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At least 163 records · Page 9

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NASA NTRS1979

Slowly moving disturbances in the X-ray corona

Sequences of soft X-ray pictures, taken aboard Skylab between May and November, 1973, have made it possible to detect slowly moving disturbances originating in disrupted filaments and causing subsequent brightenings of distant coronal structures. With speeds decreasing from approximately 400 km/sec shortly after the filament disruption to approximately 10 km/sec four or five hours later, these disturbances appear to be identical with slow waves earlier inferred by Bruzel (1952, 1969), Oehman (1953), and Yajima (1971) from chromospheric observations.

Rust, D. M.↗

NASA NTRS1981

Experimental verification of a computational procedure for the design of TWT-refocuser-MDC systems

A comparison of analytical and experimental results is presented for a high performance dual-mode helix TWT, equipped with multistage depressed collectors (MDC), and operated over conditions ranging from saturation to the linear regime. The computations are carried out with advanced multidimensional computer programs which model the electron beam as a series of disks or rings of charge and follow their trajectories from the RF input of the TWT, through the slow-wave structure, through the refocusing system, to their points of impact in the depressed collector. TWT performance, collector efficiency, and collector current distribution are computed and compared with measurements. Very good agreement is obtained between computed and measured TWT performance and collector efficiencies. The analytical techniques were subsequently applied to the design of a smaller MDC of nearly equal efficiency.

Dayton, J. A., Jr.↗

NASA NTRS1983

Resonant isolator for maser amplifier

An isolator is described for use in a low noise maser amplifier, which provides low loss across a wide bandwidth and which can be constructed at moderate cost. The isolator includes a train of garnet or ferrite elements extending along the length of a microwave channel parallel to the slow wave structure, with the elements being of staggered height, so that the thin elements which are resonant to the microwaves are separated by much thicker elements. The thick garnet or ferrite elements reduce the magnetic flux passing through the thin elements to permit altering of the shape of the thin elements so as to facilitate their fabrication and to provide better isolation with reduced loss, by increasing the thickness of the thin elements and decreasing their length and width.

Clauss, R. C.↗

NASA NTRS1983

Sleep-wake responses of squirrel monkeys exposed to hyperdynamic environments

This study examines the sleep responses of primates to acute 3 Gz environments. To investigate this question, loosely-restrained squirrel monkeys were exposed to 70 minutes of 3 Gz during the day. The animals' behavioral state was polygraphically monitored (EEG, EMG, EOG) along with video and deep body temperature. During the control period, animals exhibited slow wave sleep (SWS) napping behavior. SWS occurred during approximately 20 percent of the control period. Body temperature was maintained at 38.7 C. At 3 Gz, SWS was inhibited for 5 minutes, after which SWS occurred at levels 50 percent lower than in the control period. During the post-centrifugation period, SWS was elevated above the control (50 percent) and hyperdynamic (100 percent) levels. Body temperature was depressed 1.5 C when the animals were at 3 Gz. Thus, hyperdynamic environments are capable of modifying primate sleep behavior, at least as a result of acute exposure. Further, the increased arousal in the hyperdynamic environment is correlated with a lower body temperature. This negative correlation differs from the normal positive correlation of arousal and body temperature.

Fuller, C. A.↗

NASA NTRS1984

Plasma wave spectra near slow mode shocks in the distant magnetotail

Recently Feldman et al. (1984) used ISEE-3 data to analyze variations in charged particle and magnetic field characteristics near plasma sheet boundaries in the distant magnetotail, and they demonstrated that several of the crossings had the properties of slow mode shocks. ISEE-3 plasma wave measurements at two of these crossings are presently discussed, and it is shown that all conventional characteristics of fast collisionless shock spectra are present. Upstream (lobe side) electron plasma oscillations are detected in association with electron heat flux enhancements, and pronounced spectral peaks at lower frequencies are found within the shock layer.

