Showing papers on "Beam (structure) published in 1999"

Self-collimating phenomena in photonic crystals

Abstract: We found that self-determining collimated light is generated in a photonic crystal fabricated on silicon. The divergence of the collimated beam is insensitive to that of the incident beam and much smaller than the divergence that would be generated in conventional Gaussian optics. The incident-angle dependence of the self-collimated light propagation including lens-like divergent propagation was interpreted in terms of the highly modulated dispersion surfaces with inflection points, where the curvature changes from downward to upward corresponding to respectively a concave/convex-lens case. This demonstration is an important step towards controlling beam profile in photonic crystal integrated light circuits and towards developing “photonic crystalline optics.”

Influence of beam polarization on laser cutting efficiency

Abstract: The three-dimensional (3D) theory of laser cutting is presented. The cutting efficiency determined by its ultimate parameters at different types of polarization is estimated. The physical reasons for limitations of ultimate cutting parameters at a plane P-polarized beam are displayed. In the case of cutting metals with a large ratio of sheet thickness to width of the cut, the laser cutting efficiency for a radially polarized beam is 1.5 - 2 times larger than for plane P-polarized and circularly polarized beams. The possibility of generating the radially polarized beam is discussed.

Transfer Lengths and Bond Strengths for Composites Bonded to Concrete

Abstract: An effective method for strengthening existing concrete beams in bending consists in bonding fiber-reinforced composite laminates on the tension faces of the beams. Laboratory tests on beam specimens show, however, that it is often difficult to develop the full composite strength capacity because of premature failure due to delamination and peeling-off of the laminate. Conditions at the interface between the composite and concrete are not yet fully understood, and studies are required to determine the development lengths needed to achieve the composite's strength capacity. This paper reports results of an experimental and theoretical investigation of composite-to-concrete bonded joints. A new and fairly simple experimental apparatus capable of investigating the shear conditions between the composite and concrete is presented to determine the stress and strain distribution profiles in a composite laminate that is bonded to the concrete. A brief description of the apparatus is given and experimental results are presented; a theoretical analysis for the behavior of such composite-to-concrete bonded joints has also been developed, and predictions of the numerical model are compared to the experimental results.

Chemical and thermal freeze-out parameters from 1 A to 2 0 0 A GeV

Abstract: The present knowledge about hadrons produced in relativistic heavy ion collisions is compatible with chemical freeze-out happening when the energy density divided by the particle density reaches the value of 1 GeV. This observation is used to determine the energy dependence of the chemical freeze-out parameters ${T}_{\mathrm{ch}}$ and ${\ensuremath{\mu}}_{B}^{\mathrm{ch}}$ for beam energies varying between 1 and $200A$ GeV. The consequences of this energy dependence are studied for various particle ratios. Predictions for particle ratios at beam energy $40A$ GeV are presented. The conditions for thermal freeze-out are also determined. These correspond either to an energy density of 45 ${\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{f}\mathrm{m}}^{3}$ or to a particle density of $0.05/{\mathrm{fm}}^{3}.$

The spatial behavior of nonclassical light

Abstract: Nonclassical effects such as squeezing, antibunching, and sub-Poissonian statistics of photons have been attracting attention in quantum optics over the last decade. Up to now most theoretical and experimental investigations have been carried out exclusively in the time domain while neglecting the spatial aspects by considering only one spatial mode of the electromagnetic field. In many situations such an approximation is well justified. There are, however, problems that do not allow in principle a single-mode consideration. This is the case when one wants to investigate the quantum fluctuations of light at different spatial points in the plane perpendicular to the direction of propagation of the light beam. Such an investigation requires a complete description of quantum fluctuations of light in both time and space and cannot be done within a single-mode theory. This space-time description brings about a natural generalization into the spatial domain of such notions as the standard quantum limit, squeezing, antibunching, etc. It predicts, for example, the possibility of generating a light beam with sub-Poissonian statistics of photons not only in time but also in the beam's transverse plane. Of particular relevance to the applications is a situation in which the cross section of the light beam contains several nonoverlapping areas with sub-Poissonian statistics of photons in each. Photodetection of such a beam produces several sub-shot-noise photocurrents depending on the number of independent areas with sub-Poissonian statistics. This is in marked contrast to the case of a single-mode sub-Poissonian light beam in which any attempt to collect light from only a part of the beam deteriorates the degree of shot-noise reduction. This property of multimode squeezed light opens a range of interesting new applications in optical imaging, optical parallel processing of information, parallel computing, and many other areas in which it is desirable to have a light beam with regular photon statistics across its transverse area. The aim of this review is to describe the recent development in this branch of quantum optics.

