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At least 325 records · Page 18

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NASA NTRS1974

High-temperature mass spectrometry - Vaporization of group 4-B metal carbides

The high temperature vaporization of the metal-carbon systems TiC, ZrC, HfC, and ThC was studied by the Knudsen effusion - mass spectrometric method. For each system the metal dicarbide and tetracarbide molecular species were identified in the gas phase. Relative ion currents of the carbides and metals were measured as a function of temperature. Second- and third-law methods were used to determine enthalpies. Maximum values were established for the dissociation energies of the metal monocarbide molecules TiC, ZrC, HfC, and ThC. Thermodynamic functions used in the calculations are discussed in terms of assumed molecular structures and electronic contributions to the partition functions. The trends shown by the dissociation energies of the carbides of Group 4B are compared with those of neighboring groups and discussed in relation to the corresponding oxides and chemical bonding. The high temperature molecular beam inlet system and double focusing mass spectrometer are described.

Stearns, C. A.↗

NASA NTRS1974

Laboratory simulation of the structure of disturbed zones around bodies in space

Description of laboratory studies of certain aspects of the spacecraft/space plasma interaction regarding, in particular, the near and intermediate wake regions. The plasma wind tunnel facilities and the diagnostic techniques employed in these tests are described. Results are obtained which concern the variation of the normalized ion flux with normalized distance along the wake axis downstream from a conducting spherical body, the variation of the normalized width of the disturbed region with normalized distance downstream from the body center, and the location of the point of maximum ion current enhancement as a function of body potential.

Stone, N. H.↗

NASA NTRS1974

Parametric study of near-wake structure of spherical and cylindrical bodies in the laboratory

Some aspects of the interaction between metal bodies and streaming rarefied plasmas were studied in a newly constructed plasma wind tunnel as part of an attempt to investigate (via simulation) phenomena relevant to the spacecraft/space plasma interaction. Detailed near-wake ion current profiles for both spherical and cylindrical bodies at different body potentials and at different plasma flow parameters are presented. Various features of the profiles can be correlated, at least qualitatively, with both plasma and body characteristics. The general importance of body geometry is qualitatively demonstrated. In addition, a discussion of the relevance of the above studies to previous in situ data obtained from the Ariel I and Gemini/Agena missions is given.

Oran, W. A.↗

NASA NTRS1974

OGO-5 observations of the magnetopause

OGO-5 observations show not only that the average position of the magnetopause boundary moves in response to changes in dynamic pressure, but also that it moves in response to changes in the north-south component of the interplanetary field. Further, the boundary often oscillates about its average position as waves propagate along the boundary away from the nose region. The variation of the magnetic field through the boundary at times can be a simple rotation with no change in magnitude, and at other times can resemble a simple tangential discontinuity. However, it often displays complex patterns such as field enhancements on the magnetospheric side of the boundary or apparently uncorrelated field strength and direction changes. One particularly simple boundary crossing has been studied in detail with both positive ion data and magnetic field data. In this case, the electron and ion currents could be separately deduced and the structure agreed with that expected for a Chapman-Ferraro boundary with almost complete neutralization.

Russell, C. T.↗

NASA NTRS1977

Geometry and properties of the plasma in a quadrupole ion pump

The diameter of the plasma in a quadrupole ionization gauge and quadrupole ion pump has been measured as a function of pressure. The radio-frequency quadrupole systems are tuned in such a way that electrons remain on stable trajectories in the center of the tube. Being emitted at one end, the electrons encounter an inverted field at the other end so that they travel on spiral paths back and forth until they collide with the rest gas molecules, producing ions which are collected as an ion current at a cylinder surrounding the four hyperbolically shaped quadrupole electrodes.

Schwarz, H.↗

NASA NTRS1979

Mapping of electrical potential distribution with charged particle beams

Potentials were measured using a beam of soft X-rays in air at 2 x 10 to the -5 power Torr. Ions were detected by a continuous-dynode electron multiplier after they passed through a retarding field. Ultimate resolution depends upon the diameter of the X-ray beam which was 3 mm. When the fields in the region of interest were such to disperse the ions, only a small fraction were detected and the method of measurement was not very reliable. Yet reasonable data could be collected if the ions traveled in parallel paths toward the detector. Development should concentrate on increasing the aperture of the detector from the pinhole which was used to something measured in centimeters. Also increasing the strength of the source would provide a stronger signal and more reliable data. Measurements were made at an estimated ion current to 10 to the -15 power A from a 10 cm length of the X-ray beam, this current being several orders of magnitude below what would have a perturbing effect on the region to be measured. Consequently, the source strength can be increased and prospects for this method of measurement are good.

Robinson, J. W.↗

NASA NTRS1981

Bodies in flowing plasmas - Laboratory studies

A brief review of early rudimentary laboratory studies of bodies in flowing, rarefied plasmas is presented (e.g., Birkeland, 1908), along with a discussion of more recent parametric studies conducted in steady plasma wind tunnels, which includes the study by Hall et al. (1964), in which a strong ion density enhancement in the center of the ion void created downstream from the body was observed. Good agreement was found between the experimental results and theoretical calculations which omit ion thermal motion. Examples in which in situ data on the interaction between satellites and the ionospheric plasma have been elucidated by the laboratory results are presented, and include evidence for a midwake axial ion peak, and ion current density in the near-wake region. The application of the ionospheric laboratory to basic space plasma physics is discussed, and its application to some types of solar system plasma phenomena is illustrated.

