Showing papers on "Beam (structure) published in 2001"

Direct selective laser sintering of metals

Abstract: A method of fabricating a fully dense, three dimensional object by direct laser sintering is disclosed (flow chart). In a chamber (evacuated) with a partial pressure atmosphere, a degased powder layer is dispensed over a target surface and a beam of directed energy melts metallic powder in order to form a solid layer cross section. Another layer of powder is deposited and melted, along with a portion of the previous layer. The energy beam typically is in the form of a laser, scanning along a path resembling a parametric curve or another, arbitrary piecewise parametric curve (vector scan). In another embodiment, the previous layer is not remelted, thus creating an oxide film that acts as a clean stop to prevent unwanted downward growth.

Longitudinal Field Modes Probed by Single Molecules

Abstract: We demonstrate that a strong longitudinal, nonpropagating field is generated at the focus of a radially polarized beam mode. This field is localized in space and its energy density exceeds the energy density of the transverse field by more than a factor of 2. Single molecules with fixed absorption dipole moments are used to probe the longitudinal field. Vice versa, it is demonstrated that orientations of single molecules are efficiently mapped out in three dimensions by using a radially polarized beam as the excitation source. We also show that there is no momentum or energy transport associated with the longitudinal field.

Prestack Gaussian‐beam depth migration

Abstract: Kirchhoff migration is the most popular method of three‐dimensional prestack depth migration because of its flexibility and efficiency. Its effectiveness can become limited, however, when complex velocity structure causes multipathing of seismic energy. An alternative is Gaussian beam migration, which is an extension of Kirchhoff migration that overcomes many of the problems caused by multipathing. Unlike first‐arrival and most‐energetic‐arrival methods, which retain only one traveltime, this alternative method retains most arrivals by the superposition of Gaussian beams. This paper presents a prestack Gaussian beam migration method that operates on common‐offset gathers. The method is efficient because the computation of beam superposition isolates summations that do not depend on the seismic data and evaluates these integrals by considering their saddle points. Gaussian beam migration of the two‐dimensional Marmousi test data set demonstrates the method’s effectiveness for structural imaging in a case w...

Three Dimensional Absolute Nodal Coordinate Formulation for Beam Elements: Theory

Abstract: The description of a beam element by only the displacement of its centerline leads to some difficulties in the representation of the torsion and shear effects. For instance such a representation does not capture the rotation of the beam as a rigid body about its own axis. This problem was circumvented in the literature by using a local coordinate system in the incremental finite element method or by using the multibody floating frame of reference formulation. The use of such a local element coordinate system leads to a highly nonlinear expression for the inertia forces as the result of the large element rotation. In this investigation, an absolute nodal coordinate formulation is presented for the large rotation and deformation analysis of three dimensional beam elements. This formulation leads to a constant mass matrix, and as a result, the vectors of the centrifugal and Coriolis forces are identically equal to zero. The formulation presented in this paper takes into account the effect of rotary inertia, torsion and shear, and ensures continuity of the slopes as well as the rotation of the beam cross section at the nodal points. Using the proposed formulation curved beams can be systematically modeled.

Bent-beam electrothermal actuators-Part I: Single beam and cascaded devices

Abstract: This paper describes electrothermal microactuators that generate rectilinear displacements and forces by leveraging deformations caused by localized thermal stresses. In one manifestation, an electric current is passed through a V-shaped beam anchored at both ends, and thermal expansion caused by joule heating pushes the apex outward. Analytical and finite element models of device performance are presented along with measured results of devices fabricated using electroplated Ni and p/sup ++/ Si as structural materials. A maskless process extension for incorporating thermal and electrical isolation is described. Nickel devices with 410-/spl mu/m-long, 6-/spl mu/m-wide, and 3-/spl mu/m-thick beams demonstrate 10 /spl mu/m static displacements at 79 mW input power; silicon devices with 800-/spl mu/m-long, 13.9-/spl mu/m-wide, and 3.7-/spl mu/m-thick beams demonstrate 5 /spl mu/m displacement at 180 mW input power. Cascaded silicon devices using three beams of similar dimensions offer comparable displacement with 50-60% savings in power consumption. The peak output forces generated are estimated to be in the range from 1 to 10 mN for the single beam devices and from 0.1 to 1 mN for the cascaded devices. Measured bandwidths are /spl ap/700 Hz for both. The typical drive voltages used are /spl les/12 V, permitting the use of standard electronic interfaces that are generally inadequate for electrostatic actuators.

