Showing papers by "Applied Science Private University published in 1973"

Saturation of Stimulated Backscattered Radiation in Laser Plasmas

Abstract: Measurements of the back reflection from laser-produced plasmas have been carried out for laser intensities of up to 1.5\ifmmode\times\else\texttimes\fi{}${10}^{16}$ W/${\mathrm{cm}}^{2}$ with pulse durations of 120 psec using spherical low-$Z$ targets. The reflectivity of the plasma increases with incident intensity but saturates at a value of 10%. The intensity-dependent reflectivity is attributed to stimulated Brillouin scattering.

Coherence of Grid-Generated Turbulence

Abstract: For the purpose of predicting the dynamic response of structures and buildings to turbulent winds, a knowledge of the lateral coherence of the upstream velocity fluctuations is essential. For the special case of isotropic turbulence, the character of the coherence function is deduced from theoretical considerations and a simple method of computing this function from any arbitrary power spectrum is derived. Particular emphasis is placed on the range of validity of simple collapsing parameters, associated with certain spectral models. The theoretical predictions are confirmed by extensive hot-wire measurements taken in grid-generated turbulence. This result facilitates determination of the dynamic response of slender structures to wind loading.

Clipping and Signal Determinism: Two Algorithms Requiring Validation

Abstract: Two algorithms are proposed for recovering signals subjected to bandpass clipping and related types of nonlinear transformations. Both are based on contraction mapping principles and both seem to work experimentally, but neither has been validated formally. The conditions on signal determinism suggested by the algorithms have interesting practical implications.

Mechanisms of compaction of powdered materials

Abstract: The compaction of powdered sodium chloride, sucrose and a coal have been studied by observing changes in specific surface area, pore volume and extent of compaction with pressure. Observations have also been made of appropriate sections using a scanning electron microscope. It has been shown that the initial compaction of the three materials is not governed by the sliding of particles into new equilibrium positions. The compaction of sodium chloride is achieved by plastic deformation of the individual particles, without their fracture. This process continues into the region in which coherent compacts are produced, but work hardening of the material makes it progressively more difficult to compact as higher pressures are reached. For sodium chloride the area of contact, A , is related to the compacting pressure, P , by the equation In P = k A + K , where k and K are constants. The compaction of both sucrose and coal is dominated by particle fracture. At lower pressures, in particular, the movement of particle fragments into positions of closer proximity is an important secondary mechanism. This movement of fragments into the available pore space is particularly important in the case of sucrose. With coal there is, in addition, a tendency for fragments to remain wedged in the form of chain bridges which maintain a more open structure in the compact. At high pressures, for both materials, plastic deformation of small particle fragments may well be important.

The application of finite elements to heat conduction problems involving latent heat

Abstract: The Application of Finite Elements to Heat Conduction Problems Involving Latent Heat A finite element method is developed for the solution of heat conduction problems which involve latent heat. The method is superior to other wellknown approaches to these problems in that it allows a wider range of material properties and boundary conditions to be dealt with, such as are encountered in complex engineering operations like ground freezing. The method is applied to simple triangular finite elements in this paper, although it could be extended to other types of element including three-dimensional ones. Several example problems are discussed and illustrated, and comparisons are made with other approaches where these can also be used.

Short-pulse-laser-heated plasma experiments

Abstract: The interaction between time-tailored, high-intensity laser radiation and spherical light-element targets is investigated. Simultaneous measurements of the laser plasma electron temperature, charged-particle distribution, absorbed energy, neutron yields and backscattered light intensity and spectra were made. For 100 ps, wide laser pulses focused to intensities ranging between 3 × 1015 and 2 × 1016 W/cm2 on 150-200μm-diameter LiD and C36D74 targets, plasmas with electron temperatures up to 1.2 keV and target-generated yields of up to 4 × 104 neutrons are produced. The absorption can be accounted for by inverse bremsstrahlung and no.significant contributions due to parametric instabilities are observed. No more than 10−1 of the incident laser energy is back-reflected into spectral components at ωL, (3/2,)ωL, and 2ωL.

Linear Estimation of the Scale Parameter of the First Asymptotic Distribution of Extreme Values

Abstract: A Lagrange multiplier technique is used to obtain linear, minimum-variance, unbiased estimators for the scale parameters of the first asymptotic distributions of smallest and largest values with known mode. Coefficients for multiplying ordered observations are computed for complete and censored samples of size n = 1(1) 15. Each sample of size n is censored from above and all m-order-statistic estimators are obtained where m ? n. Then the smallest subset of # order statistics from the set of m available order statistics is found which yields a 99% efficiency relative to the m-order-statistic estimator. The Cramer-Rao lower bound for the variances of the estimators for complete samples is derived and tabled for n = 1(1) 15. For censored samples the asymptotic variances of the maximum-likelihood m-order-statistic estimators are presented for comparative purposes.