Effect Of Beam Size Research Articles

Finite laser beam size effects in thermal wave interferometry

A theoretical and experimental analysis of the finite laser beam size effects in thermal wave interferometry has been reported. The limits of the heat transfer one-dimensional approximation, that is usually employed in this measuring method, have been discussed. This study has been performed by considering the actual three-dimensional temperature distribution induced by gaussian laser heating. Measurements on yttria partially stabilized zirconia coatings plasma-sprayed onto two different metallic substrates were carried out for different values of the heating beam dimension.

Effects of gaussian beam size on diffraction measurement errors

A numerical simulation has been performed for the diffraction of a gaussian beam at a wire in the Fraunhofer approximation. The wire is symmetrically disposed with respect to the beam axis in the calculations on the shifts in the diffraction minima, while an unsymmetrical position has been used in calculating the diffraction pattern, which is characterized by change in the minima and maxima. Conditions are evaluated under which one can use the homogeneous-beam approximation in diffraction measurements.

Detecting the size effect in concrete beams using nonlinear fracture mechanics

This paper shows that, the size effect in flexure and shear strength for different concrete and reinforced concrete beam sizes subjected to concentrated and uniformly distributed loads can be successfully predicted on the basis of nonlinear fracture mechanics. The analysis was carried out by a computer simulation using the program ANACS (advanced nonlinear analysis of concrete structures), which was originally developed by the authors. In this investigation, the fictitious crack model was adopted with two orthogonal rod elements which involves the nonlinear fracture mechanics through their constitutive model to simulate the discrete crack path and represent the localized crack zone. By combining the arc-length calculation technique with the fictitious crack model, the postpeak behaviour can be predicted well even for snapback instability.

Ray-tracing model for stretcher dispersion calculation

We propose a ray-tracing model that provides a clear physical picture and simple formulas for grating pair stretcher dispersion calculations. With this model we can easily demonstrate why and to what extent the stretcher and compressor are opposite quantitatively without using a Fourier transform. The dispersion calculation shows that the spherical aberration in the stretcher decreases fourth-order dispersion compared with an aberration-free stretcher. In a chirped pulse amplification system, this fourth order can help to reduce residual fourth-order dispersion. The effect of the finite beam size and the misalignment are also considered.

Beam size and collimation effects in spectroscopic ellipsometry of transparent films with optical thickness inhomogeneity

In spectroscopic ellipsometry (SE) measurements of thin films with inhomogeneous optical thickness, the spot size of the probe beam on the sample surface is a highly critical parameter in the treatment of the experimental data. SE measurements were performed on transparent samples prepared by various techniques: plasma enhanced chemical vapor deposition of diamond film, spin-coated PMMA films and thermally grown SiO 2 film. The first two deposition techniques can produce films with inhomogeneous optical thickness. By comparing SE measurements using different set-up configurations, we demonstrate that a combination of lenses and small diaphragms is most suitable for minimizing the effects related to the optical thickness distribution and light beam divergence. In this way mistakes are avoided in extracting physical parameters from SE optical modeling of these films.

Physical mechanism of the linear beam-size effect at colliders.

We present qualitative but precise description of the linear beam-size effect predicted for the processes in which unstable but long--living particles collide with each other. We derive physically pronounced equation for the events rate which proves that the linear beam-size effect corresponds to the scattering of one beam of particles on the decay products of the other. We compare this linear beam-size effect with the known logarithmic beam-size effect measured in the experiments on a single bremsstrahlung at VEPP-4 and HERA.

Beam-size effect in time-delayed laser-induced double gratings

We have studied theoretically the effect of the probe-beam size on time-delayed laser-induced double gratings by assuming that the probe beam is described as a Gaussian beam. LettingE s1 andE s2 be the Four-Wave Mixing (FWM) signals originating from the diffraction of the probe beam by the gratings, the condition for the occurrence of the FWM signal modulation is that the divergence angle ofE s1 andE s2 is larger than half of the intersection angle between the propagation directions ofE s1 andE s2. We have also proposed methods to increase the modulation contrast.

Size effect in beams with rounded-tip V-notch

Stresses and strains in a beam with a single-edge rounded-tip V-notch subjected to bending were analyzed by the method of finite elements by using nonconforming quadrilateral elements under the assumption that the material is linearly elastic. We studied the influence of the depth of the notcha, its radius of curvature ρ, the angle of the notch ω, and the height of the beamh on the stress intensity factorKt. It is established that the relative depth of the notch ζ=a/h causes practically no influence if the notch angle ω lies within the range 0°–135°. The influence of the notch rapidly weakens as the relative radius of curvature ρd=ρ/h increases from 0.01 to 1.0 and becomes insignificant for ρd ≤ 1. For small ρd (ρd ≤ 1), the quantityKt attains its maximum value provided that ζ lies within the range 0.1–0.3. As the height of the beam increases, the quantityKt first remains practically constant, then increases, and finally, ash infinitely increases, approaches a certain constant value.