Physics of Intense Charged Particle Beams in High Energy Accelerators

Abstract: Theoretical Models of Intense Non-neutral Particle Beams Particle Orbits in Periodic Focusing Field Configurations Nonlinear Kinetic Stability Theorem, Vlasov-Maxwell Description of Periodically-Focused Intense Beam Equilibria Statistically-Averaged Rate Equations Hamiltonian Averaging Techniques Applied to the Nonlinear Vlasov-Maxwell Equations Kinetic Stability Properties and Collective Oscillations in Intense Particle Beams Warm-Fluid Stability Properties and Collective Oscillations in Intense Particle Beams Special Topics on Intense Beam Propagation.

Measurement of the Beam-Spin Azimuthal Asymmetry Associated with Deeply-Virtual Compton Scattering

Abstract: The beam-spin asymmetry in hard electroproduction of photons has been measured. The data have been accumulated by the HERMES experiment at DESY using the HERA 27.6 GeV longitudinally polarized positron beam and an unpolarized hydrogen-gas target. The asymmetry in the azimuthal distribution of the produced photons in the angle φ relative to the lepton scattering plane was determined with respect to the helicity state of the incoming positron beam. The beam-spin analyzing power in the sinφ moment was measured to be -0.23±0.04(stat)±0.03(syst) in the missing-mass range below 1.7 GeV. The observed asymmetry is attributed to the interference of the Bethe-Heitler and deeply virtual Compton scattering processes.

Partially polarized Gaussian Schell-model beams

Abstract: We consider a class of beams that are both partially polarized and partially coherent from the spatial standpoint. They are characterized by a correlation matrix whose elements have the same form as the mutual intensity of a Gaussian Schell-model beam. We focus our attention on those beams that would appear identical to ordinary Gaussian Schell-model beams in a scalar treatment. After establishing some inequalities that limit the choice of the matrix parameters, we study the main effects of propagation. Starting from the source plane, in which the beam is assumed to be uniformly polarized, we find that in the course of propagation the degree of polarization generally becomes non-uniform across a typical section of the beam. Furthermore, we find that the intensity distribution at the output of an arbitrarily oriented linear polarizer is Gaussian shaped at the source plane whereas it can be quite different at other planes.

Piezoelectric thin film micromechanical beam resonators

Abstract: Piezoelectric micromechanical beam resonators are investigated for application to electromechanical filters. The derivation of a dynamic admittance model describing the electromechanical behavior of the resonators is presented. Predictions from this model indicate that piezoelectric beam resonators offer significant potential for high-frequency applications. Using a simple three-mask fabrication process based on zinc oxide active films, doubly-clamped piezoelectric beam resonators with center frequencies ranging from 158 kHz to 1.18 MHz are reported. Resonators with both ZnO and SiO 2 substrates have been realized, and triple-beam configurations investigated for reducing mechanical anchor losses. Measured quality factors range from Q =3700 at 158 kHz to Q =930 at 1.18 MHz.

Crossed-beam studies of neutral reactions: state-specific differential cross sections.

Abstract: ▪ Abstract Crossed-molecular-beam and laser techniques have enabled experimentalists to measure the state-resolved differential cross sections of elementary chemical reactions. This article reviews recent progress in this area. Particular emphasis is placed on some intriguing physical phenomena associated with a few benchmark reactions and how these measurements help in answering fundamental questions about reaction dynamics. We examine specifically the geometric phase effects in the reaction H + D2, the dynamical resonance phenomenon in F + HD, the unusually large spin-orbit reactivity in Cl(2P) + H2, the insertion reaction O(1D) + H2, and the mode-specific reactivity in Cl + CH4(ν). The give-and-take between experiment and theory in unraveling the physical picture of the dynamics is illustrated throughout this review.