Accurate method for determining adhesion of cantilever beams

Abstract: Using surface micromachined samples, we demonstrate the accurate measurement of cantilever beam adhesion by using test structures which are adhered over long attachment lengths. We show that this configuration has a deep energy well, such that a fracture equilibrium is easily reached. When compared to the commonly used method of determining the shortest attached beam, the present method is much less sensitive to variations in surface topography or to details of capillary drying.

Working equations for piezoelectric actuators and sensors

Abstract: A new solution to the force, displacements, and charges developed in piezoelectric beams is derived. Differing from previous solutions, this development determines the neutral axis where the bending strains are zero and results in a closed form solution (without matrix inversion). With the closed form, simplifications become evident which increase understanding and facilitate calculations. These equations are than expanded to account for axial, built-in strains in the beam. A design example where axial forces exerted by the piezoelectric layer are important is presented.

VHF free-free beam high-Q micromechanical resonators

Abstract: Free-free beam, flexural-mode, micromechanical resonators utilizing non-intrusive supports to achieve measured Q's as high as 8,400 at VHF frequencies from 30-90 MHz are demonstrated in a polysilicon surface micromachining technology. The subject microresonators feature torsional-mode support springs that effectively isolate the resonator beam from its anchors via quarter-wavelength impedance transformations, minimizing anchor dissipation and allowing these resonators to achieve high Q with high stiffness in the VHF frequency range.

Ion beam therapy system and a method for operating the system

Abstract: The present invention relates to an ion beam therapy system comprising a plurality of separate patient treatment stations, a source of ions, an accelerator system for accelerating ions as a beam and an ion beam transport system for directing an ion beam from the accelerator to the treatment stations, wherein said ion beam transport system comprises at least a horizontal beam delivery system and at least a gantry (8) rotateable about an axis of rotation, wherein said gantry carries optics for receiving an ion beam on its axis of rotation transporting said ion beam away from the axis of rotation and returning the ion beam on a path intersecting the axis at a target isocentre delivering said ion beam by rotation of the gantry to said target isocentre from different angles between 0 and 360 degrees and wherein said gantry carries means for raster scanning said ion beam enclosing a vertical deflection means and a horizontal deflection means positioned upstream of a last bending magnet downstream the last quadrupole lens of said gentry, wherein said vertical deflection means and said horizontal deflection, so that the beam can be scanned over a predetermined controlled area around said target isocentre after passing said last bending magnet.

110 W fibre laser

Abstract: The authors demonstrate a Yb-doped double-clad fibre laser with a record average power output of >110 W CW in a single spatial mode beam. The conversion efficiency is /spl sim/58% with respect to the incident pump power.

Fiber Element for Cyclic Bending and Shear of RC Structures. I: Theory

Abstract: After a few years of successful application of the fiber beam element to the analysis of reinforced concrete (RC) frames, the introduction of the mechanisms of shear deformation and strength appears to be the next necessary step toward a realistic description of the ultimate behavior of shear sensitive structures This paper presents a new finite-beam element for modeling the shear behavior and its interaction with the axial force and the bending moment in RC beams and columns This new element, based on the fiber section discretization, shares many features with the traditional fiber beam element to which it reduces, as a limit case, when the shear forces are negligible The element basic concept is to model the shear mechanism at each concrete fiber of the cross sections, assuming the strain field of the section as given by the superposition of the classical plane section hypothesis for the longitudinal strain field with an assigned distribution over the cross section for the shear strain field Transve

Strengthening of Reinforced Concrete Beams with Externally Bonded Composite Laminates

Abstract: A particularly challenging problem confronting engineers in the revival of U.S. infrastructure is the rehabilitation of concrete structures. This paper summarizes the results of experimental and analytical studies concerning the flexural strengthening of reinforced concrete beams by the external bonding of high-strength, lightweight fiber reinforced plastic (FRP) plates to the tension face of the beam. Twenty-four large-scale beams were tested experimentally to evaluate the strength enhancement provided by the FRP plates. An inelastic section analysis procedure was developed that accurately predicts the load displacement response of the retrofitted beams. A nonlinear finite element method analysis was also conducted that corroborates the results from the experimental study and inelastic section analysis.

Aligning molecules with intense nonresonant laser fields

Abstract: Molecules in a seeded supersonic beam are aligned by the interaction between an intense nonresonant linearly polarized laser field and the molecular polarizability. We demonstrate the general applicability of the scheme by aligning I2, ICl, CS2, CH3I, and C6H5I molecules. The alignment is probed by mass selective two dimensional imaging of the photofragment ions produced by femtosecond laser pulses. Calculations on the degree of alignment of I2 are in good agreement with the experiments. We discuss some future applications of laser aligned molecules.

Long, narrow all-light atom guide.

