The Influence of Metal Grids on the Ion-Beam Characteristics in a Laser-Plasma Source

Instruments and Experimental Techniques - The influence of metal grids on the characteristics of particles extracted from a plasma was investigated in the course of development of the laser-plasma...     

Analysis of acoustic radiation of circular piezoelectric plates enclosed in a flat rigid shield

Acoustic radiation of circular piezoelectric plates encased in a flat rigid shield is analyzed on the basis of a combined numerical-analytical method. It is shown that a plate made of piezoelectric ceramics of the second type with a higher dynamic coefficient of electromechanical coupling ensures a higher sensitivity in the transmission mode and a wider band of radiated frequencies. In addition, a plate of this type of piezoceramics has a lower level of side lobes of the directivity characteristic because of the more uniform normal-displacement distribution on its flat surface.     

Molecular dynamic simulation of motion in solid polymers. Rotator phase of n-alkane

The method of molecular dynamic computer simulation is applied for the investigation of structure and dynamics of C₁₀-n-alkane in the solid state. Analyzed is the behaviour of individual molecules as well as of a whole crystal. The investigated system consists of 31 molecules of C₁₀-n-alkane at the temperature 150–275 K. The influence of chemical defects, such as methylene group or chlorine atom, on the dynamics of the system is considered.     

On some paradoxes in the analysis of pulsed operation mode of acoustic transducers by the method of integral transformations

Reasons for the appearance of “precursors” and “tails” in the analysis of pulsed operation mode of acoustic transducers by the method of integral transformations are considered. It is emphasized that the reason is the impossibility to make an accurate allowance for the actual dependence of the complex wave number on frequency over the entire frequency range.     

Analysis of acoustic radiation of circular piezoelectric plates with complex shapes in a flat rigid shield

Acoustic radiation from circular piezoelectric plates with complex shapes in a flat rigid shield is analyzed on the basis of a combined numerical-analytical method. It is shown that such piezoelectric plates of the first-type ceramics ((??????C-3) provide a higher sensitivity in the transmission mode in comparison to a standard plate. In addition, piezoelectric plates of this type have a lower level of the side lobes of the directivity characteristic because of a more uniform distribution of the normal displacement on a flat radiating surface.     

Characteristic Vibrational Modes of Piezoceramic Plates with Variable Thicknesses

The paper considers characteristic oscillations of finite round piezoceramic plates of variable thicknesses, with convex and concave surfaces. Characteristic oscillation spectra of these plates were studied in the regimes of resonance and antiresonance, and the dynamic coefficient of electro-mechanical coupling was calculated as a function of plate dimensions and displacement distribution over the surface. Improvements in the characteristics of thickness oscillations in the plates (the possibility of increasing the coefficient of electro-mechanical coupling and smoothing the distribution of the normal displacement component over the plate surface) owing to variable thicknesses of piezoceramic plates have been analyzed. This can be achieved only in plates with convex conical surfaces, whereas concave conical surfaces of plates lead to inferior characteristics of thickness oscillations. Improvements in characteristics of thickness oscillations in piezoelectric plates fabricated from ceramics with various compositions will be discussed.     

Analysis of Natural Vibrations of Round Bimorph Piezoelectric Ceramic Plates of Arbitrary Dimensions. I. Simply Supported Piezoelectric Plate

The problem of natural vibrations of a round bimorph simply supported piezoelectric ceramic plate (piezoplate) of an arbitrary thickness with arbitrary axisymmetric electrodes is solved in the exact formulation using the method of finite elements. The natural frequency spectra are analyzed in the resonance and antiresonance regimes. The displacement distribution over the piezoplate surfaces (vibration modes) is also analyzed and the dynamic electromechanical coupling coefficient (DCC) is investigated as a function of the relative plate thickness for various piezoelectric ceramic compositions. Round bimorph piezoplates with partial electrodes have been also considered. The results obtained make it possible to estimate the limits of applicability of an approximate one-dimensional theory and choose the optimum geometric dimensions of the plate and electrodes that ensure the maximum DCC.     

Characteristics of a pulse–periodic CO<Subscript>2</Subscript> laser for applications in the field of laser-produced plasma

The design of a pulse–periodic СО₂ laser oscillator that operates at a high level of the specific energy deposition into a self-sustained discharge is described. The laser is intended for generating pulses with a high-density radiation flux in a laser-plasma generator of multiply charged ions at the Institute of Theoretical and Experimental Physics (ITEP). The results of investigations of the spatiotemporal and energy characteristics of laser output radiation in a wide range of the discharge excitation level and the mixture composition are presented. The optimal conditions are determined under which the oscillator provides an output energy of >10 J in a pulse with a duration of ~28 ns and a record specific peak radiation power of 190 MW per liter of the active volume of a CO₂: N₂: He mixture. The high quality of the spatial characteristics was confirmed in measurements of the radial energy-density distribution in the far-field zone, whose characteristic size is close to the diffraction limit.     

Molecular dynamics of oligopeptides. 1. The use of long trajectory and high temperature data for determination of statistical weight of conformational substates

The purpose of this study was to assess the accuracy of chest x-ray (CXR) interpretation in the diagnosis of pneumocystis carinii pneumonia (PCP), bacterial pneumonia (BP), and pulmonary tuberculosis (TB) in human immunodeficiency virus (HIV)-positive patients and to identify the frequency with which these infections mimic one another radiographically. The admitting CXRs of 153 HIV-positive patients with laboratory proven BP (n = 71), PCP (n = 73), and TB (n = 9) and those of 10 HIV-positive patients with no active disease were reviewed retrospectively and independently by three radiologists who were blinded to clinical and laboratory data. Median percent accuracies were as follows: TB, 84%; PCP, 75%; BP, 64%; and no active disease, 100%. Fifteen of 153 cases (9.8%) were shown to mimic other infections radiographically. A confident and accurate diagnosis can be made radiographically in the majority of cases of PCP, BP, and TB in HIV-positive patients at the time of hospitalization. In approximately 10% of cases, these infections may mimic one another radiographically.     

Analysis of Natural Vibrations of Round Flexing Membrane-Type Piezoelectric Transducers with an Arbitrary Dimension Ratio

The problem of natural vibrations of a round exing membrane-type piezoelectric transducer (piezoelectric plate) with an arbitrary dimension ratio is solved in the exact formulation using the finite element method for two variants of fastening the membrane (simply supported and rigidly fixed plate). The natural frequency spectra are analyzed in the resonance and antiresonance regimes. The displacement distributions over the plate surfaces are also analyzed and the dependences of the dynamic electromechanical coupling coefficient are investigated as functions of the geometric dimensions of the piezoelectric transducer, the plate material, and the piezoelectric ceramic composition. The optimum geometric dimensions of the transducer which ensure the maximum dynamic electromechanical coupling coefficient are determined and the value of the coefficient is estimated.