21,703 research outputs found
Hollow cathode plasma penetration study Final report
Hollow cathode electron beam discharge for penetrating plasma sheath around reentry vehicl
Propulsion system-flight control integration and optimization: Flight evaluation and technology transition
Integration of propulsion and flight control systems and their optimization offers significant performance improvements. Research programs were conducted which have developed new propulsion and flight control integration concepts, implemented designs on high-performance airplanes, demonstrated these designs in flight, and measured the performance improvements. These programs, first on the YF-12 airplane, and later on the F-15, demonstrated increased thrust, reduced fuel consumption, increased engine life, and improved airplane performance; with improvements in the 5 to 10 percent range achieved with integration and with no changes to hardware. The design, software and hardware developments, and testing requirements were shown to be practical
The outer crust of non-accreting cold neutron stars
The properties of the outer crust of non-accreting cold neutron stars are
studied by using modern nuclear data and theoretical mass tables updating in
particular the classic work of Baym, Pethick and Sutherland. Experimental data
from the atomic mass table from Audi, Wapstra, and Thibault of 2003 is used and
a thorough comparison of many modern theoretical nuclear models, relativistic
and non-relativistic ones, is performed for the first time. In addition, the
influences of pairing and deformation are investigated. State-of-the-art
theoretical nuclear mass tables are compared in order to check their
differences concerning the neutron dripline, magic neutron numbers, the
equation of state, and the sequence of neutron-rich nuclei up to the dripline
in the outer crust of non-accreting cold neutron stars.Comment: 20 pages, 10 figures, accepted for publication in Phys. Rev.
Spin-Down of Neutron Stars and Compositional Transitions in the Cold Crustal Matter
Transitions of nuclear compositions in the crust of a neutron star induced by
stellar spin-down are evaluated at zero temperature. We construct a
compressible liquid-drop model for the energy of nuclei immersed in a neutron
gas, including pairing and shell correction terms, in reference to the known
properties of the ground state of matter above neutron drip density, . Recent experimental values and extrapolations of
nuclear masses are used for a description of matter at densities below neutron
drip. Changes in the pressure of matter in the crust due to the stellar
spin-down are calculated by taking into account the structure of the crust of a
slowly and uniformly rotating relativistic neutron star. If the initial
rotation period is ms, these changes cause nuclei, initially being in
the ground-state matter above a mass density of about , to absorb neutrons in the equatorial region where the matter
undergoes compression, and to emit them in the vicinity of the rotation axis
where the matter undergoes decompression. Heat generation by these processes is
found to have significant effects on the thermal evolution of old neutron stars
with low magnetic fields; the surface emission predicted from this heating is
compared with the observations of X-ray emission from millisecond
pulsars and is shown to be insufficient to explain the observed X-ray
luminosities.Comment: 32 pages, LaTeX, 11 Postscript figures. Accepted for publication in
Ap
Effects of neutral gas release on current collection during the CHARGE-2 rocket experiment
Observations of current collection enhancements due to cold nitrogen gas control jet emissions from a highly charged rocket payload in the ionosphere are reported. These observations were made during the second cooperative high altitude rocket gun experiment (CHARGE-2) which was an electrically tethered mother/daughter payload system. The current collection enhancement was observed at the daughter payload located 100 to 400 m away from the mother which was firing an energetic electron beam. The authors interpret these results in terms of an electrical discharge forming in close proximity to the daughter during the short periods of gas emission. The results indicate that it is possible to enhance the electron current collection capability of positively charged vehicles by means of deliberate neutral gas releases into an otherwise undisturbed space plasma. These results can also be compared with recent laboratory observations of hollow cathode plasma contactors operating in the ignited mode. Experimental observations of current collection enhancements due to cold nitrogen gas control jet emissions from a highly charged, isolated daughter payload in the nighttime ionosphere were made. These observations were derived from the second cooperative high altitude rocket gun experiment (CHARGE-2) which was an electrically tethered mother-daughter payload system. The rocket flew from White Sands Missile Range (WSMR) in December, 1985. The rocket achieved an altitude of 261 km and carried a 1 keV electron beam emitting up to 48 mA of current (Myers, et al., 1989a). The mother payload, carried the electron beam source, while the daughter acted as a remote current collection and observation platform and reached a distance of 426 m away from the main payload. Gas emissions at the daughter were due to periodic thruster jet firings to maintain separation velocity between the two payloads
Pupillometry, a bioengineering overview
The pupillary control system is examined using a microprocessor based integrative pupillometer. The real time software functions of the microprocessor include: data collection, stimulus generation and area to diameter conversion. Results of an analysis of linear and nonlinear phenomena are presented
A High Resolution Study of the Slowly Contracting, Starless Core L1544
We present interferometric observations of N2H+(1--0) in the starless, dense
core L1544 in Taurus. Red-shifted self-absorption, indicative of inward
motions, is found toward the center of an elongated core. The data are fit by a
non-spherical model consisting of two isothermal, rotating, centrally condensed
layers. Through a hybrid global-individual fit to the spectra, we map the
variation of infall speed at scales ~1400AU and find values ~0.08 km/s around
the core center. The inward motions are small in comparison to thermal,
rotational, and gravitational speeds but are large enough to suggest that L1544
is very close to forming a star.Comment: 11 pages, 2 figures Accepted for publication in Astrophysical Journal
Letter
An Ammonia Spectral Atlas of Dense Cores in Perseus
We present ammonia observations of 193 dense cores and core candidates in the
Perseus molecular cloud made using the Robert F. Byrd Green Bank Telescope. We
simultaneously observed the NH3(1,1), NH3(2,2), CCS (2_1 -> 1_0) and CC34S (2_1
-> 1_0) transitions near 23 GHz for each of the targets with a spectral
resolution of dv ~ 0.024 km/s. We find ammonia emission associated with nearly
all of the (sub)millimeter sources as well as at several positions with no
associated continuum emission. For each detection, we have measured physical
properties by fitting a simple model to every spectral line simultaneously.
Where appropriate, we have refined the model by accounting for low optical
depths, multiple components along the line of sight and imperfect coupling to
the GBT beam. For the cores in Perseus, we find a typical kinetic temperature
of T=11 K, a typical column density of N(NH3)~ 10^14.5 /cm^2 and velocity
dispersions ranging from sigma_v = 0.07 km/s to 0.7 km/s. However, many cores
with velocity dispersions > 0.2 km/s show evidence for multiple velocity
components along the line of sight.Comment: 19 pages; Accepted to ApJS; version with high resolution figures
available at http://www.cfa.harvard.edu/COMPLETE/papers/nh3-paper1.pdf ;
online data at
http://www.cfa.harvard.edu/COMPLETE/data_html_pages/GBT_NH3.htm
Large-Scale Integration of Nanoelectromechanical Systems for Gas Sensing Applications
We have developed arrays of nanomechanical systems (NEMS) by large-scale integration, comprising thousands of individual nanoresonators with densities of up to 6 million NEMS per square centimeter. The individual NEMS devices are electrically coupled using a combined series-parallel configuration that is extremely robust with respect to lithographical defects and mechanical or electrostatic-discharge damage. Given the large number of connected nanoresonators, the arrays are able to handle extremely high input powers (>1 W per array, corresponding to <1 mW per nanoresonator) without excessive heating or deterioration of resonance response. We demonstrate the utility of integrated NEMS arrays as high-performance chemical vapor sensors, detecting a part-per-billion concentration of a chemical warfare simulant within only a 2 s exposure period
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