Showing papers on "Beam (structure) published in 1997"

Novel Optical Trap of Atoms with a Doughnut Beam

Abstract: We have constructed a novel optical trap for neutral atoms by using a Laguerre-Gaussian (doughnut) beam whose frequency is blue detuned to the atomic transition. Laser-cooled rubidium atoms are trapped in the dark core of the doughnut beam with the help of two additional laser beams which limit the atomic motion along the optical axis. About ${10}^{8}$ atoms are initially loaded into the trap, and the lifetime is 150 ms. Because the atoms are confined at a point in a weak radiation field in the absence of any external field, ideal circumstances are provided for precision measurements. The trap opens the way to a simple technique for atom manipulation, including Bose-Einstein condensation of gaseous atoms.

Resonant grating waveguide structures

Abstract: Under certain conditions, a resonance phenomenon can occur in waveguide grating structures. Such structures have multilayer configuration, the most basic of which is comprised of a substrate, a thin dielectric layer or semiconductor waveguide layer, and an additional transparent layer in which a grating is etched. When such a structure is illuminated with an incident light beam, part of the beam is directly transmitted and part is diffracted and subsequently trapped in the waveguide layer. Some of the trapped light is then rediffracted outwards, so that it interferes destructively with the transmitted part of the light beam. At a specific wavelength and angular orientation of the incident beam, the structure "resonates"; namely, complete interference occurs and no light is transmitted. This paper reviews previous investigations on the resonance phenomena and presents analytic and numerical models for evaluating the resonance as a function of the geometric and optical parameters of the structures and incident radiation.

Crystal Channeling and Its Application at High-Energy Accelerators

Abstract: 1. Channeling Phenomenon.- 2. Beam Deflection by Bent Crystals.- 3. Experimental Studies of High-Energy Channeling and Bending Phenomena in Crystals.- 4. Crystal Extraction.- 5. The Use of Crystal Deflectors in Beam Lines.- 6. Application of Crystal Channeling to Particle Physics Experiments.- Epilogue.- References.

Strengthening of beams by plate bonding

Abstract: At Lulea University of Technology, Sweden, research has been carried out in the area of plate bonding, i.e., the problems that can arise when concrete members need to be strengthened using epoxy-bonded plates. Both comprehensive experimental and theoretical work have been done. In this paper a derivation of the shear and peeling stresses in the adhesive layer of a beam with a strengthening plate bonded to its soffit and loaded with an arbitrary point load are presented. The results from both theory and finite-element analysis show that the stresses are very large at the end of the plate, but they quickly diminish as we move nearer the center of the beam. The magnitude of the stresses is influenced not only by the geometrical and material parameters of the beam, but also by the adhesive and the strengthening material.

Shear and Flexural Strengthening of R/C Beams with Carbon Fiber Sheets

Abstract: The results of an experimental and analytical study of the behavior of damaged or understrength concrete beams retrofitted with thin carbon fiber reinforced plastic (CFRP) sheets are presented. The CFRP sheets are epoxy bonded to the tension face and web of concrete beams to enhance their flexural and shear strengths. The effect of CFRP sheets on strength and stiffness of the beams is considered for various orientations of the fibers with respect to the axis of the beam. Nineteen beams were fabricated, loaded beyond concrete cracking strength, and retrofitted with three different CFRP systems. The beams were subsequently loaded to failure. Different modes of failure and gain in the ultimate strength were observed, depending on the orientation of the fibers.