Scarf, F. L.↗

NASA NTRS1986

Verification of an improved computational design procedure for TWT-dynamic refocuser-MDC systems with secondary electron emission losses

A computational procedure for the design of TWT-refocuser-MDC systems was used to design a short 'dynamic' refocusing system and highly efficient four-stage depressed collector for a 200-W 8-18-GHz TWT. The computations were carried out with advanced multidimensional computer programs which model the electron beam as a series of disks of charge and follow their trajectories from the RF input of the TWT, through the slow-wave structure and refocusing section, to their points of impact in the depressed collector. Secondary emission losses in the MDC were treated semiquantitatively by injecting a representative beam of secondary electrons into the MDC analysis at the point of impact of each primary beam. A comparison of computed and measured TWT and MDC performance showed very good agreement. The electrodes of the MDC were fabricated from a particular form of isotropic graphite that was selected for its low secondary electron yield, thermal expansion characteristics, ease of machinability and vacuum properties. This MDC was tested at CW for more than 1000 h with negligible degradation in TWT and MDC performances.

Ramins, P.↗

NASA NTRS1990

Real-time measurement of mental workload: A feasibility study

The primary goal of the study was to explore the utility of event-related brain potentials (ERP) as real-time measures of workload. To this end, subjects performed two different tasks both separately and together. One task required that subjects monitor a bank of constantly changing gauges and detect critical deviations. Difficulty was varied by changing the predictability of the gauges. The second task was mental arithmetic. Difficulty was varied by requiring subjects to perform operations on either two or three columns of numbers. Two conditions that could easily be distinguished on the basis of performance measures were selected for the real-time evaluation of ERPs. A bootstrapping approach was adopted in which one thousand samples of n trials (n = 1, 3, 5 ...65) were classified using several measures of P300 and Slow Wave amplitude. Classification accuracies of 85 percent were achieved with 25 trials. Results are discussed in terms of potential enhancements for real-time recording.

Kramer, Arthur↗

NASA NTRS1990

The theory of an auto-resonant field emission cathode relativistic electron accelerator for high efficiency microwave to direct current power conversion

A novel method of microwave power conversion to direct current is discussed that relies on a modification of well known resonant linear relativistic electron accelerator techniques. An analysis is presented that shows how, by establishing a 'slow' electromagnetic field in a waveguide, electrons liberated from an array of field emission cathodes, are resonantly accelerated to several times their rest energy, thus establishing an electric current over a large potential difference. Such an approach is not limited to the relatively low frequencies that characterize the operation of rectennas, and can, with appropriate waveguide and slow wave structure design, be employed in the 300 to 600 GHz range where much smaller transmitting and receiving antennas are needed.

Manning, Robert M.↗

NASA NTRS1991

A general analysis of propagation along multiple-layer superconducting stripline and microstrip transmission lines

A rigorous spectral-domain formulation for a superconducting stripline or microstrip transmission line with a multiple-layer dielectric substrate is presented. The formulation models the strip conductor as a surface current with an equivalent surface impedance, where the surface impedance is approximated in closed form when the strip is either much thinner or much thicker than a penetration depth. In either case the surface impedance is related to the complex conductivity of the material, which is calculated from a two-fluid model. Results are presented to show the slow-wave propagation and attenuation along both microstrip and stripline packages in a realistic multiple-layer configuration, which accounts for the field penetration into the superconducting ground planes.

Nghiem, David↗

NASA NTRS1990

A general analysis of propagation along multiple-layer superconducting stripline and microstrip transmission lines

Nghiem, David↗

NASA NTRS1993

Bonding Diamond To Metal In Electronic Circuits

Improved technique for bonding diamond to metal evolved from older technique of soldering or brazing and more suitable for fabrication of delicate electronic circuits. Involves diffusion bonding, developed to take advantage of electrically insulating, heat-conducting properties of diamond, using small diamond bars as supports for slow-wave transmission-line structures in traveling-wave-tube microwave amplifiers. No fillets or side coats formed because metal bonding strips not melted. Technique also used to mount such devices as transistors and diodes electrically insulated from, but thermally connected to, heat sinks.

