Showing papers on "Beam (structure) published in 1976"

Instability, Ductility, and Size Effect in Strain-Softening Concrete

Abstract: Analysis of structural instability due to strain-softening (ie, declining branch of the stress-strain diagram) is presented In a continuum, strain-softening is impossible; it can exist only in a heterogeneous material Failure occurs by unstable localization of strain or beam curvature, in which the stored strain energy of the structure is transferred into a small strain-softening region whose size is several times the aggregate size, or the spacing of reinforcement, or the depth of the beam The existence of a lower limit on the size of this region permits ductility, along with its dependence on the size and stored energy, to be predicted by a stability analysis Calculations of limit loads and moment redistributions in strain-softening beams and frames must include instability checks of possible curvature localization The same applies to finite element analyses of reinforced concrete structures with account of tensile cracking, and predictions of limit loads of these structures which are questionable because they depend on the size of the finite elements

Ion-beam-deposited polycrystalline diamondlike films

Abstract: X‐ray and electron beam diffraction analyses have been carried out on thin films deposited from a beam of carbon ions. Results show that the films consist of a polycrystalline background of cubic diamond with a particle size of 50–100 A with single‐crystal regions up to 5 μm in diameter.

The Linear and Self-Consistent Nonlinear Theory of the Electron Cyclotron Maser Instability

Abstract: In this paper the linear and nonlinear theory of the electron cyclotron maser instability is considered. The configuration used to study the maser instability consists of relativistic electrons gyrating about and drifting along a uniform magnetic field within a parallel plate waveguide. Relativistic effects associated with the gyrating electrons are responsible for excitation of the transverse electric mode in the waveguide. Linear theory shows that the growth rate maximizes when the axial beam velocity coincides with the axial wave group velocity of the excited electromagnetic wave. This allows us to perform the nonlinear analysis in a frame where both the axial wave number and axial beam velocity vanish. We have found that the maser instability exists only if the perpendicular beam energy exceeds a threshold value. Our analysis also describes the temporal nonlinear evolution of the field amplitude and frequency of a single excited wave. The nonlinear wave dynamics are self-consistently determined from the nonlinear particle orbits through the force and wave equations. The nonlinear analysis shows that there are two possible mechanisms for the saturation of the unstable wave: 1) depletion of the available free energy associated with the rotating particles and 2) phase trapping of the gyrating electrons in the wave. The initial beam parameters determine which of the two mechanisms is responsible for saturation. Competition between the two saturation mechanisms leads to a peaking in the energy conversion efficiency as a function of beam energy. Numerical results of the nonlinear formalism show that energy conversion efficiencies from the particles to the wave can be as high as 60 percent in the beam frame. Furthermore, by appropriately contouring the external magnetic field, among other things, efficiencies as high as 70 percent can be realized.

Energetic ion distribution resulting from neutral beam injection in tokamaks

Abstract: The Fokker-Planck equation is studied for an energetic ion beam injected into a magnetized plasma consisting of Maxwellian ions and electrons with υthi ≪υb≪ υthe. The time evolution of the fast ion distribution is given in terms of an infinite sum of Legendre polynomials for distributions that are axisymmetric about the magnetic field. The effect of charge exchange is included. The resulting ion distribution is somewhat isotropic for velocities much less than the injection velocity, however, the distribution is sharply peaked in both energy and pitch angle for velocities near the injection velocity. Approximate asymptotic expressions are given for the distribution in the vicinity of the injected beam and for velocities greater than the injection velocity. The effect of a weak parallel electric field is also given.

RF beam center location method and apparatus for power transmission system

Abstract: In wireless power transmission systems, in order to provide maximum possible efficiency in the transfer of power, the receiving element of the system must intercept the greatest possible portion of the transmitted energy beam. By having a planar array of receiver elements that are symmetrically located about a physical center, it becomes possible to determine the location on the array of the center of energy of the incident beam. This information is obtained as follows. Sum the output energy of all the receiver elements to the right and left of the vertical center of the array. Determine the difference and sum of these two amounts. Divide the difference by the sum. The result is an indication of the degrees that the center of the incident beam is off in azimuth. Sum the output energy of all the receiver elements above and below the horizontal center of the array. Determine the difference and sum of these two amounts. Divide the difference by the sum. The result is an indication of the degrees that the center of the incident beam is off in elevation.

Study of the frequency distribution of the fluorescent light induced by monochromatic radiation

Abstract: The frequency distribution of the fluorescent light induced by monochromatic dye laser radiation was investigated. To exclude the influence of the Doppler width a strongly collimated atomic beam was used. The spectrum was measured by means of a piezoelectrically tunable spherical Fabry Perot. The interaction region between the laser light and the atomic beam was placed into the center of the interferometer. Thus the observed fluorescence spectrum was considerably more intense than in the case where the interferometer is used separately from the beam. The fluorescence spectrum was observed for different directions of polarization of the incident laser beam. In the case of weak excitation the spectrum consists of a sharp component essentially due to elastically scattered light. At high intensities a structure of three components is observed which is in agreement with theoretical predictions when circularly polarized light is used for excitation.