Stone, N. H.↗

NASA NTRS1981

Electron reflection and secondary emission characteristics of sputter-textured pyrolytic graphite surfaces

Measurements are presented of secondary electron emission and reflected primary electron characteristics of sputter-textured pyrolitic graphite surfaces with microstructures of various sizes and densities, made with an Auger cylindrical mirror analyzer in a high-vacuum chamber at pressures below 1.33 x 10 to the -7th N/sq m (10 to the -9th torr). A dense, tall, thin, spire-like microstructure, obtained at ion energies of 1000 eV and ion current densities of 5 mA/sq cm, is the most effective. The secondary electron emission from such a surface is lower than that of soot, whose secondary emission is among the lowest of any material. At a primary electron energy of 1000 eV, the secondary electron emission yield of smooth CU is about 350% greater than the lowest value obtained for sputter-textured pyrolitic graphite. The reflected primary electron index of smooth Cu is a factor of 80 greater. If the secondary electron emission yield is reduced to 0.3, which is possible with sputter-textured pyrolitic graphite, the traveling wave tube collector efficiency could be improved by as much as 4% over that for smooth copper.

Wintucky, E. G.↗

NASA NTRS1981

Modification of spacecraft potentials by plasma emission

The ion engine operations on Applied Technology Satellite 6 (ATS-6) altered the charge state of the spacecraft, changing the spacecraft surface potentials with respect to the distant plasmas. Plasma emission in quiet environments (plasma temperatures below 1 keV) caused the spacecraft potential to shift from a few volts positive to a few volts negative. A net ion current is emitted in such cases. The emission of a plasma or beam in energetic environments (plasma temperatures in the 5-10-keV range) in sunlight caused larger changes. Typical equilibrium potentials for ATS-6 in such environments were on the order of a hundred volts negative, with variations in potential across the spacecraft surface of comparable magnitude. Engine operations under such conditions raised the mainframe potential to near zero volts, and discharged the differential potentials on the dielectric surfaces. Plasma emission by the plasma bridge was an effective method of discharging kilovolt potentials in eclipse.

Olsen, R. C.↗

NASA NTRS1981

About the parametric interplay between ionic Mach number, body-size, and satellite potential in determining the ion depletion in the wake of the S3-2 satellite

The variation of ion current depletion in the wake of the U.S. Air Force satellite S3-2 is quantitatively determined, taking into account altitudes in the range from 300 to 1100 km. The considered investigation has the objective to present results which besides of being of scientific interest per se are useful to the planning of future experiments of body-plasma electrodynamic interactions in a supersonic and sub-Alfvenic flow regime to be conducted on board the Shuttle/Spacelab. More specifically, it is expected that the outcome of investigations of the kind presented will be useful in the planning of instrument location on ejectable ensembles of probes and on the Orbiter itself in future Shuttle/Spacelab missions.

Samir, U.↗

NASA NTRS1982

Dynamical features of moving double layers

Numerical simulations of the dynamics of double layers that form in response to an applied potential drop across a plasma are carried out for plasmas of lengths much greater than previously considered. The evolution of finite-size plasmas of lengths 100, 200 and 400 Debye lengths at the low-potential end of the simulation region was followed by the solution of the Vlasov and Poisson equations. Results show that the double layer moves towards the high-potential side of the layer, at a speed directly dependent on the length of the plasma. The moving double layer is accompanied by a moving density front which is similar to an ion acoustic shock created by the expansion of a high-density plasma into a low-density plasma, as well as plasma heating and evacuation, electron and ion current interruptions and recovery. Double layer formation, motion, and accompanying phenomena are found to repeat periodically. Results are in good agreement with laboratory experiments, and may be used to explain certain phenomena observed in auroral plasmas.

Singh, N.↗

NASA NTRS1984

Ion extraction capabilities of two-grid accelerator systems

An experimental investigation into the ion extraction capabilities of two-grid accelerator systems common to electrostatic ion thrusters is described. This work resulted in a large body of experimental data which facilitates the selection of the accelerator system geometries and operating parameters necessary to maximize the extracted ion current. Results suggest that the impingement-limited perveance is not dramatically affected by reductions in screen hole diameter to 0.5 mm. Impingement-limited performance is shown to depend most strongly on grid separation distance, accelerator hole diameter ratio, the discharge-to-total accelerating voltage ratio, and the net-to-total accelerating voltage ratio. Results obtained at small grid separation ratios suggest a new grid operating condition where high beam current per hole levels are achieved at a specified net accelerating voltage. It is shown that this operating condition is realized at an optimum ratio of net-to-total accelerating voltage ratio which is typically quite high. The apparatus developed for this study is also shown to be well suited measuring the electron backstreaming and electrical breakdown characteristics of two-grid accelerator systems.