New type of beam size effect in the W boson production at micro+ micro- colliders.

The cross section for the reactions of the type $\mu^-\mu^+ \to e\bar\nu_e X$ can not be calculated by the standard methods due to the $t$--channel singularity in the physical region. In this letter we show that accounting for the finite sizes of the colliding beams results in the regularization of this singularity. The finite cross section, which is obtained in this way, turns out to be linear proportional to the transverse sizes of the colliding beams. As an application of the above result, we calculate the cross section of the $W$ boson production at $\mu^+\mu^-$ colliders in the reaction $\mu^- \mu^+ \to e\bar\nu _e W^+$.

Observation of the beam-size effect at HERA

A precise measurement of the spectrum of the photons from $ep$ bremsstrahlung with the ZEUS luminosity monitor at HERA is reported. The measurement shows a reduced rate compared to the Bethe-Heitler spectrum for photon energies below 5~GeV. This suppression, called the beam-size effect, is explained by the finite transverse size of the beam overlap relative to the typical impact parameter in the process of $ep$ bremsstrahlung at HERA energies.

Measurement of gas-phase sound speed and thermal diffusivity over a broad pressure range using laser-induced thermal acoustics

We report on the detection and analysis of signals generated from gas-phase laser-induced gratings over a large range of static pressure (0.04-100 atm). We employed the experimental technique of laser-induced thermal acoustics and performed measurements on mixtures of NO(2) in air and CO(2) as a function of pressure. Accurate analysis of the acquired data was obtained from a full theory, including beam size effects. The theory fully reproduces the observed data for a ratio of molecular mean free path to grating wavelength extending from 1 to 4 x 10(-4). Nonlinear, least-squares f its between modeled and experimental signals provided accurate values of the sound speed and thermal diffusivity.

FEL gain calculation for imperfectly matched electron beams

We present here the details of an analytical small-signal gain calculation. The analysis builds on the basic one-dimensional analytical calculation by modeling the effects of finite electron beam size and imperfect matching of the electron beam to the wiggler. The calculation uses TRANSPORT [SLAC-91, Rev. 2 (1977)] parameters to describe the electron beam in order to easily take the output of beam transport calculations and use them as the input for FEL gain calculations. The model accepts an arbitrary TRANSPORT beam and includes the effects of energy spread, beam size, betatron oscillations, and focussing in the wiggle plane. The model has allowed us to calculate the range over which our FEL can be tuned by changing the electron energy alone (i.e., without changing any magnets).

Statistics and size effect in cementitious materials

The statistical size effect in cementitious beams is examined using the assumption that the critical threshold stress to cause a strain-softened fracture process zone to be initiated follows the Weibull distribution. The fictitious crack line model and the K-superposition theory, in combination with the statistical strain-softening relationship, are used to determine the strength of plain and notched beams. For notched beams the statistical effect on the strength is only important for small beams, but causes a large variation in all sizes of plain beams.

Three-dimensional analysis of modulated photoreflectance in a silicon wafer

The effects of probe and pumping beam size and modulation frequency on photoreflectance were investigated for a silicon wafer by considering one- and three-dimensional generation and propagation of thermal and plasma waves,PR1D andPR3D. The magnitude ofPR1D decreased as the inverse square of the effective beam radius and that ofPR3D was 100 times smaller thanPR1D at 0.1 μm effective beam radius and decreased with the effective beam radius. The phase shift ofPR1D was nearly constant at 225°, whereas that ofPR3D increased with the effective beam radius from 0° to 225°. The magnitude and phase ofPR3D become the same as those ofPR1D by satisfying the equivalence conditions, where the probe and pumping beam radii are larger than the thermal and plasma wavelengths, when the effective beam radius was larger than 112 μm.PR1D decreased with modulation frequency as ω−1/2, whereas the magnitude ofPR3D was nearly constant and 100 times smaller than that ofPR1D at 1 kHz modulation frequency. ThePR1D phase varied from 180° to 225°, but that of thePR3D increased from 0° to that ofPR1D with increase of the modulation frequency. As the modulation frequency increased, the magnitude and phase ofPR3D approached to those ofPR1D by approaching the equivalence conditions, owing to the decrease of the thermal and plasma wavelengths. The good agreements in the modulation frequency dependence of the magnitude and phase ofPR3D with those measured, justified the three-dimensional analysis of the photoreflectance.