Envelope analysis of intense relativistic quasilaminar beams in rf photoinjectors:mA theory of emittance compensation

Abstract: In this paper we provide an analytical description for the transverse dynamics of relativistic, space-chargedominated beams undergoing strong acceleration, such as those typically produced by rf photoinjectors. These beams are chiefly characterized by a fast transition, due to strong acceleration, from the nonrelativistic to the relativistic regime in which the initially strong collective plasma effects are greatly diminished. However, plasma oscillations in the transverse plane are still effective in significantly perturbing the evolution of the transverse phase space distribution, introducing distortions and longitudinal-transverse correlations that cause an increase in the rms transverse emittance of the beam as a whole. The beam envelope evolution is dominated by such effects and not by the thermal emittance, and so the beam flow can be considered quasilaminar. The model adopted is based on the rms envelope equation, for which we find an exact particular analytical solution taking into account the effects of linear space-charge forces, external focusing due to applied as well as ponderomotive RF forces, acceleration, and adiabatic damping, in the limit that the weak nonlaminarity due to the thermal emittance may be neglected. This solution represents a special mode for beam propagation that assures a secularly diminishing normalized rms emittance and it represents the fundamental operating condition of a space-charge-compensated RF photoinjector. The conditions for obtaining emittance compensation in a long, integrated photoinjector, in which the gun and linac sections are joined, as well as in the case of a short gun followed by a drift and a booster linac, are examined. @S1063-651X~97!10706-1#

Bending strength of steel beams with corrugated webs

Abstract: Beams with corrugated webs were tested to failure under uniform bending. The failure was sudden and due to vertical buckling of the compression flange into the web after the stress in the flange reached the yield stress. The test results indicate that the web offered a negligible contribution to the moment carrying capacity of the beam. The test specimens were modeled using finite elements and the computer program ABAQUS was used to perform nonlinear analysis. The finite element model was able to depict the test results to a very good degree of accuracy. Parametric analytical studies were conducted to examine the effect of the ratio between the flange and web thicknesses and yield stresses, the corrugation configuration, the panel aspect ratio, and the stress-strain relationship. Based on these studies, it was concluded that, for design, the ultimate moment capacity can be calculated based on the flange yielding ignoring any contribution from the web.

Stable azimuthal stationary state in quintic nonlinear optical media

Abstract: We analyze the structure of the first azimuthal stationary state for a nonlinear medium presenting simultaneously a cubic (focusing) and a quintic (defocusing) dependence with the light intensity in the refractive index. This solution takes the form of a dark vortex of light hosted in a compact light beam. The existence of these modes is guaranteed if the flux exceeds a certain minimum threshold and if the modes are extremely stable for fluxes larger than a critical value that we calculated. We verified the robust nature of this solution inducing internal oscillations by an initial phase chirp. Using the variational method, we obtain an approximate picture for the beam internal dynamics. We also studied numerically the interactions between two near-vortex solutions, finding that for a wide combination of beam parameters they show elastic collisions.

Internal reinforcement for hollow structural elements

Abstract: A reinforced beam (20) has an internal I-beam structure (36) with opposed adhesive foam layers (48, 50). The foam layers are extruded, cut to length and placed on opposed surfaces (38, 40) to the I-beam (20). When the reinforced beam is heated, the adhesive layers expand to secure the I-beam in place.

Theory of Self-Trapped Spatially Incoherent Light Beams

Abstract: We present a modal theory of self-trapping spatially incoherent light beams in any general nonlinear media. We find that a self-trapped incoherent beam induces a multimode waveguide which guides the beam itself by multiply populating the guided modes. The self-trapping process alters the statistics of the incoherent beam, rendering it localized. We find the conditions for self-trapping (``existence region'' in parameter space) and the correlation function of the incoherent self-trapped beam.

A Unified Beam Finite Element Model for Extension and Shear Piezoelectric Actuation Mechanisms

Abstract: This paper presents a finite element model for adaptive sandwich beams to deal with either extension or shear actuation mechanism The former corresponds to an elastic core sandwiched beam between two transversely polarized active surface layers; whereas, the latter consists of an axially polarized core, sandwiched between two elastic surface layers For both configurations, an electric field is applied through thickness of the piezoelectric layers The mechanical model is based on Bernoulli-Euler theory for the surface layers and Timoshenko beam theory for the core It uses three variables, through-thickness constant deflection, and the mean and relative axial displacements of the core's upper and lower surfaces Augmented by the bending rotation, these are the only nodal degrees of freedom of the proposed two-node adaptive sandwich beam finite element The piezoelectric effect is handled through modification of the constitutive equation, when induced electric potential is taken into account, and additio