Rovang, D. C.↗

NASA NTRS1985

Polar orbit electrostatic charging of objects in shuttle wake

A survey of DMSP data has uncovered several cases where precipitating auroral electron fluxes are both sufficiently intense and energetic to charge spacecraft materials such as teflon to very large potentials in the absence of ambient ion currents. Analytical bounds are provided which show that these measured environments can cause surface potentials in excess of several hundred volts to develop on objects in the orbiter wake for particular vehicle orientations.

Katz, I.↗

NASA NTRS1984

Ion extraction capabilities of two-grid accelerator systems

An experimental investigation into the ion extraction capabilities of two-grid accelerator systems common to electrostatic ion thrusters is described. A large body of experimental data which facilitates the selection of the accelerator system geometries and operating parameters necessary to maximize the extracted ion current is presented. Results suggest that the impingement-limited perveance is not dramatically affected by reductions in screen hole diameter to 0.5 mm. Impingement-limited performance is shown to depend most strongly on grid separation distance, accelerator hole diameter ratio, the discharge-to-total accelerating voltage ratio, and the net-to-total accelerating voltage ratio. Results obtained at small grid separation ratios suggest a new grid operating condition where high beam current per hole levels are achieved at a specified net accelerating voltage. It is shown that this operating condition is realized at an optimum ratio of net-to-total accelerating voltage ratio which is typically quite high.

Rovang, D. C.↗

NASA NTRS1984

Iterative and function-continuation Fourier deconvolution methods for enhancing mass spectrometer resolution

Mass spectrometer data in the form of ion current versus mass-to-charge ratio often include overlapping mass peaks, especially in low- and medium-resolution instruments. Numerical deconvolution of such data effectively enhances the resolution by decreasing the overlap of mass peaks. In this paper two approaches to deconvolution are presented: a function-domain iterative technique and a Fourier transform method which uses transform-domain function-continuation. Both techniques include data smoothing to reduce the sensitivity of the deconvolution to noise. The efficacy of these methods is demonstrated through application to representative mass spectrometer data and the deconvolved results are discussed and compared to data obtained from a spectrometer with sufficient resolution to achieve separation of the mass peaks studied. A case for which the deconvolution is seriously affected by Gibbs oscillations is analyzed.

Ioup, J. W.↗

NASA NTRS1987

A high flux source of swift oxygen atoms

A source of swift oxygen atoms is described which has several unique features. A high current ion beam is produced by a microwave discharge, accelerated to 10 keV and the mass selected by a modified Du Pont 21-110 mass spectrometer. The O(+) beam exciting the mass spectrometer is focused into a rectangular shape with an energy spread of less than 1 eV. The next section of the machine decelerates the ion beam into a counterpropagating electron beam in order to minimize space charge effects. After deceleration, the ion beam intersects at 90 deg, a neutral oxygen atom beam, which via resonant charge exchange produces a mixture of O(+) and O. Any remaining O(+) are swept out of the beam by an electric field and differentially pumped away while the desired O beam, collimated by slits, impinges on the target. In situ monitoring of the target surface is done by X-ray photoelectron or Auger spectroscopy. Faraday cups provide flux measurements in the ion sections while the neutral flux is determined by a special torsion balance or by a quadrupole mass spectrometer specially adapted for swift atoms. While the vacuum from the source through the mass spectrometer is maintained by diffusion pumps, the rest of the machine is UHV.

Fink, M.↗

NASA NTRS1986

Eigenfunction methods in magnetospheric radial-diffusion theory

Complete sets of orthonormal basis functions constructed according to a generalization of the quantum-mechanical WKB approximation can be used to generate a nearly-diagonal matrix representation of the radial-transport operator for ring-current ions in the presence of radial diffusion and charge exchange. The resulting eigenfunctions (constructed by weighting the basis functions in proportion to the respective components of the eigenvectors of the matrix representation) and eigenvalues provide a spatial and temporal description of the evolving phase-space density during and following a magnetospheric disturbance (e.g., a magnetic storm). A linear superposition of the basis functions can also be used to eliminate any discrepancy between the steady-state solution of the transport equation and the appropriate WKB approximation of this steady-state solution.

Schulz, Michael↗

NASA NTRS1987

Energetic neutral atom image of a storm-time ring current

Energetic neutral atom (ENA) images of a storm-time ring current produced from direct measurements and model simulations are presented and analyzed. An ENA image of the storm-time ring current was obtained with the Medium Energy Particles Instrument (MEPI) mounted on ISEE 1 on September 29, 1978, and a zero-order model of the ring current ion was employed to simulate the ENA image. The simulated instrument response is compared with actual ENA measurements. It is observed that the spatial distribution of singly charged ions detected from the ENA images displays equatorial midnight/noon asymmetry in differential ion intensity with a midnight/noon ratio of greater than or equal to 20:1; peak ion fluxs, H(+) and O(+), in the midnight sector are greater than 10 to the 6th/(sq cm s sr keV); and ENA fluxes from the brighter areas of the image exceed greater than 1000/(sq cm s sr keV). It is noted that the ENA image from MEPI data correlates with the morphology of the simulated response of MEPI calculated from the model.

Roelof, Edmond C.↗