Phase-matched gratings for enhanced forward degenerate four-wave mixing

A phase-matched grating to enhance the second-order diffracted signals generated by forward degenerate four-wave mixing has been proposed and demonstrated for the first time to the authors’ knowledge. Experiments have been performed with a cw Ar:Kr laser at λ = 488 nm, with a linear–nonlinear sandwich of glass slides and Alizarin Yellow-doped films of epoxy resin. Finite-difference simulations have been used to model the process successfully, with finite beam size effects and deviations from an ideal Kerr response of the material taken into account. The possibilities for optical processing applications are assessed in the light of these results.

Optical-coherence tomography of a dense tissue: statistics of attenuation and backscattering

Addresses fundamental issues that underlie the interpretation of images acquired from turbid tissues by optical-coherence tomography (OCT). The attenuation and backscattering properties of freshly excised rat arteries and their dependence on the focusing and collection optics of the OCT system were measured at two wavelengths in the near infrared (830 nm and 1300 nm). Determined from the ratio of the magnitudes of the reflections from glass plates placed on both sides of the arteries, the mean attenuation coefficient of the arterial wall was found to be in the range 14< mu t<22 mm-1 at 830 nm and 11< mu t<20 mm-1 at 1300 nm. The measured values of mu t were lowest for the longer source wavelength and for probe beams with the smallest average diameters. The observed dependence of mu t on beam size indicates that relatively large-scale variations in the index of refraction of the tissue contributed to degradation of the transverse spatial coherence of the beam. The authors introduce a framework for understanding and quantifying beam-size effects by way of the mutual-coherence function. The fact that spatial variations in backscattering and attenuation (which includes spatial-coherence losses) have similar effects on OCT signals makes the origin of the signals difficult to determine. Evidence is given that suggests that, in spite of this difficulty, certain features of microstructures embedded several hundred micrometres deep in a turbid tissue can still be detected and characterized.

Noise suppression for photorefractive image amplification in the LiNbO3:Fe crystal sheet

The dependence of competition between the forward scattering light and climbing light in the LiNbO3:Fe crystal sheet on the incident angle of the pump light beam is discussed. On the basis of the light climbing and beam size effects, a new method for a low-noise, high-gain image amplification in thin crystal is described.

MAPPING OF RESIDUAL STRESSES AROUND FASTENER HOLES IN HIGH STRENGTH ALUMINIUM ALLOY

Residual stress distributions around plain and cold expanded fastener holes in aluminium alloy 7050–T76 have been measured using the X-ray diffraction sin2Ψ method. Both radial and hoop stresses have been measured as a function of distance from the edge of the hole on the mandrel inlet and outlet faces. The effect of fatigue loading on the residual stress distribution around cold expanded holes has been investigated and is discussed in relation to measured fatigue endurance. The effect of the X-ray beam size on the resolution of the technique in the vicinity of the hole has been studied and life predictions incorporating the measured residual stresses have been correlated with fatigue endurance.

Size Effects in Short Beams Loaded in Shear

Size Effects in Short Beams Loaded in Shear

Dosimetric Effects of Beam Size and Collimation of Epithermal Neutrons for Boron Neutron Capture Therapy

A series of studies of "ideal" beams has been carried out using Monte Carlo simulation with the goal of providing guidance for the design of epithermal beams for boron neutron capture therapy (BNCT). An "ideal" beam is defined as a monoenergetic, photon-free source of neutrons with user-specified size, shape and angular dependence of neutron current. The dosimetric behavior of monoenergetic neutron beams in an elliptical phantom composed of brain-equivalent material has been assessed as a function of beam diameter and neutron emission angle (beam angle), and the results are reported here. The simulation study indicates that substantial differences exist in the dosimetric behavior of small and large neutron beams (with respect to the phantom) as a function of the extent of beam collimation. With a small beam, dose uniformity increases as the beam becomes more isotropic (less collimated); the opposite is seen with large beams. The penetration of thermal neutrons is enhanced as the neutron emission angle is increased with a small beam; again the opposite trend is seen with large beams. When beam size is small, the dose delivered per neutron is very dependent on the extent of beam collimation; this does not appear to be the case with a larger beam. These trends in dose behavior are presented graphically and discussed in terms of their effect on several figures of merit, the advantage depth, the advantage ratio, and the advantage depth-dose rate. Tables giving quick summaries of these results are provided.