23,909 research outputs found
Turbulence in Atomic Hydrogen
Understanding the properties of interstellar turbulence is a great
intellectual challenge and the urge to solve this problem is partially
motivated by a necessity to explain the star formation mystery. This review
deals with a recently suggested inversion technique as applied to atomic
hydrogen. This technique allows to determine 3D turbulence statistics through
the variations of 21 cm intensity. We claim that a radio interferometer is an
ideal tool for such a study as its visibility function is directly related to
the statistics of galactic HI. Next, we show how galactic rotation curve can be
used to study the turbulence slice by slice and relate the statistics given in
galactic coordinates and in the velocity space. The application of the
technique to HI data reveals a shallow spectrum of the underlying HI density
that is not compatible with a naive Kolmogorov picture. We show that the random
density corresponding to the found spectrum tends to form low contrast
filaments that are elongated towards the observer.Comment: 9 pages, 2 figures, review, to be published "Interstellar turbulence"
CU
Atomic hydrogen distribution
Several possible H2 vertical distributions in Titan's atmosphere are considered with the constraint of 5 km-A a total quantity. Approximative calculations show that hydrogen distribution is quite sensitive to two other parameters of Titan's atmosphere: the temperature and the presence of other constituents. The escape fluxes of H and H2 are also estimated as well as the consequent distributions trapped in the Saturnian system
An atomic hydrogen beam to test ASACUSA's apparatus for antihydrogen spectroscopy
The ASACUSA collaboration aims to measure the ground state hyperfine
splitting (GS-HFS) of antihydrogen, the antimatter pendant to atomic hydrogen.
Comparisons of the corresponding transitions in those two systems will provide
sensitive tests of the CPT symmetry, the combination of the three discrete
symmetries charge conjugation, parity, and time reversal. For offline tests of
the GS-HFS spectroscopy apparatus we constructed a source of cold polarised
atomic hydrogen. In these proceedings we report the successful observation of
the hyperfine structure transitions of atomic hydrogen with our apparatus in
the earth's magnetic field.Comment: 8 pages, 4 figures, proceedings for conference EXA 2014 (Exotic Atoms
- Vienna
Diamond growth in a novel low pressure flame
Diamond growth using a new low-pressure combustion technique is reported. A large-area hydrogen/oxygen flame is used as the source of atomic hydrogen. Methane diluted in hydrogen is injected into the flame near a heated silicon substrate, on which diamond crystallites nucleate and grow. This technique is potentially capable of large-area film growth, since atomic hydrogen can be generated uniformly over arbitrarily large areas
Ionisation of atomic hydrogen by positron impact
With the crossed beam apparatus the relative impact-ionization cross section of atomic hydrogen by positron impact was measured. A layout of the scattering region is given. The first measurements on the ionization of atomic hydrogen by positron impact are also given
Atomic hydrogen as a launch vehicle propellant
An analysis of several atomic hydrogen launch vehicles was conducted. A discussion of the facilities and the technologies that would be needed for these vehicles is also presented. The Gross Liftoff Weights (GLOW) for two systems were estimated; their specific impulses (I sub sp) were 750 and 1500 lb(sub f)/s/lb(sub m). The atomic hydrogen launch vehicles were also compared to the currently planned Advanced Launch System design concepts. Very significant GLOW reductions of 52 to 58 percent are possible over the Advanced Launch System designs. Applying atomic hydrogen propellants to upper stages was also considered. Very high I(sub sp) (greater than 750 lb(sub f)/s/lb(sub m)) is needed to enable a mass savings over advanced oxygen/hydrogen propulsion. Associated with the potential benefits of high I(sub sp) atomic hydrogen are several challenging problems. Very high magnetic fields are required to maintain the atomic hydrogen in a solid hydrogen matrix. The magnetic field strength was estimated to be 30 kilogauss (3 Tesla). Also the storage temperature of the propellant is 4 K. This very low temperature will require a large refrigeration facility for the launch vehicle. The design considerations for a very high recombination rate for the propellant are also discussed. A recombination rate of 210 cm/s is predicted for atomic hydrogen. This high recombination rate can produce very high acceleration for the launch vehicle. Unique insulation or segmentation to inhibit the propellant may be needed to reduce its recombination rate
Production of atomic hydrogen by cosmic rays in dark clouds
The presence of small amounts of atomic hydrogen, detected as absorption dips
in the 21 cm line spectrum, is a well-known characteristic of dark clouds. The
abundance of hydrogen atoms measured in the densest regions of molecular clouds
can be only explained by the dissociation of H due to cosmic rays. We want
to assess the role of Galactic cosmic rays in the formation of atomic hydrogen,
by using recent developments in the characterisation of the low-energy spectra
of cosmic rays and advances in the modelling of their propagation in molecular
clouds. We model the attenuation of the interstellar cosmic rays entering a
cloud and compute the dissociation rate of molecular hydrogen due to collisions
with cosmic-ray protons and electrons as well as fast hydrogen atoms. We
compare our results with the available observations. The cosmic-ray
dissociation rate is entirely determined by secondary electrons produced in
primary ionisation collisions. These secondary particles constitute the only
source of atomic hydrogen at column densities above cm. We
also find that the dissociation rate decreases with column density, while the
ratio between the dissociation and ionisation rates varies between about 0.6
and 0.7. From comparison with observations we conclude that a relatively flat
spectrum of interstellar cosmic-ray protons, as the one suggested by the most
recent Voyager 1 data, can only provide a lower bound for the observed atomic
hydrogen fraction. An enhanced spectrum of low-energy protons is needed to
explain most of the observations. Our findings show that a careful description
of molecular hydrogen dissociation by cosmic rays can explain the abundance of
atomic hydrogen in dark clouds. An accurate characterisation of this process at
high densities is crucial for understanding the chemical evolution of
star-forming regions.Comment: 7 pages, 7 figures, accepted by Astronomy and Astrophysic
Atomic hydrogen storage method and apparatus
Atomic hydrogen, for use as a fuel or as an explosive, is stored in the presence of a strong magnetic field in exfoliated layered compounds such as molybdenum disulfide or an elemental layer material such as graphite. The compounds maintained at liquid helium temperatures and the atomic hydrogen is collected on the surfaces of the layered compound which are exposed during delamination (exfoliation). The strong magnetic field and the low temperature combine to prevent the atoms of hydrogen from recombining to form molecules
ATOMIC HYDROGEN STORAGE METHOD AND APPARATUS
Atomic hydrogen, for use as a fuel or as an explosive, is stored in the presence of a strong magnetic field in exfoliated layered compounds such as molybdenum disulfide or an elemental layer material such as graphite. The compound is maintained at liquid helium temperatures and the atomic hydrogen is collected on the surfaces of the layered compound which are exposed during delamination (exfoliation). The strong magnetic field and the low temperature combine to prevent the atoms of hydrogen from recombining to form molecules
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