Beam cooler for low-energy radioactive ions

Abstract: An ion beam cooler for mass-separated radioactive ion beams has been developed and tested at the IGISOL-type mass separator facility Technical description and characteristic properties are presented An energy spread below 1 eV and transmission efficiency of 60% were measured

Non-linear vibrations and stability of an axially moving beam with time-dependent velocity

Abstract: Non-linear vibrations of an axially moving beam are investigated. The non-linearity is introduced by including stretching effect of the beam. The beam is moving with a time-dependent velocity, namely a harmonically varying velocity about a constant mean velocity. Approximate solutions are sought using the method of multiple scales. Depending on the variation of velocity, three distinct cases arise: (i) frequency away from zero or two times the natural frequency, (ii) frequency close to zero, (iii) frequency close to two times the natural frequency. Amplitude-dependent non-linear frequencies are derived. For frequencies close to two times the natural frequency, stability and bifurcations of steady-state solutions are analyzed. For frequencies close to zero, it is shown that the amplitudes are bounded in time.

Thermal effects and their mitigation in end-pumped solid-state lasers

Abstract: Degradation in beam quality due to aberrated thermal lensing and depolarization loss due to stress induced birefringence in end-pumped edge-cooled solid-state lasers are investigated. A simple model for the dependence of thermal lensing and degradation in beam quality on the transverse intensity profile of the pump beam is presented. Experimental measurements of thermal lensing, degradation in beam quality and depolarization loss in a diode-bar-end-pumped Nd:YAG rod support the main predictions of the model and, in addition, show that there can be significant extra heating under non-lasing conditions compared to lasing conditions. The role of energy transfer upconversion as a mechanism for additional heat loading under non-lasing conditions is considered. Finally, various strategies for limiting the impact of thermal lensing and thermally induced birefringence on laser performance in simple end-pumped cavity configurations are reviewed.

THz interconnect with low-loss and low-group velocity dispersion

Abstract: We report the demonstration of a physically flexible, practicable THz interconnect with minimal pulse distortion and loss. The interconnect is a parallel-plate waveguide, with the TEM mode excited, constructed of two thin copper strips. The incoming 0.22 ps THz pulse broadens to 0.39 ps after propagating 250 mm in the waveguide and is also attenuated by a factor of ten. We show that this attenuation is mainly due to the finite conductivity of copper with some additional loss caused by the beam spread in the unguided dimension. The pulse broadening is due to the frequency-dependent loss since the group velocity dispersion is negligible.

Contact structure and production method thereof and probe contact assembly using same

Abstract: A contact structure for establishing electrical connection with contact targets. The contact structure is formed of a contactor carrier and a plurality of contactors. The contactor has an upper end oriented in a vertical direction, a straight beam portion oriented in a direction opposite to the upper end and having a lower end which functions as a contact point for electrical connection with a contact target, a return portion returned from the lower end and running in parallel with the straight beam portion to create a predetermined gap therebetween, a diagonal beam portion provided between the upper end and the straight beam portion to function as a spring.

Significance of midspan debonding failure in frp-plated concrete beams

Abstract: Reinforced concrete beams enhanced in flexure with adhesively-bonded fibre reinforced polymer plates are susceptible to a brittle form of failure, defined by delamination of the cover concrete attached to the adhesive that causes the plates to debond from the beam. This paper demonstrates that, while previous research has focused almost singularly on one debond mode in which concrete delamination progresses from the ends of the plates inwards, there exists another critical debond mode that initiates near flexural cracks in the midspan region of the plated beam and propagates out to the ends of the plates. Data from large-scale experimental work are presented to show that midspan debond action is triggered by high shear stresses transmitted from the plates through the adhesive to the cover concrete. These stresses arise initially from tension stiffening in the cracked concrete and corrosion of the embedded steel. It is shown that strain gauge data are required from both the bonded and exposed surfaces of t...

Description and dosimetric verification of the PEREGRINE Monte Carlo dose calculation system for photon beams incident on a water phantom.

