3,781 research outputs found
Solar sailing - mission opportunities and innovative technology demonstration
Solar sailing is a unique and elegant form of propulsion that transcends reliance on reaction mass. Rather than carrying propellant, solar sails acquire momentum from photons, the quantum packets of energy from which sunlight is composed. In addition, since solar sails are not limited by reaction mass, they can provide continual acceleration, limited only by the lifetime of the sail film in the space environment. Therefore, solar sails can expand the envelope of possible missions, enabling new high-energy mission concepts that are essentially impossible with conventional reaction propulsion, and enhancing current mission concepts by lowering launch mass and reducing trip times
The DCU laser ion source
Laser ion sources are used to generate and deliver highly charged ions of various masses and energies. We present details on the design and basic parameters of the DCU laser ion source (LIS). The theoretical aspects of a high voltage (HV) linear LIS are presented and the main issues surrounding laser-plasma formation, ion extraction and modeling of beam transport in relation to the operation of a LIS are detailed. A range of laser power densities (I ⌠108â1011âWâcmâ2) and fluences (F = 0.1â3.9âkJâcmâ2) from a Q-switched ruby laser (full-width half-maximum pulse duration ⌠35âns, λ = 694ânm) were used to generate a copper plasma. In âbasic operating mode,â laser generated plasma ions are electrostatically accelerated using a dc HV bias (5â18 kV). A traditional einzel electrostatic lens system is utilized to transport and collimate the extracted ion beam for detection via a Faraday cup. Peak currents of up to I ⌠600âÎŒA for Cu+ to Cu3+ ions were recorded. The maximum collected charge reached 94 pC (Cu2+). Hydrodynamic simulations and ion probe diagnostics were used to study the plasma plume within the extraction gap. The system measured performance and electrodynamic simulations indicated that the use of a short field-free (L = 48âmm) region results in rapid expansion of the injected ion beam in the drift tube. This severely limits the efficiency of the electrostatic lens system and consequently the sources performance. Simulations of ion beam dynamics in a âcontinuous einzel arrayâ were performed and experimentally verified to counter the strong space-charge force present in the ion beam which results from plasma extraction close to the target surface. Ion beam acceleration and injection thus occur at âhigh pressure.â In âenhanced operating mode,â peak currents of 3.26 mA (Cu2+) were recorded. The collected currents of more highly charged ions (Cu4+âCu6+) increased considerably in this mode of operation
Simulated Extragalactic Observations with a Cryogenic Imaging Spectrophotometer
In this paper we explore the application of cryogenic imaging
spectrophotometers. Prototypes of this new class of detector, such as
superconducting tunnel junctions (STJs) and transition edge sensors (TESs),
currently deliver low resolution imaging spectrophotometry with high quantum
efficiency (70-100%) and no read noise over a wide bandpass in the visible to
near-infrared. In order to demonstrate their utility and the differences in
observing strategy needed to maximize their scientific return, we present
simulated observations of a deep extragalactic field. Using a simple analytic
technique, we can estimate both the galaxy redshift and spectral type more
accurately than is possible with current broadband techniques. From our
simulated observations and a subsequent discussion of the expected migration
path for this new technology, we illustrate the power and promise of these
devices.Comment: 30 pages, 10 figures, accepted for publication in the Astronomical
Journa
The Angular Power Spectrum of EDSGC Galaxies
We determine the angular power spectrum, C_l, of the Edinburgh/Durham
Southern Galaxy Catalog (EDSGC) and use this statistic to constrain
cosmological parameters. Our methods for determining C_l, and the parameters
that affect it are based on those developed for the analysis of cosmic
microwave background maps. We expect them to be useful for future surveys.
