Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “electron localization”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Inert gas thrusters

Inert gases, particularly argon and xenon, are of interest as possible alternatives to the usual electric thruster propellants of mercury and cesium. Hollow cathode data were obtained for a wide range of operating conditions. Some test conditions gave plasma coupling voltages at or below the sputtering threshold, hence should permit long operating lifetimes. All observations of hollow cathode operation were consistent with a single theory of operation, in which a significant amount of the total electron emission is from localized areas within the orifice. This mode of emission is also supported by scanning electron microscope photographs that indicate local temperatures at or near the melting temperature of the tungsten tip. Experimental hollow cathode performance was correlated for two orifice diameters, three inert gas propellants, and a range of flow rates for each propellant. The basic theory for the production of doubly ionized argon and xenon was completed. Experimental measurements of the doubly ionized fraction agree with theory within about plus or minus 20 percent. High voltage isolators were studied for the propellant feed line. The breakdown voltage per segment ranged from 300 to over 500 V with argon.

Kaufman, H. R.↗
NASA NTRS1987

Implementation of variable time step stochastic dynamics for electronically inelastic gas-surface collisions

A variable time step algorithm has been implemented for solving the stochastic equations of motion for gas-surface collisions. It has been tested for a simple model of electronically inelastic collisions with an insulator surface in which the phonon manifold acts as a heat bath and electronic states are localized. In addition to reproducing the accurate nuclear dynamics of the surface atoms, numerical calculations have shown the algorithm to yield accurate ensemble averages of physical observables such as electronic transition probabilities and total energy loss of the gas atom to the surface. This new algorithm offers a gain in efficieny of up to an order of magnitude compared to fixed time step integration.

Garrett, Bruce C.↗
NASA NTRS1980

Spontaneous emission of radiation from localized Langmuir perturbation

The radiation at the first and second harmonics of the electron plasma frequency from localized Langmuir oscillations is computed for a field-free plasma. The localized perturbations are assumed to have cylindrical symmetry about the direction of propagation of the localized perturbation. It is shown that when the wavelengths of the excited modes are much greater than the scale lengths of the Langmuir perturbation, a quadrupole radiation pattern is recovered for emission at twice the electron plasma frequency. An application to type III solar radio bursts is discussed briefly.

Freund, H. P.↗
NASA NTRS1990

Localization effects in radiationally disordered high-temperature superconductors: Theoretical interpretation

Theoretical interpretation of recent experiments on radiationally disordered high-temperature superconductors is presented, based on the concepts of mutual interplay of Anderson localization and superconductivity. Microscopic derivation of Ginzburg-Landau coefficients for the quasi-two-dimensional system in the vicinity of localization transition is given in the framework of the self-consistent theory of localization. The 'minimal metallic conductivity' for the quasi-two-dimensional case is enhanced due to a small overlap of electronic states on the nearest neighbor conducting planes. This leads to a stronger influence of localization effects than in ordinary (three-dimensional) superconductors. From this point of view even the initial samples of high-temperature superconductors are already very close to Anderson transition. Anomalies of H(c2) are also analyzed, explaining the upward curvature of H(c2)(T) and apparent independence of dH(c2)/dT (T = T(sub c)) on the degree of disorder as due to localization effects. Researchers discuss the possible reasons of fast T(sub c) degradation due to the enhanced Coulomb effects caused by the disorder induced decrease of localization length. The appearance and growth of localized magnetic moments is also discussed. The disorder dependence of localization length calculated from the experimental data on conductivity correlates reasonably with the theoretical criterion for suppression of superconductivity in the system with localized electronic states.

Goshchitskii, B. N.↗
NASA NTRS2005

Autonomous Assembly of Modular Structures in Space and on Extraterrestrial Locations

The fulfillment of the new US. National Vision for Space Exploration requires many new enabling technologies to accomplish the goal of utilizing space for commercial activities and for returning humans to the moon and extraterrestrial environments. Traditionally, flight structures are manufactured as complete systems and require humans to complete the integration and assembly in orbit. These structures are bulky and require the use of heavy launch vehicles to send the units to the desired location, e.g. International Space Station (ISS). This method requires a high degree of safety, numerous space walks and significant cost for the humans to perform the assembly in orbit. For example, for assembly and maintenance of the ISS, 52 Extravehicular Activities (EVA's) have been performed so far with a total EVA time of approximately 322 hours. Sixteen (16) shuttle flights haw been to the ISS to perform these activities with an approximate cost of $450M per mission. For future space missions, costs have to be reduced to reasonably achieve the exploration goals. One concept that has been proposed is the autonomous assembly of space structures. This concept is an affordable, reliable solution to in-space and extraterrestrial assembly operations. Assembly is autonomously performed when two components containing onboard electronics join after recognizing that the joint is appropriate and in the precise position and orientation required for assembly. The mechanism only activates when the specifications are correct and m a nominal range. After assembly, local sensors and electronics monitor the integrity of the joint for feedback to a master controller. To achieve this concept will require a shift in the methods for designing space structures. In addition, innovative techniques will be required to perform the assembly autonomously. Monitoring of the assembled joint will be necessary for safety and structural integrity. If a very large structure is to be assembled in orbit, then the number of integrity sensors will be significant. Thus simple, low cost sensors are integral to the success of this concept. This paper will address these issues and will propose a novel concept for assembling space structures autonomously. The paper will present Several autonomous assembly methods. Core technologies required to achieve in space assembly will be discussed and novel techniques for communicating, sensing, docking and assembly will be detailed. These core technologies are critical to the goal of utilizing space in a cost efficient and safe manner. Finally, these technologies can also be applied to other systems both on earth and extraterrestrial environments.