Satellite communication service with non-congruent sub-beam coverage

Abstract: A method for use in a satellite communications system (20) of a type that has at least one satellite (22) having forward and reverse beams each comprised of sub-beams for relaying user communications between a ground station (24) and user terminals (30). In a first embodiment forward and reverse antenna systems are separately optimized for their intended applications without regard for maintaining congruence between antenna beam sub-beams at the surface of the earth. In a second embodiment a method optimizes signal flow between the ground station and the user terminals and includes the steps of: (a) determining user terminal RF signal conditions within the sub-beams of at least one of the forward and reverse beams; and (b) in response to the determined RF signal conditions, re-allocating sub-beams of at least one of the forward and reverse beams such that the totality of the sub-beams of the forward beam are non-congruent with the totality of the sub-beams of the reverse beam, at the surface of the earth, while maintaining the forward beam substantially congruent with the reverse beam, at the surface of the earth. The step of determining includes a step of maintaining a database that stores a map of the sub-beams. In one embodiment of this invention the steps of determining and maintaining are executed at the ground station, while in another embodiment the steps of determining and maintaining are executed on-board the satellite.

High-frequency response of atomic-force microscope cantilevers

Abstract: Recent advances in atomic-force microscopy have moved beyond the original quasistatic implementation into a fully dynamic regime in which the atomic-force microscope cantilever is in contact with an insonified sample. The resulting dynamical system is complex and highly nonlinear. Simplification of this problem is often realized by modeling the cantilever as a one degree of freedom system. This type of first-mode approximation (FMA), or point-mass model, has been successful in advancing material property measurement techniques. The limits and validity of such an approximation have not, however, been fully addressed. In this article, the complete flexural beam equation is examined and compared directly with the FMA using both linear and nonlinear examples. These comparisons are made using analytical and finite difference numerical techniques. The two systems are shown to have differences in drive-point impedance and are influenced differently by the interaction damping. It is shown that the higher modes must be included for excitations above the first resonance if both the low and high frequency dynamics are to be modeled accurately.

Gamma-ray production in a storage ring free-electron laser

Abstract: A nearly monochromatic beam of 100% linearly polarized g rays has been produced via Compton backscattering inside a free electron laser optical cavity. The beam of 12.2 MeV g rays was obtained by backscattering 379.4 nm free-electron laser photons from 500 MeV electrons circulating in a storage ring. A detailed description of the g-ray beam and the outlook for future improvements are presented. [S0031-9007(97)03322-X]

Deflection of Neutral Molecules using the Nonresonant Dipole Force

Abstract: The ac Stark shift produced by nonresonant radiation creates a potential minimum for a ground state molecule at the position where the laser intensity is maximum. The gradient of this potential exerts a force on the molecule. We experimentally observe this force when a beam of CS{sub 2} molecules is redirected by sending it through the intensity gradient near the focus of a laser beam. We trace the direction of the molecules in the molecular beam, showing that the molecules that pass near the center of the high intensity laser beam will focus. {copyright} {ital 1997} {ital The American Physical Society}

Accurate characterization of Monte Carlo calculated electron beams for radiotherapy

