Showing papers on "Beam (structure) published in 1975"

Electromagnetic beam propagation in turbulent media

Abstract: The most recent developments on the propagation of optical beams in a turbulent medium, such as the clear atmosphere, are reviewed. Among the phenomena considered are beam spreading, beam wander, loss of coherence, scintillations, angle-of-arrival variations, and short-pulse effects.

Excitation of Alfven waves by high-energy ions in a tokamak

Abstract: It is shown that shear Alfv\'en waves can be destabilized by resonance with high-energy "beam" ions near the magnetic axis in a tokamak, if the beam is radially nonuniform.

Backscattering of 10-100 keV electrons from thick targets

Abstract: Experimental measurements of backscattering as a function of incident electron beam energy, angle of beam incidence and atomic number of scatterer, together with energy spectra of backscattered electrons, are discussed within the framework of a simple, qualitative theory. A semi-empirical expression for the variation of backscattering coefficient with angle of beam incidence is derived. Spectra integrated over energy and angle of take off also tabulated from experimental measurements for several elements. The use of the experimental data to derive backscattering corrections for X-ray elemental analysis, particularly at non-normal incidence, is briefly described.

Intense, mixed-energy hydrogen beams for ctr injection

Abstract: Present high-current ion sources for neutral injection experiments accelerate a mixture of atomic and molecular hydrogen species that are converted into neutral particles with different energies and neutralization efficiencies. Beam composition can have important effects on injection system efficiency, vacuum design, and first-wall loading. Beam composition measurements of the 20-keV LBL high-current sources are used to calculate the relative power in the various hydrogen and deuterium beam components expected at higher energies.

Method for improving the crystallinity of semiconductor films by laser beam scanning and the products thereof

Abstract: A method is disclosed for improving the crystallinity of semiconductor films by scanning the surface of such films with a shaped, focused laser beam The laser is matched to the film so that the beam delivers sufficient energy thereto to heat the film above a temperature at which crystallization occurs along the scan track

Ion−beam etching

Abstract: In the course of developing techniques for ion−beam etching of solid bulk materials and thin deposited films, we undertook a systematic investigation of etch rates of several materials and the dependence of etch rate on beam parameters and beam angle of incidence. Results of these measurements are presented. Under certain conditions, cones formed on the surface being etched. An explanation for this instability phenomenon is given. In most practical applications of ion−beam etching, suitable masking techniques are necessary. We have developed procedures for photoresist masking for two classes of etching: (a) high−resolution, shallow−depth etching (typical dimension ∠1 μ); (b) low−resolution, very deep etching (typical dimension ∠50 μ). The methods used will be discussed, and in particular we will consider the influence of redeposition of sputtered material and the profile of the photoresist on the final−etched wall shape.

Nonlinear stabilization of beam plasma interactions by parametric effects

Abstract: The nonlinear stabilization of the kinetic stage of electron beam plasma instabilities by parametric effects is investigated. It is found that within a definite range of plasma parameters parametric instatilities induced by the beam generated waves can stabilize the system at a level of wave energy density substantially lower than expected by quasi‐linear theory. This occurs because at a certain level of beam‐generated plasma waves, the transfer rate of wave energy outside the spectral region in resonance with the beam exceeds the beam plasma instability growth rate. A model of a quasi‐steady state for the case of continuous beam injection is proposed. The possibility of utilizing ultrarelativistic electron beams for achieving ignition temperatures in a tokamak is discussed.

Normal Modes in the Ion-Beam-Plasma System

Abstract: Two types of dispersion relations are experimentally shown to exist in an ion-beam-plasma system. For beam velocities close to the ion-sound speed, two normal modes, one unstable, are seen. For larger beam velocities, the interferometer output exhibits beating between the three predicted stable normal modes, which are the "fast" and "slow" ion-beam modes and the plasma ion-acoustic wave.

Linear theory of a cold relativistic beam propagating along an external magnetic field

Abstract: The linear theory of a cold relativistic electron beam propagating parallel to an external magnetic field and through a cold, homogeneous plasma is investigated. The electromagnetic dispersion relation is solved numerically and compared with analytical predictions based on the electrostatic approximation. It is found that electromagnetic effects are important for determining the entire unstable spectrum. However, except for the strong magnetic field regime, the maximum growth rates and corresponding frequencies are in agreement with those predicted by the electrostatic approximation. In the strong magnetic field regime the two−stream spectrum is found to be much narrower in angle than predicted by the electrostatic approximation. In the moderate and strong magnetic field regime the growth rate of waves propagating at large angles with respect to the beam are independent of beam energy.

Boron neutron capture therapy for the treatment of cerebral gliomas. I. Theoretical evaluation of the efficacy of various neutron beams.