Alhorn, Dean C.↗
NASA NTRS1975

Local time variations of particle flux produced by an electrostatic field in the magnetosphere

The local time variations of particle distributions, which can be produced by the sudden enhancement of a cross-tail electric field, are examined by using adiabatic theory. The strong coupling between acceleration, inward motion, and east-west drift is emphasized. The model is used to interpret recent measurements of delay times of newly accelerated electrons at different local times at geostationary orbit. Observations of the asymmetric storm time 'ring current' are interpreted in terms of the model without invoking particle loss.

Kivelson, M. G.↗
NASA NTRS2013

Integrated High-Speed Torque Control System for a Robotic Joint

A control system for achieving high-speed torque for a joint of a robot includes a printed circuit board assembly (PCBA) having a collocated joint processor and high-speed communication bus. The PCBA may also include a power inverter module (PIM) and local sensor conditioning electronics (SCE) for processing sensor data from one or more motor position sensors. Torque control of a motor of the joint is provided via the PCBA as a high-speed torque loop. Each joint processor may be embedded within or collocated with the robotic joint being controlled. Collocation of the joint processor, PIM, and high-speed bus may increase noise immunity of the control system, and the localized processing of sensor data from the joint motor at the joint level may minimize bus cabling to and from each control node. The joint processor may include a field programmable gate array (FPGA).

Davis, Donald R.↗
NASA NTRS1972

Atlas of electron content values observed at Urbana, Illinois, 1 December 1967 - 30 December 1970

Ionospheric electron content versus local time data deduced from Faraday rotation observations of ATS-III geostationary satellite signals at Urbana, Illinois are reported. The data are presented in two forms. Values of subionospheric latitude (SILAT) and subionospheric longitude (SILON) are in degrees north and degrees west, respectively. These are computed on the basis of 350 km for the mean ionospheric height, which value is also used for the calculation of the geometric-magnetic factor, required for the conversion of the measured Faraday rotation angle to electron content. Entries of zero for the electron content in the tables represent no data for those times.

Flaherty, B. J.↗
NASA NTRS1973

Electron avalanche structure determined by random walk theory

A self-consistent avalanche solution which accounts for collective long range Coulomb interactions as well as short range elastic and inelastic collisions between electrons and background atoms is made possible by a random walk technique. Results show that the electric field patterns in the early formation stages of avalanches in helium are close to those obtained from theory based on constant transport coefficients. Regions of maximum and minimum induced electrostatic potential phi are located on the axis of symmetry and within the volume covered by the electron swarm. As formation time continues, however, the region of minimum phi moves to slightly higher radii and the electric field between the extrema becomes somewhat erratic. In the intermediate formation periods the avalanche growth is slightly retarded by the high concentration of ions in the tail which oppose the external electric field. Eventually the formation of ions and electrons in the localized regions of high field strength more than offset this effect causing a very abrupt increase in avalanche growth.

Englert, G. W.↗
NASA NTRS1976

The MPD arc as a plasmadynamic laser source

The paper reports results of measurements of local plasma properties (electron density and temperature, radiance, and population density) in the recombining plasma flow of a high power (100 kW) quasi-steady MPD arc designed as a plasmadynamic laser source. The profiles of electron density and temperature and the populations of excited states of Ar II show large deviations from LTE and indicate the existence of population inversions between the 4d and 4p states.

Campbell, E. M.↗
NASA NTRS1979

Isis 1 observations at the source of auroral kilometric radiation

Observations of auroral kilometric radiation (AKR) were made by Isis 1 in the source region. The radiation is found to be generated in the extraordinary mode just above the local cut-off frequency and to emanate nearly perpendicular to the magnetic field. It occurs within local depletions of electron density, where the ratio of plasma frequency to cyclotron frequency is less than 0.2. The density depletion is restricted to altitudes above about 2000 km, and the upper AKR frequency limit corresponds to the extraordinary cut-off frequency at this altitude. AKR is observed from Isis 1 above the nighttime auroral zone over a wider extent in longitude than in latitude with an intense source region observed most often near 2200 LMT and 70 deg invariant latitude. It is directly related to inverted V electron precipitation events with an electron-to-wave energy conversion efficiency of the order of 0.1 to 1%.