Assuming flat cold dark matter models with a cosmological constant (constrained
by COBE/DMR and local cluster abundances), and a scale--independent bias, b, we
find good fits to the EDSGC angular power spectrum with 1.11 < b < 2.35 and 0.2
< Omega_m < 0.55 at 95% confidence. These results are not significantly
affected by the ``integral constraint'' or extinction by interstellar dust, but
may be by our assumption of Gaussianity.Comment: 11 pages, 9 figures, version to appear in Ap
Recommended from our members
Trade issues background paper: The impact of dumping on trade in fisheries products
Put simply, dumping is the exporting of produce at less than production cost to the material detriment of competitor industries in the importing country. The recourse by the plaintiff â the importing country - is to impose import bans and/or compensating duties (duty orders) on the rogue products in legitimate protection of their own industry â i.e. a re-levelling of the playing field. These are called Anti- Dumping Measures (ADMs). Theoretically this is a technical issue that should be subject to economic logic and legal argument. In practice it is highly political, can be arbitrary and is often raised at the behest of an industry finding its self-unable to compete for harsh but fair economic reasons
The EXOSAT medium-energy slew survey catalog
We present a catalog of X-ray sources observed during slew maneuvers by the
Medium Energy Detector Array onboard the EXOSAT Observatory. The EXOSAT Medium
Energy slew-survey catalog (EXMS) provides a unique record of the 1--8 keV
X-ray sky between 1983 and 1986. 98% of the sky was observed, with 85%
receiving an exposure of >60 s. 1210 sources were detected. By comparing these
source positions with other catalogs, identifications are given for 992
detections (82% of the sample). These identifications consist of 250 distinct
objects, including 95 different X-ray binary systems, and 14 different AGN. A
further 58 detections have multiple candidates, while 160 detections remain
unidentified. Collimator transmission corrected 1-8 keV count rates are given
for the identified sources, together with raw count rates for the other
detections. The construction of the EXMS and the checks performed to ensure the
validity of the derived source properties are discussed. A publically available
version of this catalog is maintained on the EXOSAT database and archive system
(telnet://[email protected]).Comment: 52 pages. 22 Figures. To be published in A&AS. For more information,
see http://astro.estec.esa.nl/SA-general/Projects/Exosat/exmsintro.htm
Statistics of Weak Gravitational Lensing in Cold Dark Matter Models; Magnification Bias on Quasar Luminosity Functions
We compute statistical properties of weak gravitational lensing by
large-scale structure in three Cold Dark Matter models. We use a PM
-body code to simulate the formation and evolution of large-scale structure
in the universe. We perform ray-tracing experiments for each
model using the multiple lens-plane algorithm. From the results of these
experiments, we calculate the probability distribution functions (PDF) of the
convergences, shears, and magnifications, and their root-mean-square (rms)
values. We find that the rms values of the convergence and shear agree with the
predictions of a nonlinear analytical model. We also find that the PDFs of the
magnifications have a peak at values slightly smaller than , and
are strongly skewed toward large magnifications. In particular, for the
high-density model, a power-law tail appears in the magnification distribution
at large magnifications for sources at redshifts . The rms values of the
magnifications essentially agree with the nonlinear analytical predictions for
sources at low redshift, but exceed these predictions for high redshift
sources, once the power-law tail appears.
We study the effect of magnification bias on the luminosity functions of
high-redshift quasars, using the calculated PDFs of the magnifications. We show
that the magnification bias is moderate in the absence of the power-law tail in
the magnification distribution, but depends strongly on the value of the
density parameter. In presence of the power-law tail, the bias becomes
considerable, especially at the bright end of the luminosity functions.Comment: 24 pages, 9 figures, LaTex using epsfig.sty. Submitted to the The
Astrophysical Journa
Stochastic Biasing and Weakly Non-linear Evolution of Power Spectrum
Distribution of galaxies may be a biased tracer of the dark matter
distribution and the relation between the galaxies and the total mass may be
stochastic, non-linear and time-dependent. Since many observations of galaxy
clustering will be done at high redshift, the time evolution of non-linear
stochastic biasing would play a crucial role for the data analysis of the
future sky surveys. In this paper, we develop the weakly non-linear analysis
and attempt to clarify the non-linear feature of the stochastic biasing. We
compute the one-loop correction of the power spectrum for the total mass, the
galaxies and their cross correlation. Assuming the local functional form for
the initial galaxy distribution, we investigate the time evolution of the
biasing parameter and the correlation coefficient. On large scales, we first
find that the time evolution of the biasing parameter could deviate from the
linear prediction in presence of the initial skewness. However, the deviation
can be reduced when the initial stochasticity exists. Next, we focus on the
quasi-linear scales, where the non-linear growth of the total mass becomes
important. It is recognized that the scale-dependence of the biasing
dynamically appears and the initial stochasticity could affect the time
evolution of the scale-dependence. The result is compared with the recent
N-body simulation that the scale-dependence of the halo biasing can appear on
relatively large scales and the biasing parameter takes the lower value on
smaller scales. Qualitatively, our weakly non-linear results can explain this
trend if the halo-mass biasing relation has the large scatter at high redshift.Comment: 29pages, 7 postscript figures, submitted to Ap
Nonlinear internal wave penetration via parametric subharmonic instability
6 pages, 5 figuresInternational audienceWe present the results of a laboratory experimental study of an internal wave field generated by harmonic, spatially-periodic boundary forcing from above of a density stratification comprising a strongly-stratified, thin upper layer sitting atop a weakly-stratified, deep lower layer. In linear regimes, the energy flux associated with relatively high frequency internal waves excited in the upper layer is prevented from entering the lower layer by virtue of evanescent decay of the wave field. In the experiments, however, we find that the development of parametric subharmonic instability (PSI) in the upper layer transfers energy from the forced primary wave into a pair of subharmonic daughter waves, each capable of penetrating the weakly-stratified lower layer. We find that around of the primary wave energy flux penetrates into the lower layer via this nonlinear wave-wave interaction for the regime we study
- âŠ