Showing papers on "Beam (structure) published in 1984"

Negative dispersion using pairs of prisms.

Abstract: We show that pairs of prisms can have negative group-velocity dispersion in the absence of any negative material dispersion. A prism arrangement is described that limits losses to Brewster-surface reflections, avoids transverse displacement of the temporally dispersed rays, permits continuous adjustment of the dispersion through zero, and yields a transmitted beam collinear with the incident beam.

Electromagnetic ion beam instabilities

Abstract: The linear theory of electromagnetic instabilities driven by an energetic ion beam streaming parallel to a magnetic field in a homogeneous Vlasov plasma is considered. Numerical solutions of the full dispersion equation are presented. At propagation parallel to the magnetic field, there are four distinct instabilities. A sufficiently energetic beam gives rise to two unstable modes with right-hand polarization, one resonant with the beam, the other nonresonant. A beam with sufficiently large T (perpendicular to B)/T (parallel to B) gives rise to the left-hand ion cyclotron anisotropy instability at relatively small beam velocities, and a sufficiently hot beam drives unstable a left-hand beam resonant mode. The parametric dependences of the growth rates for the three high beam velocity instabilities are presented here. In addition, some properties at oblique propagation are examined. It is demonstrated that, as the beam drift velocity is increased, relative maxima in growth rates can arise at harmonics of the ion cyclotron resonance for both right and left elliptically polarized modes.

Distortion and harmonic generation in the nearfield of a finite amplitude sound beam

Abstract: Distortion and harmonic generation in the nearfield of a finite amplitude sound beam are considered, assuming time‐periodic but otherwise arbitrary on‐source conditions. The basic equations of motion for a lossy fluid are simplified by utilizing the parabolic approximation, and the solution is derived by seeking a Fourier series expansion for the sound pressure. The harmonics are governed by an infinite set of coupled differential equations in the amplitudes, which are truncated and solved numerically. Amplitude and phase of the fundamental and the first few harmonics are calculated along the beam axis, and across the beam at various ranges from the source. Two cases for the source are considered and compared: one with a uniformly excited circular piston, and one with a Gaussian distribution. Various source levels are used, and the calculations are carried out into the shock region. The on‐axis results for the fundamental amplitude are compared with results derived using the linearized solution modified with various taper functions. Apart from a nonlinear tapering of the amplitude along and near the axis, the results are found to be very close to the linearized solution for the fundamental, and for the second harmonic close to what is obtained from a quasilinear theory. The wave profile is calculated at various ranges. An energy equation for each harmonic is obtained, and shown to be equivalent within our approximation to the three‐dimensional version of Westervelt’s energy equation. Recent works on one‐dimensional propagation are reviewed and compared.

Diffuse ions produced by electromagnetic ion beam instabilities

Abstract: The evolution of the electromagnetic ion beam instability driven by the reflected ion component backstreaming away from the earth's bow shock into the foreshock region is studied by means of computer simulation. The linear and quasi-linear stages of the instability are found to be in good agreement with known results for the resonant mode propagating parallel to the beam along the magnetic field and with theory developed in this paper for the nonresonant mode, which propagates antiparallel to the beam direction. The quasi-linear stage, which produces large amplitude delta B approximately B, sinusoidal transverse waves and 'intermediate' ion distributions, is terminated by a nonlinear phase in which strongly nonlinear, compressive waves and 'diffuse' ion distributions are produced. Additional processes by which the diffuse ions are accelerated to observed high energies are not addressed. The results are discussed in terms of the ion distributions and hydromagnetic waves observed in the foreshock of the earth's bow shock and of interplanetary shocks.

Alignment of resonant optical cavities

Abstract: When an input Gaussian beam is improperly aligned and mode-matched to a stable optical resonator, the electric field in the resonator couples to off-axis spatial eigenmodes. We show that a translation of the input axis or a mismatch of the beam waist to the resonator waist size causes a coupling of off-axis modes which is inphase with the input field. On the other hand, a tilt of the input beam or a mismatch of the beam waist position to cavity waist position couples to these modes in quadrature phase. We also propose a method to measure these coupling coefficients and thereby provide a means to align and mode-match a resonant optical cavity in real time.