Abstract: PEREGRINE is a three-dimensional Monte Carlo dose calculation system written specifically for radiotherapy. This paper describes the implementation and overall dosimetric accuracy of PEREGRINE physics algorithms, beam model, and beam commissioning procedure. Particle-interaction data, tracking geometries, scoring, variance reduction, and statistical analysis are described. The BEAM code system is used to model the treatment-independent accelerator head, resulting in the identification of primary and scattered photon sources and an electron contaminant source. The magnitude of the electron source is increased to improve agreement with measurements in the buildup region in the largest fields. Published measurements provide an estimate of backscatter on monitor chamber response. Commissioning consists of selecting the electron beam energy, determining the scale factor that defines dose per monitor unit, and describing treatment-dependent beam modifiers. We compare calculations with measurements in a water phantom for open fields, wedges, blocks, and a multileaf collimator for 6 and 18 MV Varian Clinac 2100C photon beams. All calculations are reported as dose per monitor unit. Aside from backscatter estimates, no additional, field-specific normalization is included in comparisons with measurements. Maximum discrepancies were less than either 2% of the maximum dose or 1.2 mm in isodose position for all field sizes and beam modifiers.

Headlamp system with selectable beam pattern

Abstract: A headlamp system stores a plurality of beam patterns on a microprocessor 32 that selects a predetermined beam pattern based on driving conditions and sends the predetermined beam pattern to a digital micromirror device (DMD) 18 that is capable of generating the desired beam pattern by adjusting the position of individual pixels on the DMD 18. Several vehicle parameters are inputs 36, 37, 38, 29, 70 to the microcontroller 32 and are used to determine the appropriate beam pattern to be produced by the DMD 18 and projected by the headlamp system of the present invention.

Brittle Failure and Bond Development Length of CFRP-Concrete Beams

Abstract: The performance of a series of 1.5 m-long reinforced concrete beams with CFRP plates bonded to the soffit is presented. Effects of the plate length, the reinforcing steel ratio, and the thickness of the concrete cover on behavior of the beams are discussed with particular emphasis on the brittle failure mode of the concrete ripping. It was found that before the brittle failure, the composite action of the beam could be divided into three distinct zones based on the distribution of the strains along the CFRP plate: (1) The “destressed” zone, (2) the “bond development” zone; and (3) the “fully composite” zone. Concrete ripping may be prevented by limiting the strain at the transition point of the composite zone of the plate. An analytical formulation to predict the failure load corresponding to the concrete ripping failure mode is proposed based on composite theory in combination with a strain limiting criterion. The strain limit can be determined based on simple bond tests with various bond lengths for a g...

Jamin-type interferometers and components therefor

Abstract: A retroreflector (32) has three mutually-orthogonal reflective surfaces arranged around an optical axis (12). The reflective surfaces stop short of the optical axis to provide a central region of the retroreflector which transmits incident light (M,N) and a peripheral region of the retroreflector which retroreflects incident light (C,D). When the reflector is used in a Jamin-type interferometer (10) with another reflector (34), this enables the interferometer to be used for measuring displacement between the reflectors. In the interferometer, a projected beam (M) is disposed between a pair of return beams (D,N) and/or one of the return beams (N) is disposed between a pair of the projected beams (C,M). This enables a first contiguous area of a face of a beam splitter (22) to be provided with a phase-shifting coating (28) to produce a phase quadrature relationship between a pair of interferogram beams (G,I). This simplifies the masking required when applying the coating (28). In manufacture of the beam splitting member (22), a thin-film, beam-splitting, metal coating is applied to the member, and the member and coating are baked so as to modify the phase shift produced by the coating to enable the phase quadrature relationship. During baking a beam of light is projected at the coating with an angle of incidence of substantially π/4 radians so that the beam is split into a transmitted component and a reflected component. The intensities or phases of the transmitted and reflected components are monitored during baking, and the baking is terminated when the monitored intensities or phases have a predetermined relationship. This improves the reliability and/or accuracy of the resulting phase shift.

Diagonal Shear Failure and Size Effect in RC Beams without Web Reinforcement

Abstract: This paper describes a formal theory, which explains the mechanism of shear failure in reinforced concrete (RC) ordinary (slender) beams without web reinforcement. The theory determines the type of splitting of concrete responsible for the diagonal shear failure of beams under concentrated loads and the location along the shear span where this splitting occurs. A simple expression is derived for the shear stress at failure as the product of the ratio of the neutral axis depth to the effective depth of beam times the splitting tensile strength of concrete. It is also shown how the nominal shear strength can be adjusted to take into account the size effect. The derived formula from this analysis is verified by comparisons to extensive sets of experimental data from the literature, which have been obtained on slender beams with various strengths of concrete (low and high), steel ratios, shear span to depth (a/d) ratios, and geometrical sizes.