Abstract: Monte Carlo studies of dose distributions in patients treated with radiotherapyelectron beams would benefit from generalized models of clinical beams if such models introduce little error into the dose calculations. Methodology is presented for the design of beam models, including their evaluation in terms of how well they preserve the character of the clinical beam, and the effect of the beam models on the accuracy of dose distributions calculated with Monte Carlo. This methodology has been used to design beam models for electron beams from two linear accelerators, with either a scanned beam or a scatteredbeam.Monte Carlo simulations of the accelerator heads are done in which a record is kept of the particle phase-space, including the charge, energy, direction, and position of every particle that emerges from the treatment head, along with a tag regarding the details of the particle history. The character of the simulated beams are studied in detail and used to design various beam models from a simple point source to a sophisticated multiple-source model which treats particles from different parts of a linear accelerator as from different sub-sources. Dose distributions calculated using both the phase-space data and the multiple-source model agree within 2%, demonstrating that the model is adequate for the purpose of Monte Carlotreatment planning for the beams studied. Benefits of the beam models over phase-space data for dose calculation are shown to include shorter computation time in the treatment head simulation and a smaller disk space requirement, both of which impact on the clinical utility of Monte Carlotreatment planning.

Observation of a single-beam gradient-force optical trap for dielectric particles in air

Abstract: A single-beam gradient-force optical trap for dielectric particles, which relies solely on the radiation pressure force of a TEM(00)-mode laser light, is demonstrated in air for what is believed to be the first time. It was observed that micrometer-sized glass spheres with a refractive index of n=1.45 remained trapped in the focus region for more than 30 min, and we could transfer them three dimensionally by moving the beam focus and the microscope stage. A laser power of ~40 mW was sufficient to trap a 5- microm -diameter glass sphere. The present method has several distinct advantages over the conventional optical levitation method.

Proton beam digital imaging system

Abstract: A proton beam digital imaging system comprising an X-ray source which is movable into the treatment beam line that can produce an X-ray beam through a region of the patient. An X-ray receiver receives the X-ray beam after it has passed through the patient and the X-ray receiver generates photons that correspond to the X-ray image. The photons are directed to a TEC CCD camera to produce a patient orientation image which displays the orientation of the center of the beam relative to selected monuments in the patient's skeletal structure. The system also receives a master prescription image which displays the target isocenter with respect to the same selected monuments of the patient's skeletal structure. By comparison of the relative positions of the center of the beam in the patient orientation image and the isocenter in the master prescription image with respect to the selected monuments, the amount and direction of movement of the patient to make the beam center correspond to the target isocenter is determined.

Voice monitoring system using laser beam

Abstract: An infrared laser beam (B1) is applied from a transmitter (100) to a window pane (410) of a building (400), and a reflected beam (B2) is received by a receiver (200). On each of the target displays (113, 213), a laser spot (Q) picked up by a CCD camera is displayed. The receiver is provided therein with a photocell adapted to output an electric signal in accordance with the luminous energy of the reflected beam, and a knife edge for shielding a part of the reflected beam, and a component of vibration of the window pane due to an interior voice is detected as positional fluctuation of the reflected beam. When a place of installation of the receiver is to be determined, the reflected beam is detected by using a beam detector (300) comprising a plurality of light receiving elements. The transmitter scans an object with the laser beam (B1) modulated by an audio-frequency so that the reflected beam (B) is easily detected. When the reflected beam hits the beam detector, the detector beeps vibratorily with an audio-frequency. The XY coordinates showing an actual scanning position are transmitted from the transmitter to the receiver, and shown on a coordinates display (212).

Optical quadrature Interferometer

Abstract: An optical quadrature interferometer is presented. The optical quadrature interferometer uses a different state of polarization in each of two arms of the interferometer. A light beam is split into two beams by a beamsplitter, each beam directed to a respective arm of the interferometer. In one arm, the measurement arm, the light beam is directed through a linear polarizer and a quarter wave plate to produce circularly polarized light, and then to a target being measured. In the other arm, the reference arm, the light beam is not subject to any change in polarization. After the light beams have traversed their respective arms, the light beams are combined by a recombining beamsplitter. As such, upon the beams of each arm being recombined, a polarizing element is used to separate the combined light beam into two separate fields which are in quadrature with each other. An image processing algorithm can then obtain the in-phase and quadrature components of the signal in order to construct an image of the target based on the magnitude and phase of the recombined light beam. The system may further be used for lensless imaging.