Mechanism to determine position and orientation in space

Abstract: A mechanism to determine position and orientation in space of a robot linkage or the like. The mechanism typically includes a plurality of structural beams that form the linkage and each structural beam has an associated measuring beam which, in the preferred embodiment, is housed within the structural beam in a way that deflection of the structural beam does not impose loads on the measuring beam. Angular measuring devices and linear measuring devices serve to locate endpoints of the measuring beam and the information derived serves to locate the free end or endpoint (typically a gripper or the like) relative to an anchor to establish position and orientation at the endpoint.

Apparatus and methods for ion implantation

Abstract: A system for implanting ions into a target element including a source arrangement for producing an ion beam; a beam analyzing arrangement for receiving the ion beam and selectively separating various ion species in the beam on the basis of mass to produce an analyzed beam; and a beam resolving arrangement disposed in the path of the analyzed beam for permitting a preselected ion species to pass to the target element. The analyzing arrangement has an ion dispersion plane associated therewith. The source arrangement has an associated ion emitting envelope including an area of substantial extension in a plane parallel to the ion dispersion plane and producing ions entering said analyzing arrangement which are travelling substantially either toward or from a common apparent line object lying in a plane perpendicular to the ion dispersion plane.

Stability of Fracturing Process in RC Beams

Abstract: A reinforced concrete beam section with a through‐thickness edge crack in the stretched part is considered. The eccentric axial force transmitted by the reinforcement to the concrete beam element, is estimated by means of a rotation congruence condition able to provide this statically undetermined reaction. In the field of linear elastic fracture mechanics, such a force increases linearly by increasing the external bending moment, until the limit force of yielding or slippage is reached. From this point on a perfectly plastic behavior of the reinforcement is considered. Once the bending moment of steel plastic flow is exceeded, the cracked beam segment presents a linear‐hardening behavior, until the concrete fracture occurs. It is shown how the stability of the process of concrete fracture and steel plastic flow depends on the mechanical and geometrical (scale included) properties of the beam cross section.

Charged Beam Interaction with Solids

Abstract: Yoshi-Hiko Ohtsuki 1983 London: Taylor and Francis viii + 248 pp price £22.50 ISBN 0 85066 239 7 This book provides a useful and concise review of theoretical treatments of scattering and energy loss processes for electrons or ions in solids. Particular emphasis is given to inelastic scattering in crystals and the phenomena of channelling and dechannelling, to the theory of which Professor Ohtsuki has himself made notable contributions.

Toward a consistent beam theory

Abstract: It is well known that the Euler-Bernoulli theory of the bending of beams makes use of a contradicting assumption of zero shear strains and nonzero shear stresses. Sometimes, this type oJ assumption is also carried over to more refined shear deformation theories. This paper outlines a theory thai avoids this assumption. With the aid of the specific example of a tip loaded cantilever beam, it is shown that the present theory gives Euler Bernoulli solutions in that part of the beam where shear deformation is unimportant and a shear deformation type of solution in the pari of the beam where shear deformation is important, with transition stress patterns between the two. Numerical studies, with a shear modulus representative of sandwich beams, bring out the usefulness of the present theory for the analysis of such soft-cored beams.

Temperature distributions produced by scanning Gaussian laser beams.

Abstract: A general solution is presented for the temperature rise produced by the absorption of a scanning Gaussian laser beam in a solid target. In normalized coordinates, the temperature rise is found to depend only on the ratio of the scan speed to the rate of heat diffusion in the solid and the ratio of the beam radius to the absorption depth. For slow scan speeds the solution simplifies to the steady-state approximation in which the power input is balanced by heat conduction into the solid. For fast scan speeds the solution approaches the energy density limit in which the temperature rise is proportional to the integrated beam intensity. For highly absorbing materials the solution simplifies to the surface absorption approximation. The general solution demonstrates the conditions under which each approximation can be used. Similar solutions are found for the related case of pulsed exposure by a stationary beam. The solution is demonstrated experimentally by exposing thermal paper with a CO2 laser.

