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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
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 NTRS1974

Vibrationally excited nitrogen in stable auroral red arcs and its effect on ionospheric recombination

The time-dependent continuity equations, including diffusion, were solved for the first six energy levels of molecular nitrogen for conditions in the thermosphere corresponding to stable auroral red (SAR) arcs. The results show that molecular nitrogen is excited vibrationally to the degree that the rate constant for the ionospheric loss process, O(+) + N2 yields NO(+) + N, is increased by as much as a factor of 7.6 at F2 region altitudes. It was found that deviations from the energetically equivalent Boltzmann distribution were large, causing the rate constant to be as much as 1.6 times the rate constant calculated for the Boltzmann distribution. These results indicate that SAR arc intensities as small as 58 R can produce noticeable increases in the ionosphere ion-atom interchange reaction rate and hence in the rate of loss of ionospheric electrons. It is suggested that the observed decrease of electron density in the F2 region in SAR arcs can probably be explained by enhanced reaction rates for ion-atom interchange between O(+) and N2 caused by vibrational excitation of molecular nitrogen by electron impact.

Newton, G. P.↗
NASA NTRS1981

Cold plasma boundaries and auroral arcs

Auroral arcs often extend for more than a thousand kilometers with little deviation of their relative position within the auroral oval. At high altitudes, the outer limits of the plasmasphere are usually marked by sharp decreases in the cold plasma densities. It is suggested that some auroral arcs follow the ionospheric trace of these boundary shells.

Mcilwain, C. E.↗
NASA NTRS1976

The quantitative properties of three soft X-ray flare kernels observed with the AS&E X-ray telescope on Skylab

The physical parameters for the kernels of three solar X-ray flare events have been deduced using photographic data from the S-054 X-ray telescope on Skylab as the primary data source and 1-8 and 8-20 A fluxes from Solrad 9 as the secondary data source. The kernels had diameters of about 5-7 seconds of arc and in two cases electron densities at least as high as 0.3 trillion per cu cm. The lifetimes of the kernels were 5-10 min. The presence of thermal conduction during the decay phases is used to argue: (1) that kernels are entire, not small portions of, coronal loop structures, and (2) that flare heating must continue during the decay phase. We suggest a simple geometric model to explain the role of kernels in flares in which kernels are identified with emerging flux regions.

Kahler, S. W.↗
NASA NTRS1978

Some properties of low-vapor-pressure braze alloys for thermionic converters

Property measurements were made for arc-melted, rod-shaped specimens. Density and dc electrical resistivity at 296 K were measured for various binary eutectic alloys. Thermal conductivity was inferred from the electrical conductivity using the Wiedemann, Franz, Lorenz relation. Linear thermal expansion from 293 K to two-thirds melting point, under a helium atmosphere, was measured for Zr, 21.7-wt percent Ru; Zr, 13-wt percent W; Zr, 22.3-wt percent Nb; Nb, 66.9-wt percent Ru; and Zr, 25.7-wt percent Ta.

Bair, V. L.↗
NASA NTRS1994

Quiet-time intensifications along the poleward auroral boundary near midnight

Radar and optical measurements from Sondrestrom are combined with satellite and Goose Bay data in a study of the poleward edge of the nightside auroral oval during a quiet period. The B(sub y) and B(sub z) components of the interplanetary magnetic field were close to zero, and the B(sub x) component was approximately 8 nT for more than 24 hours. On a large scale, the convection and precipitation patterns remained almost constant during this period; on a small scale, however, the conditions were quite dynamic. At 10- to 20-min intervals the arc that marked the poleward auroral boundary intensified, and a new arc appeared poleward of it. About once per hour, stronger intensifications were observed. One such event is examined in detail. The auroral arcs first appeared to dim, and then they brightened, with a factor of 10 increase in E region electron density. At the time of the brightening a new arc formed poleward of all the arcs. The arcs then drifted southward at velocities of approximately 270 m/s. A plasma drift disturbance, characterized by a doubling of the southward velocity and a reversal in the east-west component, propagated westward at 900 m/s through the fields of view of the Sondrestrom and Goose Bay radars. A simultaneous satellite overpass close to the radars revealed the presence of an energetic ion event similar to the 'velocity dispersed ion structures' observed on the Aureol satellite and presumed to be the signature of fast ion beams within the plasma sheet boundary layer. The stronger arc intensification events observed by the Sondrestrom radar are associated with an increase in plasma flow across the boundary between open and closed magnetic field lines. We interpret this increased flow as the ionospheric signature of abrupt, localized increases in the reconnection rate in the midnight sector.

