Showing papers on "Beam (structure) published in 1974"

Precipitating electron fluxes formed by a magnetic field aligned potential difference

Abstract: A model is developed in which a magnetic field aligned potential difference is assumed to accelerate electrons downward into the atmosphere. It is pointed out that the upgoing backscattered electrons produced by this electron beam may process insufficient kinetic energy to overcome the hypothetical potential difference. These electrons will be reflected downward to appear as members of a precipitating electron population. A numerical model was constructed in order to describe the total precipitating electron flux in terms of a primary accelerated beam and backscatter from the atmosphere. It is pointed out that many features that appear in the model beam are observed in the auroral electron beam as well. An example of an auroral beam, measured by Frank and Ackerson (1971), is compared favorably with a model electron beam formed by a parallel potential.

cw Self-Focusing and Self-Trapping of Light in Sodium Vapor

Abstract: We report steady-state self-focusing, self-trapping, and self-defocusing of a cw dye laser beam in sodium vapor for frequencies within several Doppler widths of the $D$-line resonance transitions. We measured the variation of the beam profile as the light propagates through the vapor. We observed a 20-mW beam self-trapped in a 12-cm-long filament having a half-power diameter of 70 \ensuremath{\mu}m.

Intense Coherent Cherenkov Radiation Due to the Interaction of a Relativistic Electron Beam with a Slow-Wave Structure

Abstract: Extremely high-power microwave emission is generated when a pulsed relativistic electron beam propagates down the axis of a corrugated-wall wave guide. The radiation occurs at a frequency such that the phase velocities of the negative-energy space charge wave on the beam and a low-order TM mode in the wave guide are equal. Power levels of 500 MW are generated. The conversion efficiency of electron energy into electromagnetic energy radiation is 17%.

Powerful relativistic electron beams in a plasma and in a vacuum (theory)

Abstract: The possibility of using intense relativistic electron beams (REBs) for heating plasmas in open systems is discussed. Within this context the following three sets of problems are discussed: REB transport in a vacuum with a strong magnetic field; beam equilibrium, stability and critical currents in a vacuum. Beam transport in a plasma; charge and current neutralization of the beam; reverse-current heating of the plasma, and macroscopic REB instabilities in the plasma. The theory of collective relaxation of REBs in a plasma, including quasi-linear and non-linear relaxation models; the role of plasma non-uniformity, and macroscopic effects during REB relaxation.

Observations of radiation from an electron beam artificially injected into the ionosphere

Abstract: This paper reports the observations of waves generated by a controlled beam of particles artificially injected into the ionosphere and magnetosphere. The measurements were made during the Electron Echo 1 experiment, in which an electron accelerator was carried to a height of 350 km in the ionosphere from Wallops Island, Virginia, on an Aerobee 350 sounding rocket. It injected into the earth's magnetic field over 3000 16-ms pulses of electrons with 40-keV energy and a current of 70 mA at pitch angles between 70 and 110 deg. The ejected fiber glass nose cone carried antennas and receivers to measure the electric field of waves generated by the beam. Associated with the electron beam was radiation detected at frequencies near the electron plasma frequency of the background ionosphere, near twice the electron cyclotron frequency in the whistler mode, and near zero frequency. Associated with the operation of an argon plasma generator used to keep the accelerator neutralized were continuous emissions detected at frequencies near the lower-hybrid resonance (LHR).

Hole and electron transport in SiO2 films

Abstract: Carrier transport in SiO2 has been studied by exciting hole‐electron pairs in an oxide film by a pulsed electron beam. The energy of the beam (4–8 kV) was chosen to minimize excitation in the Si substrate upon which the SiO2 was grown. Measurements of oxide current vs applied voltage were made with beam intensity and energy as parameters for SiO2 layers of three thicknesses. It is demonstrated that normalized current‐vs‐field curves are independent of beam intensity, beam energy, and film thickness over the range studied. These results indicate that the analysis used by various workers to determine the mobility‐lifetime (μτ) product for SiO2 is invalid. Observed dependences of current on applied field can best be explained by geminate and/or columnar recombination. The present findings indicate that both holes and electrons traverse most of the SiO2 without appreciable permanent trapping. The amount of positive charge trapped at or near the SiO2–Si interface is a significant fraction of the ``collected'' charge, indicating that some of the holes that move to the interface do not penetrate it.