Field-curvature effect on the diffraction ring pattern of a laser beam dressed by spatial self-phase modulation in a nematic film.

Abstract: The wave-front curvature of an incident laser beam can modify the spatial phase modulation of the beam, and hence the diffraction of the beam, traversing a liquid-crystal film. It is shown both theoretically and experimentally that this explains the anomalous fine structure in the diffraction ring pattern observed with laser intensities above the Freederickscz transition threshold.

Geometrical theory of diffraction, evanescent waves, complex rays and Gaussian beams

Abstract: Summary. The Gaussian beam method has recently been introduced into synthetic seismology to overcome shortcomings of the ray method, especially in transition regions due to focusing or diffraction where ray theory fails. One proceeds by discretizing the initial data as a superposition of paraxial Gaussian beams, each of which is then traced through the seismic environment. Since Gaussian beam fields do not diverge in ray transition regions, they are ‘uniformly regular’ although the quality of this regularity depends on the beam parameters and on the ‘numerical distance’ which defines the extent of the transitional domain. However, when Gaussian beam patches are used to simulate non-Gaussian initial data, there arise ambiguities due to choice of patch size and location, beam width, etc., which are at the user's disposal. The effects of this arbitrariness have customarily been explored by trial and error numerical experiment but no quantitative recommendations have emerged as yet. As a step toward a priori predictive capability, it is proposed here to perform a systematic study on analytically tractable prototype models of how the parameters and location of a single beam affect the quality of the observed seismic field, especially in ray transition regions. The conversion of ordinary ray fields into beam fields in canonical configurations can be accomplished conveniently by displacing a real source point into a complex coordinate space. Thus, the desired beam solutions can be obtained directly from available ray, and even paraxial ray, fields. Complex ray theory and its implications are reviewed here, with an emphasis on improvements of beam tracking schemes employed at present.

Design of megawatt gyrotrons

Abstract: The design parameters of a 120 GHz gyromonotron capable of output powers in excess of 1 MW are determined. A nonlinear model of the interaction between the beam and rf field is used in which the efficiency is a function of only three normalized variables. By expressing the technological constraints in terms of these variables, permissible design parameters yielding high efficiency operation can be calculated. Constraints that are considered include ohmic heating of the walls, voltage depression of the beam, reduced coupling between the beam and rf field due to beam thickness, and efficiency degradation due to space charge forces within the beam. An analysis of the tradeoffs between current and voltage at the 1 MW level indicates that lower order modes can be utilized at lower voltages, but the constraints based on current limitations are difficult to satisfy. An 80 kV, 29 A design is presented that achieves a total efficiency of 44%. The primary uncertainty of these designs is the severity of competition due to parasitic modes. However, a number of isolated asymmetric modes appear capable of single mode emission at 1 MW based on present experimental results. Multimegawatt operation is also considered. It is shown that powersmore » exceeding 20 MW are possible if single mode operation can be achieved in very high order modes. The methodology presented in this paper is general and can be easily adapted to other frequencies and output powers.« less

Resolution beyond the diffraction limit in the acoustic microscope: A nonlinear effect

Abstract: By operating the reflection mode scanning acoustic microscope at nonlinear power levels, resolution beyond the linear diffraction limit can be achieved. To demonstrate this effect, imaging experiments were performed in liquid nitrogen, liquid argon, and water at frequencies between 2000 and 2800 MHz. The spatial frequency response of the nonlinear imaging system was found to have a cutoff frequency which is at least 1.4 times higher than the cutoff frequency of the linear system. It is proposed that the resolution improvement is the result of interaction between the transmitted fundamental beam and harmonics. The harmonics are generated in the converging portion of the beam and have focal spots which are significantly smaller than the focal spot of the fundamental beam. The harmonic power reflected by the object influences the detected fundamental power due to nonlinear (parametric) interaction and results in the improved imaging resolution. A simple physical model and a gaussian beam analysis are presented to elucidate energy exchange between the focused fundamental beam and the higher harmonics.