De La Beaujardiere, O.↗
NASA NTRS2003

RF Frequency Oscillations in the Early Stages of Vacuum Arc Collapse

RF frequency oscillations may be produced in a typical capacitive charging / discharging pulsed power system. These oscillations may be benign, parasitic, destructive or crucial to energy deposition. In some applications, proper damping of oscillations may be critical to proper plasma formation. Because the energy deposited into the plasma is a function of plasma and circuit conditions, the entire plasma / circuit system needs to be considered as a unit To accomplish this, the initiation of plasma is modeled as a time-varying, non-linear element in a circuit analysis model. The predicted spectra are compared to empirical power density spectra including those obtained from vacuum arcs.

Griffin, Steven T.↗
NASA NTRS2018

The Interplay Between Radiation Pressure and the Photoelectric Instability in Optically Thin Disks of Gas and Dust

In optically thin disks, dust grains are photoelectrically stripped of electrons by starlight, heating nearby gas and possibly creating a dust clumping instability-the photoelectric instability (PeI)-that significantly alters global disk structure. In the current work, we use the Pencil Code to perform the first numerical models of the PeI that include stellar radiation pressure on dust grains in order to explore the parameter regime in which the instability operates. In some models with low gas and dust surface densities, we see a variety of dust structures, including sharp concentric rings. In the most gas- and dust-rich models, nonaxisymmetric clumps, arcs, and spiral arms emerge that represent dust surface density enhancements of factors of ∼5-20. In one high gas surface density model, we include a large, low-order gas viscosity and find that it observably smooths the structures that form in the gas and dust, suggesting that resolved images of a given disk may be useful for deriving constraints on the effective viscosity of its gas. Our models show that radiation pressure does not preclude the formation of complex structure from the PeI, but the qualitative manifestation of the PeI depends strongly on the parameters of the system. The PeI may provide an explanation for unusual disk morphologies, such as the moving blobs of the AU Mic disk, the asymmetric dust distribution of the 49Ceti disk, and the rings and arcs found in the HD 141569A disk.

photoelectric instability (PeI)↗
NASA NTRS1976

On the emission mechanism in high current hollow cathode arcs

Large hollow cathodes have been operated in a magnetoplasmadynamic arc over wide ranges of current and mass flow, with orifice current densities and mass fluxes encompassing those encountered in low current steady state hollow cathode arcs. Detailed cathode interior measurements of current and potential distributions show that maximum current penetration into the cathode is about one diameter axially upstream from the tip, with peak inner surface current attachment up to one cathode diameter upstream of the tip. The spontaneous attachment of peak current upstream of the cathode tip is suggested as a criterion for characteristic hollow cathode operation. This empirical criterion was verified by experiment.

Krishnan, M.↗
NASA NTRS1989

Arc restrike in the rail accelerator

One of the causes of the degradation in rail accelerator performance is the formation of a secondary arc. Experimental evidence of arc restrike and the subsequent growth of this secondary arc is presented. A simple analytical treatment of arc restrike is developed in terms of breakdown of residual vapor atoms. It is found that after the passage of the primary arc, the bore volume contains a large number of residual neutral vapor atoms. If the density of these atoms is in excess of the critical density, then for a certain length of time the condition exists in the bore for the formation of a secondary arc. Evaporation of atoms from the bore surfaces cannot provide a sufficient number of atoms for an arc restrike. A likely source of the high residual atom density is the leakage of a portion of the ablated material that is added to the trailing edge of the primary arc.

Ray, Pradosh K.↗
NASA NTRS1972

Coordinated observations of the magnetosphere - The development of a substorm.