Abstract: A 1-mm-diameter all-light atom guide capable of transporting ultracold atoms tens of centimeters with high efficiency is described. We made the atom tunnel, a dark hollow beam that is blue detuned from resonance, by passing a few tens of milliwatts of power from a TEM00 diode laser beam through an optical sequence composed of three axicons and a simple lens. We demonstrate transport of 108 Cs atoms approximately 20 cm with minimal heating. We show that it is possible for one to control the direction and speed of the atoms in the tunnel by varying the detuning of the tunnel beam.

The beam emission spectroscopy diagnostic on the DIII-D tokamak

Abstract: A beam emission spectroscopy system has been installed on DIII-D to provide localized density fluctuation measurements for long-wavelength turbulent modes with k⩽3 cm−1 which are typically associated with anomalous radial transport. High signal-to-noise fluctuations measurements are accomplished through use of high speed electronics to maintain a frequency response of over 500 KHz and cryogenically cooled amplifiers and detectors to reduce electronic noise. The optics and neutral beam-sightline geometry have been optimized to allow for spatial resolution of Δr⩽1 cm. In addition, a half-scale two-dimensional (2D) fiber array to measure the 2D turbulent density field, necessary to measure the full S(kr,kθ) wavenumber spectra, has been implemented and initial results obtained.

Dynamics and flow structures in the turbulent wake of rigid and flexible cylinders subject to vortex-induced vibrations

Abstract: We present simulation results of vortex-induced vibrations of an infinitely long flexible cylinder at Reynolds number Re = 1000, corresponding to a ‘young’ turbulent wake (i.e. exhibiting a small inertial subrange). The simulations are based on a new class of spectral methods suitable for unstructured and hybrid grids. To obtain different responses of the coupled flow–structure system we vary the structure's bending stiffness to model the behaviour of a vibrating inflexible (rigid) cylinder, a cable, and a beam. We have found that unlike the laminar flow previously studied, the amplitude of the cross-flow oscillation is about one diameter for the cable and the beam, close to experimental measurements, but is lower for the rigid cylinder. We have also found that for the latter case the flow response corresponds to parallel shedding, but for the beam and cable with free endpoints a mixed response consisting of oblique and parallel shedding is obtained, caused by the modulated travelling wave motion of the structure. This mixed shedding pattern which alternates periodically along the span can be directly related to periodic spatial variation of the lift force. In the case of structures with pinned endpoints a standing wave response is obtained for the cylinder; lace-like flow structures are observed similar to the ones seen in the laminar regime. Examination of the frequency spectra in the near wake shows that at Re = 1000 all cases follow a −5/3 law in the inertial range, which extends about half a decade in wavenumber. However, these spectra are different in all three cases both in low and high frequencies, with the exception of the beam and cable, for which the high-frequency portion is identical despite the differences in the displacement time history and the large-scale features of the corresponding flow.

An acoustical helicoidal wave transducer with applications for the alignment of ultrasonic and underwater systems

Abstract: A simple four‐panel transducer capable of producing a beam with a screw‐dislocation along its axis was constructed and evaluated. A screw‐dislocation in a wave front is characterized by a phase dependence about the dislocation axis that varies as exp(−imφ), where m is an integer and φ is the angle about the axis. At the axis the phase is indeterminate and as a result there is a corresponding null in the pressure magnitude. To generate a wave front with these characteristics, a four‐panel 3‐1 composite piezoelectric transducer was driven with the appropriate phasing of the panels to create dislocation along the beam axis. As a result the beam does not possess cylindrical symmetry, however, the dislocation is found to exist in both the far and near fields of the transducer. This null then clearly indicates the axis of the beam at all distances and has the potential to be used as an aid in the alignment of objects in sonar experiments or other similar applications. A related transducer was summarized previously [J. Acoust. Soc. Am. 103, 2971 (1998)] and is also discussed here for the purposes of comparison. [Work supported by the Office of Naval Research.]

Monte Carlo simulation of a typical 60Co therapy source.

Abstract: The BEAM Monte Carlo code is used to simulate the 60Co beam from an Eldorado 6 radiotherapy unit and to calculate the relative air-kerma output factors as a function of field size. The unit is realistically modeled, including source capsule, housing and collimator assembly. The calculated relative air-kerma output factors at SSD=80.5 cm agree to within 0.1% with measured values. It is shown that the variation of the output factor is almost entirely due to scattered photons from the fixed and adjustable collimators and there is no effect of shadowing primary photons. The influence of the geometry of the collimation system on the photon spectra on-axis is shown to be small but finite. The calculated buildup region of a depth-dose curve in a water phantom irradiated by a narrow and a broad 60Co beam is shown to agree with experimental data at the 2% to 3% level. Unlike previous calculations, the results accurately predict the effects of electron contamination from the surface to dose maximum. The variation of electron contamination with field size is also presented, as are spectra as a function of field size.

