Surveying the Dynamic Radio Sky with the Long Wavelength Demonstrator Array

This paper presents a search for radio transients at a frequency of 73.8 MHz (4 m wavelength) using the all-sky imaging capabilities of the Long Wavelength Demonstrator Array (LWDA). The LWDA was a 16-dipole phased array telescope, located on the site of the Very Large Array in New Mexico. The field of view of the individual dipoles was essentially the entire sky, and the number of dipoles was sufficiently small that a simple software correlator could be used to make all-sky images. From 2006 October to 2007 February, we conducted an all-sky transient search program, acquiring a total of 106 hr of data; the time sampling varied, being 5 minutes at the start of the program and improving to 2 minutes by the end of the program. We were able to detect solar flares, and in a special-purpose mode, radio reflections from ionized meteor trails during the 2006 Leonid meteor shower. We detected no transients originating outside of the solar system above a flux density limit of 500 Jy, equivalent to a limit of no more than about 10^{-2} events/yr/deg^2, having a pulse energy density >~ 1.5 x 10^{-20} J/m^2/Hz at 73.8 MHz for pulse widths of about 300 s. This event rate is comparable to that determined from previous all-sky transient searches, but at a lower frequency than most previous all-sky searches. We believe that the LWDA illustrates how an all-sky imaging mode could be a useful operational model for low-frequency instruments such as the Low Frequency Array, the Long Wavelength Array station, the low-frequency component of the Square Kilometre Array, and potentially the Lunar Radio Array.Comment: 20 pages; accepted for publication in A

Les propriétés rhéologiques de la craie et leur influence sur le percement de galeries

Research including laboratory and in-situ tests down to depth of 220 m on the behaviour of chalk was carried out for an underground pumping plant scheme in the Champagne region of France, but it has now been abandoned. Chalk is a soft, light, highly porous material and is saturated with water below the water table. Its particularly fine structure was described with the aid of an electron microscope. Elastic modulus measurements produced the first evidence of remarkably close agreement between laboratory and in-situ data. Chalk shows no scale effect, which indicates a total absence of cracks. The formation is continuous even at a scale of 1 decametre. Modules and strength are affected by moderate axial anisotropy ; strength of the rock is greater in a direction perpendicular to its stratification. Creep and the behaviour of chalk under tri-axial load were also investigated, and the latter was found to result in a considerable reduction in volume. Reworking by impact or vibration converts this brittle rock into a soil near its liquidity limit. The exceptional continuity and homogeneity of chalk and initial stresses very close to a hydrostatic distribution are good conditions for studies of the stability of underground works in elastic-plastic media. The exploration shaft showed the same rock-bursting below a depth of 120 m as in the Mont Blanc tunnel. though this 'danger signal' does not mean that the works are impracticable. It should be possible to avoid any drilling difficulties, especially by drilling vent shafts ahead of the work face to induce decompression. And, as in the Mont Blanc tunnel, the lining should not be designed to take the total load

First Sky Tests of Adaptive Optics Predictive Control

One of the main limitations to adaptive optics (AO) correction efficiency is the time lag in the loop due to wave-front time averaging, digital-to-analog conversion of deformable mirror control voltages and read-out of the wave-front sensor (WFS) detector. It has been shown that Shack-Hartmann WFS measurements are predictible. More recently, we studied closed-loop prediction for AO control and synthesized a linear modal predictor. In this paper, the theory of modal predictive control in AO is reminded. Numerical and laboratory results are presented, where the ONERA AO system including a modal predictive control is used with a turbulence simulation cell. Finally, first results obtained on sky at the focus of the 1.52 m telescope at Observatoire de Haute-Provence are given and discussed. They show the significant gain which can be brought by using modal predictive control instead of conventional control algorithms. 1 Introduction The performance of the servo loop of an adaptive optics (..

A COMPARISON OF NUMERICAL-SOLUTIONS OF THE BOLTZMANN TRANSPORT-EQUATION FOR HIGH-ENERGY ELECTRON-TRANSPORT SILICON

In this work we have undertaken a comparison of several previously reported computer codes which solve the semi-classical Boltzmann equation for electron transport in silicon. Most of the codes are based on the Monte Carlo particle technique, and have been used here to calculate a relatively simple set of transport characteristics, such as the average electron energy. The results have been contributed by researchers from Japan, Europe, and the United States, and the results were subsequently collected by an independent observer. Although the computed data vary widely, depending on the models and input parameters which are used, they provide for the first time a quantitative (though not comprehensive) comparison of Boltzmann Equation solutions