Optical information recording apparatus

Abstract: An optical information recording and reproducing apparatus is disclosed in which a predetermined signal is recorded on a track of a rotating recording medium by a recording light beam and the recorded information is reproduced by a reproducing light beam. The information recorded on the last record track is reproduced by the recording beam and the reproducing beam and radiating position of the recording beam on the recording medium is displaced by a predetermined distance from the last record track.

Elastic interaction of local and lateral buckling in beams

Abstract: A nonlinear finite strip method of analysis is described for the post-local buckling of geometrically imperfect plate assemblies. The method is used to provide an accurate alternative to the Winter effective width formula for obtaining the effective section of a simply supported I-beam in the post-local buckling range of structural response. The effective section of a locally buckled beam with thin flange outstands is used to investigate the resistance of the beam to flexural-torsional buckling. The analytical methods developed to assess the nonlinear interaction of local and lateral buckling are compared with experimental tests performed by Cherry.

Gas source molecular beam epitaxy of GaxIn1−xPyAs1−y

Abstract: The use of P2 and As2 beams generated by several different beam sources for the growth of InP, GaAs, and GaxIn1−xPyAs1−y has been investigated. Accommodation coefficients for As2 and P2 were determined for heated GaAs and InP surfaces. It is demonstrated that a source utilizing decomposition of the hydrides over the range 200–2000 Torr and providing a leak of the resulting P2, As2, and H2 molecules into a molecular beam epitaxy (MBE) system can be used for the growth of GaxIn1−xPyAs1−y layers lattice matched to InP. Heterostructure lasers emitting at 1.5 μm with room temperature threshold current densities of 2000 A/cm2 and differential quantum efficiencies of 17%–19% were fabricated to demonstrate the quality of the epitaxy by this method. Initial studies of the cracking of AsH3 and PH3 at low pressures in contact with heated Ta suggest that Ta acts as a catalyst for the decomposition and that low pressure beam sources may also be useful for gas source MBE.

Hydrodynamics of thermal self‐focusing in laser plasmas

Abstract: Two‐dimensional Eulerian hydrodynamics simulations have been carried out to investigate the thermal self‐focusing of laser beams in plasmas with long scale lengths. Realistic modeling of the plasma dynamics and the time dependence of the plasma heating is found to be necessary to follow the fully nonlinear development of self‐focusing. The laser propagation is treated using a self‐consistent ray‐tracing model. The simulations presented illustrate the dependence of thermal self‐focusing on beam size, plasma scale length and initial temperature, laser wavelength and intensity, and flux limiter. For the interaction of a finite‐sized beam with a preformed long‐scale‐length plasma, whole‐beam self‐focusing is found to occur when the beam diameter is smaller than the plasma scale length. For beams with a hot spot imposed on a uniform background, self‐focusing occurs only for hot spots of sufficient amplitude (≳a few percent in typical cases). Self‐focusing is less likely to occur in an initially hot plasma. Low‐density self‐focusing channels formed in the underdense plasma by the expulsion of plasma from laser‐heated regions may collimate the reflected light back towards the laser. Small‐scale self‐focusing has also been simulated, along with a subsequent evolution into whole‐beam self‐focusing.

Frequency modulated lasar radar

Abstract: An apparatus and a method for measuring the distance to an arbitrary target includes a radiation source producing a beam of coherent radiation the frequency of which is continuously varied. The beam is divided into a ranging beam and a reference beam. The ranging beam is coupled to a ranging interferometer, which directs a portion of the ranging beam at the target. The ranging interferometer produces a first signal indicative of the phase difference between a portion of the ranging beam directed at and scattered by the target and another portion of the ranging beam which has traveled over a path of fixed length. The reference beam is coupled to a reference interferometer. A portion of the reference beam is directed by the reference interferometer along a reference path of a predetermined length, and the phase difference between the portion of the reference beam directed along the reference path and another portion of the reference beam which has travelled over a path of a fixed length is measured. The number of fringes resulting from the wave interference produced in the ranging interferometer and the number of fringes in the wave interference pattern produced by the reference interferometer are counted and used, together with the known length of the reference path, to determine the distance of the target from the ranging interferometer.