Electromagnetic instabilities driven by unequal proton beams in the solar wind

Abstract: The paper sets forth a numerical investigation of the linear dispersion relation for typical solar wind conditions at 1 AU during those times (high-speed streams) when a secondary beam of protons drifting relative to the main proton component is present. Three beam-driven instabilities were found to occur as the beam drift velocity approaches the Alfven speed: (1) a pure, field-aligned magnetosonic wave that is most important at relatively high beta and/or high beam drift speeds; (2) an oblique magnetosonic wave having highest growth rates 15-30 deg from the magnetic field; and (3) an oblique Alfven wave having maximum growth rates at increasing angle to the magnetic field. The linear growth rates for the field-aligned magnetosonic and the Alfven oblique modes are investigated as a function of relative beam density, varying anisotropic pitch angle distributions for the various components, electron temperature, and electron heat flux.

Pulsar average wave forms and hollow-cone beam models

Abstract: An analysis of pulsar average waveforms at radio frequencies from 40 MHz to 15 GHz is presented. The analysis is based on the hypothesis that the observer sees one cut of a hollow-cone beam pattern and that stationary properties of the emission vary over the cone. The distributions of apparent cone widths for different observed forms of the average pulse profiles (single, double/unresolved, double/resolved, triple and multiple) are in modest agreement with a model of a circular hollow-cone beam with random observer-spin axis orientation, a random cone axis-spin axis alignment, and a small range of physical hollow-cone parameters for all objects.

Optimization of Vibrating Beams with Respect to Higher Order Natural Frequencies

Abstract: We seek the design of a transversely vibrating beam that yields a maximum value of a higher natural frequency ωn of specified order n. The beams considered are thin, elastic and of geometrically similar cross sections. The volume, length, and boundary conditions of the beams are assumed to be given. The governing equations are derived by the calculus of variations and solved numerically by successive iterations for low values of n. A simple method of obtaining optimal solutions corresponding to any higher value of n by aid of scaled optimal beam elements is developed. The paper presents the optimal solutions for beams with any combination of free, simply supported and clamped ends for any given value of n.

Method and apparatus for laser treatment of geological formations

Abstract: A method and apparatus are disclosed for drilling gas, oil or geothermal wells in geological formations and for "fracing" the pay zones of such wells to increase recovery, using a laser beam projected into the well bore along a beam guide so as to make available laser energy adequate to melt or vaporize the formation under down-hole conditions. Fluid circulation is established via the beam guide to keep the beam path adequately free of contaminants to permit drilling. A novel orifice window is provided to allow transmitting the high energy beam from the surface into a high-pressure, down-hole environment.

Optical focussing device

Abstract: A device for focussing a read-out light beam on a moving data carrier. It comprises an objective lens which causes said read-out beam to converge on the data carrier; a cylindrical lens arranged in the path of a light beam reflected by the data carrier; and photo-electric means for detecting the reflected beam and arranged in such a fashion that the light spot obtained there, normally substantially circular in shape, is distorted by elongation if focussing of the read-out beam on the data carrier is incorrect.

Elastic and rotationally inelastic diffraction of hydrogen molecular beams from the (001) face of LiF at 80°K

Abstract: The in‐plane scattering of H2 molecules at thermal energies from the (001) face of LiF was studied at low temperatures, by means of nozzle beam techniques. Both elastic and rotationally inelastic diffraction peaks were resolved over a wide range of incident and final angles. Taking into account the properties of the supersonic beam, diffraction probabilities were derived for the observed peaks. A discussion of the results in light of the present theories is given. The large probability of occurrence of rotational transitions in the presence of the surface is pointed out.

Kinetic theory of a relativistic beam

Abstract: A Fokker–Planck equation is derived to study the evolution of a stable, low‐current beam propagating in a gas‐plasma medium. Small‐angle scattering of the beam particles by the medium causes diffusion in the phase space projected transverse to the direction of propagation. The projected components of dynamical friction vanish. As a result, there is a continued input of energy into the transverse particle motions, which is taken up in expansion against the pinch field. A quasi‐static Bennett equilibrium, with isothermal distribution of transverse momenta, is shown to be a similarity solution of the Fokker–Planck equation with scale radius increasing in accord with Nordsieck’s formula. An H theorem is proved and the Bennett distribution is shown to minimize both H and −dH/dt; hence, it is the time‐dependent asymptotic state. The predicted current profile and radius are shown to be in fair agreement with experiment.