Strengthening Reinforced Concrete Beams Using Fiber Reinforced Polymer (FRP) Laminates

Abstract: The behavior of reinforced concrete beams strengthened with various types of fiber reinforced polymer (FRP) laminates is presented. The experimental program included strengthening and testing 24 simply supported rectangular cross section beams. Each beam was initially loaded above its cracking load. The cracked beams were strengthened with FRP laminates and then tested until complete failure. Five available strengthening systems of various types of carbon/glass fiber reinforced polymer (CFRP/GFRP) strengthening materials were used. These materials included two types of CFRP sheets, bi- and unidirectional GFRP sheets, and CFRP plates. The effects of strengthening on deflection, failure load and failure mode, strain, and beam ductility are discussed. In addition, the influence of different numbers of FRP layers, type of epoxy, and strengthening pattern on the behavior of beams was examined. The ratio of absorbed energy at failure to total energy, or energy ratio, was used as a measure of beam ductility. It is concluded that, in addition to the longitudinal layers, the fibers oriented in the vertical direction forming a U-shape around the beam cross section significantly reduce beam deflections and increase beam load carrying capacity. Furthermore, the presence of vertical and horizontal sheets, together with a proper epoxy, can lead to a doubling of the ultimate load carrying capacity of the beam. However, all the strengthened beams experienced brittle failure, mandating a higher factor of safety in design.

Medium Effects in Kaon and Antikaon Production in Nuclear Collisions at Subthreshold Beam Energies

Abstract: Production cross sections of ${K}^{+}$ and ${K}^{\ensuremath{-}}$ mesons have been measured in $\mathrm{C}+\mathrm{C}$ collisions at beam energies per nucleon below and near the nucleon-nucleon threshold. At a given beam energy, the spectral slopes of the ${K}^{\ensuremath{-}}$ mesons are significantly steeper than the ones of the ${K}^{+}$ mesons. The excitation functions for ${K}^{+}$ and ${K}^{\ensuremath{-}}$ mesons nearly coincide when correcting for the threshold energy. In contrast, the ${K}^{+}$ yield exceeds the ${K}^{\ensuremath{-}}$ yield by a factor of about 100 in proton-proton collisions at beam energies near the respective nucleon-nucleon thresholds.

Monte Carlo modelling of electron beams from medical accelerators

Abstract: Monte Carlo simulation of radiation transport is considered to be one of the most accurate methods of radiation therapy dose calculation. With the rapid development of computer technology, Monte Carlo based treatment planning for radiation therapy is becoming practical. A basic requirement for Monte Carlo treatment planning is a detailed knowledge of the radiation beams from medical accelerators. A practical approach to obtain the above is to perform Monte Carlo simulation of radiation transport in the medical accelerator. Additionally, Monte Carlo modelling of the treatment machine head can also improve our understanding of clinical beam characteristics, help accelerator design and improve the accuracy of clinical dosimetry by providing more realistic beam data. This paper summarizes work over the past two decades on Monte Carlo simulation of clinical electron beams from medical accelerators.

Characterization of contact electromechanics through capacitance-voltage measurements and simulations

Abstract: Electrostatically actuated polysilicon beams fabricated in the multiuser MEMS process (MUMPs) are studied, with an emphasis on the behavior when the beam is in contact with an underlying silicon nitride dielectric layer. Detailed two-dimensional (2-D) electromechanical simulations, including the mechanical effects of stepups, stress-stiffening and contact, as well as the electrical effects of fringing fields and finite beam thickness, are performed. Comparisons are made to quasi-2-D and three-dimensional simulations. Pull-in voltage and capacitance-voltage measurements together with 2-D simulations are used to extract material properties. The electromechanical system is used to monitor charge buildup in the nitride which is modeled by a charge trapping model. Surface effects are included in the simulation using a compressible-contact-surface model. Monte Carlo simulations reveal the limits of simulation accuracy due to the limited resolution of input parameters.

Thermal arched beam microelectromechanical switching array

Abstract: A MEMS actuator is provided that produces significant forces and displacements while consuming a reasonable amount of power. The MEMS actuator includes a microelectronic substrate, spaced apart supports on the substrate and a metallic arched beam extending between the spaced apart supports. The MEMS actuator also includes a heater for heating the arched beam to cause further arching of the beam. In order to effectively transfer heat from the heater to the metallic arched beam, the metallic arched beam extends over and is spaced, albeit slightly, from the heater. As such, the MEMS actuator effectively converts the heat generated by the heater into mechanical motion of the metallic arched beam. A family of other MEMS devices, such as relays, switching arrays and valves, are also provided that include one or more MEMS actuators in order to take advantage of its efficient operating characteristics. In addition, a method of fabricating a MEMS actuator is further provided.