Coordinated observations of a substorm are reported by using data from all-sky camera (ASCA) stations near the northern conjugate of the ATS 5 geostationary satellite, plasma and magnetic-field experiments on the ATS 5 satellite, Vela 5B at 18 earth radii in the magnetotail, the Heos 1 interplanetary probe, and ground-based magnetograms. The substorm event occurred after a very quiet day and was preceded by a development period during which the interplanetary field turned southward and the plasma energy density increased near the earth on the nightside. This period was also evidenced by a depression of the midlatitude H component of the geomagnetic field at the earth's surface. The auroral breakup was preceded by the appearance of quiet arcs, the leveling off of the plasma energy density increase at ATS, and the disappearance of the tail plasma at 18 earth radii.

Mende, S. B.↗
NASA NTRS1976

Measurements of field-aligned currents in a multiple auroral arc system

A rocket-borne experiment to study the currents associated with a system of multiple auroral arcs was conducted at Poker Flat, Alaska, at 1122 UT on Feb. 2, 1972. The magnetic field in the vicinity of the auroral system was measured with a cesium vector magnetometer. Possible configurations were inferred by constructing model current systems that reproduced the magnetic field variations measured along the flight path. The data are interpreted in terms of a model current system consisting of two eastward electrojets and one westward electrojet and three pairs of oppositely directed Birkeland sheet currents, all lying in a plane approximately parallel to the auroral arcs. Sheet thicknesses ranged from 20 to 60 km and current densities from 10 to 45 microampere/sq m; the electrojet currents ranged from 1000 to 2000 A. A possible alternate model consisted of four pairs of sheets whose thicknesses range from 10 to 40 km with current densities from 10 to 90 microampere/sq m. There was quite good agreement between the locations of the visual arcs and the upward current sheets. The overall current configuration is discussed in view of the theoretical models constructed by Atkinson and Sato and Holzer and of other observations.

Sesiano, J.↗
NASA NTRS1973

Quasi-steady plasma acceleration.

A coaxial plasma accelerator driven by protracted pulses of current in the range of 10,000 to 100,000 A and synchronized mass flows from 1.0 to 36 g/sec argon attains, after some tens of microseconds, a stable magnetoplasmadynamic acceleration mode. This 'quasi-steady' discharge form is characterized by constant terminal voltage and current, a diffuse, fixed current distribution within the discharge, and a steady plasma efflux at velocities of approximately 20 km/sec. Measured potential distributions reveal that the bulk of the arc voltage gradient, exclusive of the electrode falls, occurs within two diameters of the cathode, and is normal to it. The anode fall voltage varies inversely with local current density, implying substantially lower anode losses at higher power arc operation. Spectroscopic, potential, and velocity measurements indicate the existence of a characteristic mass flow rate for a given current, below which arc operation becomes erratic.

Jahn, R. G.↗
NASA NTRS2002

The X-ray attenuation characteristics and density of human calcaneal marrow do not change significantly during adulthood

Changes in the material characteristics of bone marrow with aging can be a significant source of error in measurements of bone density when using X-ray and ultrasound imaging modalities. In the context of computed tomography, dual-energy computed techniques have been used to correct for changes in marrow composition. However, dual-energy quantitative computed tomography (DE-QCT) protocols, while increasing the accuracy of the measurement, reduce the precision and increase the radiation dose to the patient in comparison to single-energy quantitative computed tomography (SE-QCT) protocols. If the attenuation properties of the marrow for a particular bone can be shown to be relatively constant with age, it should be possible to use single-energy techniques without experiencing errors caused by unknown marrow composition. Marrow was extracted by centrifugation from 10 mm thick frontal sections of 34 adult cadaver calcanei (28 males, 6 females, ages 17-65 years). The density and energy-dependent linear X-ray attenuation coefficient of each marrow sample were determined. For purposes of comparing our results, we then computed an effective CT number at two GE CT/i scan voltages (80 and 120 kVp) for each specimen. The coefficients of variation for the density, CT number at 80 kVp and CT number at 120 kVp were each less than 1%, and the parameters did not change significantly with age (p > 0.2, r2 < 0.02, power > 0.8 where the minimum acceptable r2 = 0.216). We could demonstrate no significant gender-associated differences in these relationships. These data suggest that calcaneal bone marrow X-ray attenuation properties and marrow density are essentially constant from the third through sixth decades of life.

NASA Discipline Musculoskeletal↗