Electron beam focusing and application to pulsed fusion

Abstract: This paper reviews recent work on the focusing of high-power relativistic electron beams in diodes and discusses concepts for pulsed fusion based on this technology. The physics of high-current relativistic electron beam focusing using plasmas in high-current diodes is studied experimentally and with computer simulation. The physics of the beam interaction with dense targets and the requirements for break-even are briefly discussed.

Neutral-beam injection into a tokamak, part I: fast-ion spatial distribution for tangential injection

Abstract: The production processes and spatial distribution of fast ions resulting from tangential injection of a diffuse neutral beam into a tokamak are discussed. The spatial distribution of fast ions for various injection trajectories and absorption mean free paths are calculated and discussed in detail. Maximum beam absorption for a parabolic density profile is shown to occur for injection roughly halfway between the inner wall of the torus and the magnetic axis; however, since this maximum is near unity and only weakly dependent on the injection trajectory, this is not the most important possible optimization. Since the drift orbit surface area over which the fast ions are distributed is roughly proportional to the distance from the magnetic axis, the fast ion density is found to be strongly peaked at the magnetic axis for present experiments where the absorption mean free path λ is comparable to the plasma radius a. This geometric peaking effect is strong enough to overcome the exponential beam attenuation and cause the fast-ion density and consequent beam energy deposition to be peaked at the plasma centre as long as λ0 a/4. Charge exchange of the fast ions with neutrals in the plasma can deplete the fast-ion population, particularly near the plasma edge. When charge exchange is an important loss mechanism, beam injection nearly tangent to the magnetic axis is found to maximize the beam effectiveness in heating.

Effects of neutral injection heating upon toroidal equilibria

Abstract: Effects of neutral beam injection upon the equilibrium of a toroidal plasma are considered. The distribution function of energetic ions produced by the beam in the plasma is calculated for injection both parallel and perpendicular to the magnetic field taking account of effects due to the toroidal geometry. The effect of trapped particles on the current induced in the plasma by such a beam is calculated, together with the associated cross-field diffusion. Loss mechanisms for the momentum deposited in the plasma by the neutral beam are considered. Ripples in the toroidal magnetic field strength are particularly efficient at destroying toroidal momentum and lead to flow velocities much less than the sound speed for typical injection parameters.

Transverse vibrations of double‐tapered cantilever beams with end support and with end mass

Abstract: The free vibrations of a double‐tapered cantilever beam with (1) end support and (2) end mass have been investigated using the Bernoulli‐Euler equation. The beam was tapered linearly in the horizontal and in the vertical planes simultaneously with the taper ratio in the horizontal plane equal to that in the vertical plane. A table is presented for the first case from which the fundamental frequency, second, third, fourth, and fifth harmonic can easily be obtained for various taper ratios. A chart, plotted from this table, shows the effect of taper ratio on the various harmonics. For the second case, a table and resulting charts show the effect of taper ratio and ratio of end mass to beam mass on the fundamental frequency and higher harmonies. Although previously presented, the case of the beam with free end is also included for purposes of comparison.

On Computation of the Motion of Elastic Rods

Abstract: A computational method is developed for the finite-amplitude three-dimensional motion of inextensible elastic rods with equal principal stiffnesses. The method also applies to the two-dimensional motion of such rods with unequal principal stiffnesses. For these two classes of problems the equations of the classical theory of rods are reduced to a non-linear vector equation of motion together with the inextensibility condition and appropriate boundary and initial conditions. Consistent finite-difference approximations are introduced and a semi-explicit method of solution is devised. The approximate limitation for numerical stability of the method is shown to be the same as for the usual explicit method in linear beam dynamics. By way of example the method is applied to the free fall of a circular pipe through water onto a rigid plane from a suspended initial configuration.

Gaussian Beam Interaction with a Planar Dielectric Interface

Abstract: The problem of electromagnetic Gaussian beam scattering from a planar interface separating two lossless dielectric media is considered. Employing a modal plane wave expansion and the continuity of the fields across the boundary, the reflected and transmitted beams are obtained using the Fresnel approximation, and a Taylor series expansion of the reflection and transmission coefficients, for both polarizations. Particular emphasis is given to the beam shift of both the reflected and refracted beams, the change in width of the transmitted beam, and the behavior at polarizing incidence.

Laser drilling mechanics

Abstract: Many laser drilling applications require that the shape of the drilled hole be carefully controlled. In order to successfully do this it is necessary to understand the laser drilling process. This paper reports on a quantitative model which predicts the depth and shape of a hole drilled in alumina ceramic by a ruby laser. Both experimental and theoretical results indicate that the predominant drilling mechanism for this application is not one of surface absorption and conduction inward, but one in which laser energy is absorbed throughout the bulk of the ceramic. The depth and shape of holes drilled in ceramic have been accurately predicted from the measured beam energy density distribution. Finally, it is shown that the radiation pressure of the focused beam plays an important role in the romoval of molten material from the heated region.

