addition, we have begun an analysis of microwave emission maps of Venus obtained at the Very Large Array (VLA) in April, 1996. This report is organized as follows: A summary of results is presented, giving representative examples of the various physical profiles retrieved from all the Magellan radio occultation studies, emphasizing latitudinal variations evident in the data. Next, a preliminary analysis of the VLA maps is given. A summary of activities follows the scientific results, detailing papers published and presented at various conferences. This report concludes with a "Conclusions and Suggestions for Future Work" section.

Jenkins, Jon M.↗
DOE OSTI2026

Dynamic evolution of ultracold electron beams from nanostructured cathodes

Dynamic Coulomb expansion of dense, nonuniform particle beamlets can lead to causticlike formations in the charge density profile of the resultant beam. Such features only occur for cold beams where thermal diffusion of the particles is minimized. These phenomena have been observed experimentally for select cases with ion beams but not yet for electron beams as the electron dynamics occur over far faster timescales. We simulate the interaction of electron bunches while varying the initial transverse temperature and density profiles to determine the thresholds that characterize this pattern formation. Furthermore, we extend the simulations to geometric arrangements that are more pertinent to the nanotip cathode application and consider the effects of asymmetries and the impact of a low-density halo on the overall process. The simulations are conducted using a previously documented high-fidelity algorithm for collisional particle dynamics. These studies identify the temperature and density thresholds required for caustic formation and provide guidance for optimizing the design of nanoscale structured cathodes. We also demonstrate that the correlation between the final density patterns and the underlying beam parameters enables a virtual diagnostic tool for measuring ultralow emittance beams solely based on a transverse charge density profile.

43 PARTICLE ACCELERATORS↗
DOE OSTI2024

Plasma potential shaping using end-electrodes in the Large Plasma Device

We perform experiments in the Large Plasma Device at the University of California, Los Angeles, studying how different end-electrode biasing schemes modify the radial potential profile in the machine. We impose biasing profiles of different polarities and gradient signs on a set of five concentric electrodes placed 12 m downstream from the plasma source. We find that imposing concave-down profiles (negative potential radial gradient) on the electrodes creates radial potential profiles halfway up the plasma column that are comparable to those imposed on the electrodes and a few electron temperature in height, regardless of the biasing polarity. On the other hand, imposing concave-up profiles (positive potential radial gradient) leads to non-monotonic radial potential profiles. This observation can be explained by the current drawn through the electrodes and the parallel plasma resistivity, highlighting their important role in controlling the rotation of plasma. Concave-down plasma potential profiles, obtained by drawing electrons on the axis, are predicted to drive azimuthal drift velocities that can approach significant fractions of the ion sound speed in the central region of the plasma column.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