Jacquez, Andrew E.↗

NASA NTRS1993

Flight crew sleep during multiple layover polar flights

This study investigated changes in sleep after multiple transmeridian flights. The subjects were 12 B747 airline pilots operating on the following polar flight: Tokyo (TYO)-Anchorage (ANC)-London (LON)-Anchorage-Tokyo. Sleep polysmonograms were recorded on two baseline nights (B1, B2), during layovers, and, after returning to Tokyo, two recovery nights were recorded (R1, R2). In ANC (outbound), total sleep time was reduced and, sleep efficiency was low (72.0 percent). In London, time in bed increased slightly, but sleep efficiency was still reduced. On return to ANC (inbound), there was considerable slow wave sleep rebound and multiple awakenings reduced sleep efficiency to 76.8 percent. Sleep efficiency on R2 was significantly lower than on B1 but not different from R1. To sum up, sleep of aircrews flying multiple transmeridian flights is disrupted during layovers and this effect persists during the two recovery nights. As a result, there is a marked cumulative sleep loss during multilegs polar route trip in comparison to single leg flights. These findings suggest that following such extensive transmeridian trips, crews should have at least three nights of recovery sleep in their home time zone before returning to duty.

Sasaki, Mitsuo↗

NASA NTRS1991

A high-efficiency 59- to 64-GHz TWT for intersatellite communications

The design of a 75-W, 59- to 64- GHz TWT with a predicted overall efficiency in excess of 40 percent is described. This intersatellite communications TWT, designated Model 961HA, employs a coupled-cavity slow-wave structure with a two-step velocity taper and an isotropic graphite multistage depressed collector (MDC). Because the RF efficiency of this TWT is less than 8 percent, an MDC design providing a very high collector efficiency was necessary to achieve the overall efficiency goal of 40 percent.

Wilson, Jeffrey D.↗

NASA NTRS1995

0n the dynamics of warped accretion disks

We present some analyses of warping modes in inviscid near Keplerian disks taking the three-dimensional structure fully into account. The results of this investigation verify the validity of a vertical averaging approximation for thin disks when the radial wavelength is significantly longer than the disk thickness. They also indicate that long wavelength disturbances may persist for long times. Shorter wavelength disturbances in non-self gravitating disks are found to propagate with little dispersion at a speed related to the sound speed. When self-gravity becomes important, fast and slow waves are found which also propagate with little dispersion. When a small viscosity is included, the evolution of the disturbances becomes more diffusive in character.

Papaloizou, J. C. B.↗

NASA NTRS2001

Sleep, performance, circadian rhythms, and light-dark cycles during two space shuttle flights

Sleep, circadian rhythm, and neurobehavioral performance measures were obtained in five astronauts before, during, and after 16-day or 10-day space missions. In space, scheduled rest-activity cycles were 20-35 min shorter than 24 h. Light-dark cycles were highly variable on the flight deck, and daytime illuminances in other compartments of the spacecraft were very low (5.0-79.4 lx). In space, the amplitude of the body temperature rhythm was reduced and the circadian rhythm of urinary cortisol appeared misaligned relative to the imposed non-24-h sleep-wake schedule. Neurobehavioral performance decrements were observed. Sleep duration, assessed by questionnaires and actigraphy, was only approximately 6.5 h/day. Subjective sleep quality diminished. Polysomnography revealed more wakefulness and less slow-wave sleep during the final third of sleep episodes. Administration of melatonin (0.3 mg) on alternate nights did not improve sleep. After return to earth, rapid eye movement (REM) sleep was markedly increased. Crewmembers on these flights experienced circadian rhythm disturbances, sleep loss, decrements in neurobehavioral performance, and postflight changes in REM sleep.