Focused arc beam transducer-reflector

Abstract: An acoustic transducer-reflector using a single disc-shaped piezoceramic element radiating into a compound reflector for obtaining a line focus sound beam that is characteristic of focused-arc transducers that maybe used for underseas applications and for non-destructive materials testing. The single transducer emits acoustic energy into the compound reflector which reshapes a circular beam into a focused arc beam. A shadow mask or aperture stop positioned in front of the reflector assures a beam such as would be radiated from a ring source.

Control system using multiplexed laser beams

Abstract: One or more reference beams from a low-power HeNe laser are multiplexed onto the optical path of a pulsed or Q-switched highpower YAG laser and thereby suffer the same deflection in galvanometer-type deflection apparatus used to deflect the main beam as does the main beam. The reference beams are next split off, prior to the impingement of the YAG beam on the workpiece to be machined, and passed through reference grids. After the beams pass through the reference grids, photodiodes generate signals which can be used to determine precisely where the YAG beam will impinge upon the workpiece when next energized.

Ion beam sputtering - the effect of incident ion energy on atomic mixing in subsurface layers

Abstract: Secondary ion mass spectrometric methods were used to study the effect of incident ion energy on atomic mixing in subsurface layers. A Ta2O5 film containing a 50 A 3lP-rich layer 230 A below the surface was depth profiled for phosphorus using normal incidence 160 primary particles of various energies (1.75 to 18.5 keV). A pronounced energy effect was observed in the widths of 31P profiles generated by >4 keV 16O. For 18.5 keV 16O, the observed profile contained two distinct components-the 31P-rich layer and the 31P recoil distribution. This later condition, prevails when the peak of the incident ion damage distribution occurs at a depth which equal or exceeds the distance from the surface to the 31P-rich layer. The desirable primary ion beam energy for characterizing the true elemental distribution is dictated by the shape and location of the layer to be profited. In most instances, a low energy beam is preferred.

Optical device for producing a minute light beam

Abstract: An optical device for producing a minute beam comprises beam forming means such as laser generator or the like, annular beam forming means such as lens or annular aperture mask for forming the beam from the beam forming means into a beam of annular cross section, and a condenser lens for condensing the annular beam from the annular beam forming means.

A beam-modulated time-of-flight mass spectrometer. II. Experimental work

Abstract: For pt.I see ibid., vol.6, 785 (1973). A straight path beam-modulated time-of-flight mass spectrometer has been constructed. The instrument consists of an ion source, a drift tube and a collector. The ion source is constructed by combining a conventional ionization chamber with an electron gun structure from a cathode ray tube incorporating an XY deflection system. The ion beam is modulated by applying to the X or Y plates a voltage varying in accordance with a step function. The mathematical analysis of the beam-modulation process has shown that the theoretical resolving power of this instrument should be inversely proportional to the beam plus collector slit width and proportional to the square of the flight path.

Fine Structure, Hyperfine Structure and Lamb Shift Measurements by the Beam-foil Technique

Abstract: A survey on high resolution experiments with atomic systems prepared by the beam-foil or beam-gas interaction is presented. RF-resonance, field-quenching, quantum-beat and level-crossing experiments are described in elementary terms whereas more advanced theoretical techniques are sketched for quantum beats in an appendix.

The modified Dugdale-Barenblatt model adapted to various fracture configurations in metals

Abstract: The Dugdale-Barenblatt model is extended to encompass the influence of strain hardening on the plastic enclaves developed at the tips of a crack in a plate subjected to tension at infinity. While in the DB-model the distribution of internal stresses along the plastic zone in the extension of the crack length was taken constant and equal to the yield stress, in the modified version of this model this distribution is taken variable with a minimum value the yield stress (σ0) and a maximum value (σmax depending on the loading step and the amount of strain-hardening of the material. Six different configurations of stress distribution in the plastic enclaves were considered with various values of the ratio σmax/σ0. For each stress configuration and for various loading steps the shape of the respective caustic corresponding to the singularity created at the plastic zone near the crack tip was computed by modifying appropriately Dugdale's theory to each of the six configurations in the plastic enclave. The caustic was formed by reflections of a parallel coherent light beam at the vicinity of the crack-tip.