Microwave and radiofrequency Stark spectrum of ArHCN: A highly nonrigid molecule

Abstract: The rotational spectrum of the weakly bound complex ArHCN has been observed using molecular beam electric resonance spectroscopy. The spectrum is superficially characteristic of that of a linear molecule with both unusually large centrifugal distortion (requiring a J6 dependent distortion term to fit the data) and an unexpectedly large bending amplitude. The spectroscopic constants are The centrifugal distortion constant DJ is remarkably large and abnormally sensitive to isotopic substitution. Using the usual model, the stretching and bending force constants obtained from these data are an order of magnitude smaller than those similarly computed for the hydrogen halide complexes of argon. The calculated stretching and bending frequencies are 10 cm−1, predicting that excited vibrational levels should be populated in the beam. Three transitions have been observed which appear to correspond to an excited vibrational level of ArDCN, but poor signal‐to‐noise has prohibited their unambiguous assignment. W...

Beam propagation and optical power limiting with nonlinear media

Abstract: A simple model for absorption and refraction of a laser beam by a bounded nonlinear medium and for diffraction during the beam’s subsequent free-space propagation is used to calculate analytically the steady-state transverse-field profile in the far-field limit using a number of different laser input profiles. Good qualitative agreement with features found in experiments with indium antimonide at cryogenic temperatures is obtained when the input amplitude is assumed to possess a parabolic cross section. Power-transfer curves for a simple and entirely passive power-limiting device operating below the saturation level for nonlinear absorption have been calculated numerically, and it is concluded that the mechanism for limiting is largely determined by the ratio of nonlinear refraction to nonlinear absorption coefficients. Applications to the protection of sensitive optical components are envisaged.

An experimental method for determining the dynamic contact law

Abstract: An experimental method is investigated whereby the strain response from an impacted beam is sufficient to determine the contacting force. Once the force is known, it is shown how the contact law can be determined. Experimental results for an impacted aluminum beam are demonstrated.

The inverse problem for the vibrating beam

Abstract: The beam is modelled by a system of rigid rods joined together by rotational springs and with masses at the joints. The left end is clamped while the right end is subject to one of a number of conditions: it is free, pinned, sliding or clamped. It is shown that the resonant and antiresonant frequencies of the system may be almost completely ordered. Necessary and sufficient conditions are found that these frequencies must satisfy to correspond to an actual system, with positive parameters, and two stripping procedures are devised for the determination of these system parameters, from a knowledge of certain frequency spectra.

Stability of laminar electron layers

Abstract: The stability of a finite thickness, laminar cylindrical shell of electrons rotating azimuthally and enclosed in a coaxial waveguide is considered. The equilibrium rotation of the electrons is supported either by a radial electric field, an axial magnetic field, or a combination of both. The stability problem is formulated exactly as an eigenvalue problem, including all relativistic and electromagnetic effects as well as all effects of self and applied equilibrium fields. An approximate dispersion relation, valid for thin beams, is obtained analytically and the classical results for the ‘‘longitudinal’’ modes, i.e., the negative mass, cyclotron maser, and diocotron instabilities and for the ‘‘transverse’’ mode are recovered in appropriate limits. The dispersion relation is relatively simple and is valid for arbitrary values of the equilibrium electric and magnetic fields and for arbitrary beam energy. It therefore provides a ready comparison of the small signal properties of such devices as the Astron, gyrotron, orbitron, heliotron and the various cross field devices. It may also be of interest in accelerator and space physics applications. Some heretofore unnoticed effects on beam stability of equilibrium fields are reported; one such effect in particular leads to formulation of a simple, effective method either to maximize or eliminate altogether the longitudinal mode growth. Results from the dispersion relation compare favorably to results obtained from a numerical solution of the eigenvalue problem.