Coupled photorefractive spatial-soliton pairs

Abstract: We provide a comprehensive experimental and theoretical study of incoherently coupled photorefractive spatial-soliton pairs in all three possible realizations: bright–bright, dark–dark, and dark–bright. We also show that when the total intensity of two coupled solitons is much lower than the effective dark irradiance, the coupled soliton pair is reduced to Manakov solitons. In all cases, mutual trapping of both components in the coupled soliton pair is verified by analyzing, experimentally and numerically, the beam evolution after decoupling.

X-ray generator

Abstract: An X-ray generator comprises an evacuated and sealed X-ray tube, an electron gun, an X-ray target, an internal electron mask, and an X-ray window consisting of a thin tube of material with low X-ray absorption and high mechanical strength, for example beryllium. The window connects the tube to the target assembly containing the X-ray target. The generator preferably also includes a system for focusing and steering the electron beam onto the target, a cooling system to cool the target material, kinematic mounts to allow precise and repeatable mounting of X-ray devices for focusing the X-ray beam, and X-ray focusing devices of varying configurations and methods. The X-ray generator of the invention produces an X-ray source having a focal spot or line of very small dimensions and is capable of producing a high intensity X-ray beam at a relatively small point of application using a low operating power.

Sinusoidal-Gaussian beams in complex optical systems

Abstract: Sinusoidal-Gaussian beam solutions are derived for the propagation of electromagnetic waves in free space and in media having at most quadratic transverse variations of the index of refraction and the gain or loss. The resulting expressions are also valid for propagation through other real and complex lens elements and systems that can be represented in terms of complex beam matrices. The solutions are in the form of sinusoidal functions of complex argument times a conventional Gaussian beam factor. In the limit of large Gaussian beam size, the sine and cosine factors of the beams are dominant and reduce to the conventional modes of a rectangular waveguide. In the opposite limit the beams reduce to the familiar fundamental Gaussian form. Alternate hyperbolic-sinusoidal-Gaussian beam solutions are also found.

A four-point bending technique for studying subcritical crack growth in thin films and at interfaces

Abstract: A technique was developed to obtain the subcritical crack growth velocity in a 4-point bending sample by analyzing the load-displacement curve. This was based on the observation that the compliance of a beam increases as the crack grows. Beam theory was used to analyze the general configuration where two cracks propagated in the opposite directions. A simple equation relating the crack velocity to the load and displacement was established, taking advantage of the fact that the compliance was linearly proportional to the crack lengths; thus the absolute crack length was not important. Two methods of obtaining crack velocity as a function of load were demonstrated. First, by analyzing a load-displacement curve, a corresponding velocity curve was obtained. Second, by changing the displacement rate and measuring the corresponding plateau load, a velocity value was calculated for each plateau load. While the former was capable of obtaining the dependence of crack velocity versus load from a single test, the latter was found to be simpler and more consistent. Applications were made to a CVD SiO2 system. In both cases of crack propagation either inside the SiO2 layer or along its interface with a TiN layer, the crack growth velocity changed with the stress intensity at the crack tip exponentially. As a result, a small crack will grow larger under essentially any tensile stresses typically existing in devices, provided that chemical agents facilitating stress corrosion mechanisms are also present.

An Impedance Method for Dynamic Analysis of Active Material Systems

Abstract: This paper describes a new approach to analyzing the dynamic response of active material systems with integrated induced strain actuators, including piezoelectric, electrostrictive, and magnetostrictive actuators. This approach, referred to as the impedance method, has many advantages compared with the conventional static approach and the dynamic finite element approach, such as pin force models and consistent beam and plate models. The impedance approach is presented and described using a simple example, a PZT actuator-driven one-degree-of-freedom spring-massdamper system, to demonstrate its ability to capture the physics of adaptive material systems, which is the impedance match between various active components and host-structures, and its utility and importance by means of an experimental example and a numerical case study.The conventional static and dynamic finite element approaches are briefly summarized. The impedance methodology is then discussed in comparison with the static approach. The basic e...