An exposure model for electron-sensitive resists

Abstract: We present a mathematical model for the exposure of electron-sensitive resists where an electron beam is incident normal to a substrate coated with a thin layer of resist. We include both the scattering of the incident electrons as they penetrate the resist and the electrons backscattered from within the resist and from the substrate. The calculations yield contours of equal absorbed energy density, and these are interpreted as the contours which bound the resist after development. The absorbed energy density is found as the sum, for all electrons, of the product of the energy absorbed per unit length of trajectory and the flux density of electrons at the point in question. We first calculate the absorbed energy density for an electron beam of vanishingly small cross section (an incident delta function) and then convolve that result with a beam of Gaussian current-density distribution to obtain the reSult for a single beam location. For poly(methyl methacrylate) resist, we study the achievable dot resolution, as a function of the incident charge, for various incident energies-and substrates. Since our main interest is in computer-controlled resist exposures in which patterns are generated as a succession of dots, we calculate the absorbed energy density contours for a line generated in that manner. Detailed comparison is made with the experimental results of Wolf et al., by fitting a single point on one contour at one beam energy to account for the unknown developer sensitivity. The resulting contours predict the undercutting effect experimentally observed for the 5-20-keV beam energies studied. The developed shape and linewidth are found to be nonlinear functions of the incident charge per unit length. Experimental data for the linewidth at 20 keV are presented and compared with theory.

Effect of In-Plane Deformation on Lateral Buckling

Abstract: In the classical analysis of the flexural-torsional buckling of beams, beam columns and rigid-jointed plane frames, it is assumed that the major axis rigidity is very large, so that the small in-plane deformations can be neglected. The effects of the in-plane deformations on lateral buckling are investigated in this paper for determinate beams and cantilevers, beam columns, continuous beams, and portal frames. This is done by deriving more accurate governing differential equations, and by obtaining closed form or numerical solutions of these. The results obtained indicate that the classical critical loads or moments are generally conservative, except for the members which are highly restrained laterally. The sources of error in the classical analysis are also studied, and their effects are demonstrated. The results of experiments on small scale beams, which are in close agreement with the theoretical predictions, are reported.

Phase compensation for thermal blooming.

Abstract: Appropriate correction of the initial phase of a laser beam is shown by numerical computation to be capable of appreciably reducing thermal blooming for a cw beam.

Astigmatizer for laser recording and reproducing system

Abstract: In a laser recording system, a high energy intensity laser beam is focused to form a circular write-spot on a rotating thermoplastic disc. This spot creates tracks of grooves and flat areas or lands in accordance with input data. To read the data from the disc a reduced energy beam is focused on the previously recorded track, and the pattern of grooves and lands is determined from the reflected energy. The optics associated with the reading process include an astigmatizer which elongates the read-spot in a direction transverse to the track. This reduces mechanical tolerances required to insure that the read-spot will impinge upon the desired track. The astigmatizer may also serve to reduce the beam spot energy intensity so that the same laser, without the astigmatizer, can be used for the recording process. In one embodiment, the astigmatizer is a movable plate having a single slit. For reading, the astigmatizer is positioned in the beam path and the slit creates an orthogonal diffraction pattern which provides the elongated read-spot of reduced energy intensity.

Lever-action mountings for beam steerer mirrors

Abstract: A mirror-type light beam deflector or steerer utilizes a hinged mounting linkage to couple a beam deflecting mirror to a mirror driving apparatus. In a preferred form, flexible hinges are employed to avoid any introduction of play or looseness between the mirror and mirror driver.

Particle detecting system

Abstract: Submicron-sized particles are detected by a system which observes emission, as by scattering or by fluorescence from particles undergoing Brownian motion in a region of a fluid medium excited by an evanescent wave created adjacent an interface between the fluid medium and a multiple internal totally reflecting cell or light guide. The excited region can range in depth between several wavelengths and a fraction of the wavelength of the exciting beam, hence serves as an "aperture" having a dimension of about the same order of magnitude as the particles being detected. The particles can be classified according to size by examining the amplitude modulation arising out of the motion of the particles through the aperture.

Inclusion of Transverse Shear Deformation in Finite Element Displacement Formulations

Abstract: A stiffness matrix is derived for a beam element with transverse shear deformation. It is shown that straightforward energy minimization yields the correct stiffness matrix in displacement formulations when transverse shear effects are considered. Since the TIM4 beam element does not represent the geometric boundary conditions for a cantilever beam the rotation of the normal must be retained as a grid point degree of freedom.