Non-NASA Center↗

NASA NTRS1995

Comparison of electric and growth responses to excision in cucumber and pea seedlings. II. Long-distance effects are caused by the release of xylem pressure

Excision of a growing stem causes local wound responses, such as membrane depolarization and growth inhibition, as well as effects at larger distances from the cut. In this study, cucumber hypocotyls were excised 100 mm below the hook, so that the growing region was beyond the reach of the wound-induced depolarization (up to 40 mm). Even at such a distance, the cut still caused a considerable and rapid drop in the hypocotyl growth rate. This growth response is not a direct wound response because it does not result from the cut-induced depolarization and because it can be simulated by root pressure manipulation (using a pressure chamber). The results indicate that the growth response resulted from the rapid release of the xylem pressure upon excision. To test this conclusion we measured the xylem pressure by connecting a pressure probe to the cut surface of the stem. Xylem pressure (Px) was found to be +10 to +40 kPa in cucumber hypocotyls and -5 to -10 kPa or lower in pea epicotyls. Excision of the cucumber hypocotyl base led to a rapid drop in Px to negative values, whereas excision in pea led to a rapid rise in Px to ambient (zero) pressure. These fast and opposite Px changes parallel the excision-induced changes in growth rate (GR): a decrease in cucumber and a rise in pea. The sign of the endogenous xylem pressure also determined whether excision induced a propagating depolarization in the form of a slow wave potential (SWP). Under normal circumstances pea seedlings generated an SWP upon excision whereas cucumber seedlings failed to do so. When the Px in cucumber hypocotyls was experimentally inverted to negative values by incubating the cumber roots in solutions of NaCN or n-ethylmaleimide, excision caused a propagating depolarization (SWP). The experiment shows that only hydraulic signals in the form of positive Px steps are converted into propagating electric SWP signals. These propagating depolarizations might be causally linked to systemic 'wound' responses, which occur independently of the short-distance or direct wound responses.

Non-NASA Center↗

NASA NTRS1999

Frontal predominance of a relative increase in sleep delta and theta EEG activity after sleep loss in humans

The effect of sleep deprivation (40 h) on topographic and temporal aspects of electroencephalographic (EEG) activity during sleep was investigated by all night spectral analysis in six young volunteers. The sleep-deprivation-induced increase of EEG power density in the delta and theta frequencies (1-7 Hz) during nonREM sleep, assessed along the antero-posterior axis (midline: Fz, Cz, Pz, Oz), was significantly larger in the more frontal derivations (Fz, Cz) than in the more parietal derivations (Pz, Oz). This frequency-specific frontal predominance was already present in the first 30 min of recovery sleep, and dissipated in the course of the 8-h sleep episode. The data demonstrate that the enhancement of slow wave EEG activity during sleep following extended wakefulness is most pronounced in frontal cortical areas.

NASA Discipline Space Human Factors↗

NASA NTRS2001

The effects of age and gender on sleep EEG power spectral density in the middle years of life (ages 20-60 years old)

The effects of age and gender on sleep EEG power spectral density were assessed in a group of 100 subjects aged 20 to 60 years. We propose a new statistical strategy (mixed-model using fixed-knot regression splines) to analyze quantitative EEG measures. The effect of gender varied according to frequency, but no interactions emerged between age and gender, suggesting that the aging process does not differentially influence men and women. Women had higher power density than men in delta, theta, low alpha, and high spindle frequency range. The effect of age varied according to frequency and across the night. The decrease in power with age was not restricted to slow-wave activity, but also included theta and sigma activity. With increasing age, the attenuation over the night in power density between 1.25 and 8.00 Hz diminished, and the rise in power between 12.25 and 14.00 Hz across the night decreased. Increasing age was associated with higher power in the beta range. These results suggest that increasing age may be related to an attenuation of homeostatic sleep pressure and to an increase in cortical activation during sleep.

NASA Discipline Regulatory Physiology↗