Benson, R. F.↗
NASA NTRS1981

Auroral kilometric radiation source region observations from ISIS 1

The ISIS 1 observations of the high-frequency portion of the auroral kilometric radiation (AKR) spectrum are considered, that is, from the minimum frequency encountered for the extraordinary mode cut-off (approximately 450 kHz) to the upper frequency cut-off (approximately 800 kHz). AKR is found to be generated in the extraordinary mode just above the local cutoff frequency and to emanate in a direction that is nearly perpendicular to the magnetic field. It occurs within local depletions of electron density, where the ratio of plasma frequency to cyclotron frequency is below 0.2. The density depletion is restricted to altitudes above approximately 2,000 km, and the upper AKR frequency limit corresponds to the extraordinary cutoff frequency at this altitude.

Benson, R. F.↗
NASA NTRS1984

Detection of a radio emission at 3 kHz in the outer heliosphere

A radio source in the outer heliosphere has been detected by the plasma wave receivers on Voyagers 1 and 2. The radio emission is observed in the frequency range 2-3 kHz, and is above the local solar wind electron plasma frequency whenever supporting plasma density data are available. The maximum spectral density of the emission recorded is about 10 to the -14th V-squared/m-squared/Hz. The bandwidth of the radio noise is about 1 kHz. Possible sources include continuum radiation from Jupiter's distant magnetotail and radiation at the second harmonic of the plasma frequency at the heliopause. If the latter interpretation is correct, these data represent the first remote observations of the heliopause.

Kurth, W. S.↗
NASA NTRS1986

Dynamics of a discrete auroral arc

Porcupine Flight 4 data were used to determine the field-aligned currents associated with a southward moving discrete auroral arc in the postmidnight sector. Three different methods were used for determining the field-aligned current which should give identical results if the arcs are quasi-stationary and no parallel electric field exists between the payload and the dynamo region of the ionosphere. As long as the rocket is above the arc, the three methods agree. The integral of precipitating electron flux, the local magnetic field perturbations, and the divergence of the horizontal Pedersen current all indicate an upward current of 5 + or - 3 microamperes/sq m. Immediately north of the arc a strong downward current of about 10-20 microamperes/sq m is detected. The magnitude, however, is not well known because the rocket's velocity relative to the arc cannot be clearly established. Further north of the southward moving arc, the two methods that can be applied (magnetic field perturbations and divergence of the horizontal Pedersen current) yield contradictory results not only about the magnitude of the current but also about the direction of the current. It is suggested that this discrepancy is due to time-dependent electric field.

Bruening, K.↗
NASA NTRS1987

Ground-state properties of the periodic Anderson model

The ground-state energy, hybridization matrix element, local moment, and spin-density correlations of a one-dimensional, finite-chain, periodic, symmetric Anderson model are obtained by numerical simulations and compared with perturbation theory and strong-coupling results. It is found that the local f-electron spins are compensated by correlation with other f-electrons as well as band electrons leading to a nonmagnetic ground state.

Blankenbecler, R.↗
NASA NTRS1989

A multipoint case study of high-latitude daytime ULF pulsations

A multipoint case study approach is used to delineate significant morphological differences between, and distinct sources for, two kinds of high-latitude dayside ULF wave activity in the Pc 3 to Pi 1 frequency range. Evidence is presented that narrow-band Pc 3-4 pulsations appear roughly simultaneously at widely spaced locations in the dayside magnetosphere and polar cap and that their occurrence/intensity is controlled by the cone angle of the IMF, while broadband Pi 1 type activity appears to be related closely to precipitation of energetic plasma sheet electrons, hence more localized ionospheric activity. Additional evidence is provided that the cusp/cleft regions play a significant part in the transmission to low altitudes of at least some of the wave energy of solar wind-controlled pulsations.

Engebretson, M. J.↗
NASA NTRS1996

Plasma and energetic particle properties of dispersionless substorm injections at geosynchronous orbit

Using a year's worth of geosynchronous spacecraft data, the substorm associated behavior of the thermal plasma in the plasma sheet in relation to energetic particle injections is reviewed. Five classes of injection events were found: ion injections without accompanying electron injection at 21 local time (LT); ion injection followed a few minutes later by an electron injection at 22 LT; simultaneous ion and electron injection close to midnight; electron injections followed by an ion injection at 01 LT; and pure electron injections at 02 LT. The thermal electrons show a significant increase in temperature and pressure at substorm onset, while the density and thermal ion signatures are typically weak. Energetic ions are found to contribute to the total ion pressure and temperature. Preexisting perpendicular anisotropies in the thermal electrons and ions are reduced during the substorm growth phase, but are re-enhanced after onset.

Birn, J.↗