Lower hybrid instability driven by a spiraling ion beam

Abstract: A lower-hybrid instability with ion cyclotron harmonics is observed to be resonantly driven by an ion beam injected obliquely to the confining magnetic field, in agreement with a linear, warm-plasma theory. Quasilinear velocity-space diffusion of the beam is observed. (AIP)

Ultrasonic fan beam scanner for computerized time-of-flight tomography

Abstract: A fast ultrasonic scanner for soft tissue imaging and/or characterization has a transmitter with a divergent beam pattern and an arcuate array of receivers. The propagation delay time of a short acoustic pulse traversing a water bath and immersed specimen depends on the velocity distribution and is detected at each receiver. Digital data is derived from the receiver signals at many angular positions of the scan apparatus and is used to compute a reconstruction of the velocity distribution in the specimen. The instrument provides quantitative measurements of bulk tissue and has application in breast examination and mass screening for breast cancer.

Return-beam-induced oscillations in self-coupled semiconductor lasers

Abstract: When a stripe-geometry heterostructure GaAlAs laser is irradiated by its own output beam, oscillations in the 1–100 MHz range are generated in the laser, and the frequency f of the oscillation varies with the length 2L of the optical path of the beam returned as f = α(c/2L), with α ≪ 1, where c is the velocity of light.

Picture quality of computed tomography

Abstract: Some limitations that impair the picture quality of computed tomography are presented. Picture grain is analyzed in detail and its relationship to different matrix sizes is demonstrated. The choices of matrix size for viewing various parts of the body are defined, and the need for higher resolution in the future is debated. Comparison is made between two scanning systems: the moving fan beam with rotation and the simple rotating fan beam. Possible artifacts in the picture caused by drift and delay in dector response are discussed.

Atomic and ionic beam source utilizing pulsed laser blow off

Abstract: When a pulsed laser is focused on a material surface and sufficient energy is suddenly deposited in that surface, local vaporization occurs and surface material ’’blows off.’’ If this occurs in a vacuum, intense beams of even refractory materials can be formed by collimating at distances where the expanding gas bubble has become collisionless. A simple model is presented which predicts experimentally observed particle velocity distributions and angular distributions. Of particular significance is the implication from these distributions that the equilibrium temperature of the initial dense gas bubble is much lower than a cursory on‐axis velocity measurement might imply. This is critical in evaluating effects due to beam particle internal energy. Laser parameters and experimental configurations determine bubble temperature and hence internal energy and in the extreme whether atoms or ions (plasma) are produced; particle velocities may be effectively varied by choosing appropriate angles and arrival times.

Bragg diffraction of Gaussian beams by periodically modulated media

Abstract: Analytical and numerical results are given on the diffraction of optical beams by a periodically modulated dielectric medium, which represents a thick hologram or an acoustic column. By using a rigorous representation for the field of a realistically bounded beam incident at a Bragg angle, we examine both the refracted beam and the beam due to Bragg scattering inside the periodic medium. The Bragg-scattered beam is formed by continuous coupling of energy from the refracted beam into the Bragg-scattered wave. As the Bragg-scattered wave also couples part of its energy back to the refracted wave, the continuous coupling of energy back and forth between these two waves results in a diffusion of energy over a wide region. For a sufficiently thick modulated layer, both the refracted and the Bragg-scattered beams split into two beams. Because of this distortion of the beam profiles, the diffraction efficiency is found to be smaller than that accounted for by previous approaches using a single incident plane-wave model.

Radiation-sensitive record with protected sensitive surface

Abstract: A record structure for recording with a radiation beam uses a transparent disc or drum, a coaxial backing disc or drum and two coaxial resilient ring-shaped seals to provide an enclosure between the discs or drums. Radiation-sensitive recording material on the side of the disc or drum within the enclosure is thereby protected from ambient dust particles and other contaminations, while the gaseous or solid by-products of the inter-action between the recording material and the radiation beam in the area around which the beam impinges are substantially contained between the disc surfaces and cannot produce deposits on an objective lens used to focus the radiation beam or poison the user.

Analysis of Reinforced Fibrous Concrete Beams

Abstract: A method has been developed to predict the ultimate strength of concrete beams reinforced with a combination of steel bars and randomly distributed steel fibers. Strengths of three full-size beams were obtained experimentally to verify the analytical method. The method, which applies to beams using a cement-rich mortar mix with the steel fibers, shows good correlation with experimental results. A dynamic bond stress of 333 psi was assumed for the fibers. Fiber lengths were from 1-1/2 in. to 2-1/4 in. and the fiber amount was varied from 1.22% to 1.51% by volume. The test programs showed that: (1) Steel fibrous concrete increased the flexural load capacity of the reinforced beams by about 25%; (2) crack width and crack spacing were less than in a conventional reinforced beam; and (3) the post-cracking stiffness of the fibrous beams was greater than a conventional beam.