Abstract: The technique of boronneutron capture therapy in the treatment of cerebral gliomas depends upon the selective loading of the tumor with a 10B‐enriched compound and subsequent irradiation of the brain with low‐energy neutrons. The charged particles produced in the 10B (n,α) 7Li reaction have ranges in tissue of less than 10 μm so that the dose distribution closely follows the 10B distribution even to the cellular level. The effectiveness of this therapy procedure is dependent not only on the 10B compound but on the spectral characteristics of the neutron source as well. Hence, an optimization of these characteristics will increase the chances of therapeutic success. Transport calculations using a neutral particle transport code have been made to determine the dose–depth distributions within a simple head phantom for five different incident neutron beams. Comparison of these beams to determine their relative therapeutic efficacy was made by the use of a maximum useable depth criterion. In particular, with presently available compounds, the MIT reactor (MITR) therapy beam (a) is not inferior to a pure thermal neutron beam, (b) would be marginally improved if its gamma‐ray contamination were eliminated, (c) is superior to a partially 10B‐filtered MITR beam, and (d) produces a maximum useable depth which is strongly dependent upon the tumor‐to‐blood ratio of 10B concentrations and weakly dependent upon the absolute 10B concentration in tumor. A pure epithermal neutron beam with a mean energy of 37 eV is shown to have close to the optimal characteristics for boronneutron capture therapy. Furthermore, these optimal characteristics can be approximated by a judiciously D2O moderated and 10B‐filtered 252Cf neutron source. This tailored 252Cf source would have at least a 1.5 cm greater maximum useable depth than the MITR therapy beam for realistic 10B concentrations. However, at least one gram of 252Cf would be needed to make this a practical therapy source. If the moderated 252Cf source is not 10B filtered, the resultant neutron beam has characteristics similar to those of the MITR beam with no gamma‐ray contamination. For such a beam, 100 mg of 252Cf would produce a flux of 2.4×108 neutrons/(cm2 sec), which is an intensity suitable for therapy applications.

Energy absorbing device

Abstract: This invention relates to an energy absorbing device intended to be used as a composite bumper. Said bumper comprises an energy absorbing beam, a deformable unit comprising at least two energy absorbing members located between the beam and the vehicle chassis, said beam being supported by said members, and a flexible shield wrapping either both aforesaid constituting units or a portion thereof.

Theory of Microwave Emission by Velocity-Space Instabilities of an Intense Relativistic Electron Beam

Abstract: The stability of waveguide modes in a waveguide along which is propagated an unneutralized, intense beam of relativistic electrons guided by an applied axial magnetic field is considered. It is found that the waveguide mode can interact unstably with the beam by resonating with the Doppler shifted cyclotron frequency of the beam. This instability appears to be the mechanism by which microwaves are produced in some recent experiments which are discussed.

Height sensing device

Abstract: A height sensing device is provided which comprises a beam interrupter which swings into and out of the path of a beam passing from a light source to a photodiode. The beam interrupter is a disc rotatably mounted on a swinging sensor. The disc has a slot eccentrically located about the rotational axis of the disc. For adjustment, the disc may be rotated relative to the sensor to bring a portion of the slot into the beam path and then locked in such a position onto the sensor.

System for controlling attitude of laser beam plane

Abstract: A system is disclosed for controlling the plane of a laser beam in either horizontal, vertical or selected grade angle attitudes. The system includes an assembly comprising a housing which carries the operative components in a water-tight enclosure. The laser beam is generated by a laser tube carried on a chassis which is mounted for a limited range of universal movement about orthogonally intersecting Y and X axes by means of a gimbal frame. Optics structure is provided to route the beam in a collimated path along a Z-axis longitudinally of the chassis. A beam reflector unit rotatably mounted on an end of the chassis reflects the beam in a plane orthogonal to the Z-axis through a lighthouse structure provided in an end of the housing. The system for controlling the beam plane attitude includes three level sensing vials which are carried on the chassis, as well as a pair of Y- and X-axis motors and associated drive trains.

Electrostatic lens to focus an ion beam to uniform density

Abstract: A focusing lens for an ion beam having a gaussian or similar density profile is provided. The lens is constructed to provide an inner zero electrostatic field, and an outer electrostatic field such that ions entering this outer field are deflected by an amount that is a function of their distance from the edge of the inner field. The result is a beam that focuses to a uniform density in a manner analogous to that of an optical ring lens. In one embodiment, a conically-shaped network of fine wires is enclosed within a cylindrical anode. The wire net together with the anode produces a voltage field that re-directs the outer particles of the beam while the axial particles pass undeflected through a zero field inside the wire net. The result is a focused beam having a uniform intensity over a given target area and at a given distance from the lens.

Ion-beam implosion of fusion targets

Abstract: The performance of ion-beam: irradiated fusion targets consisting of a high-density spherical shell containing DT gas has been calculated. Breakeven with 10-MeV protons irradiating 1: 2-mm-diam targets can be produced with a beam current around 10 MA. Results for various target sizes and other beam particles and voltages are also discussed.

Study of a field‐ionization source for microprobe applications

Abstract: Operating parameters for a field‐ionization source have been measured. Sensitivities of 5×10−5 A sr−1 Torr−1 were found at 77 K. Angular distributions are uniform near ϑ=0° and show the beam to be confined to ±20°. The beam signal‐to‐noise ratio was found to increase with increasing current. Calculations based on parameters of lenses in use indicate a resolution of 0.1 μm at ≳10−10 A is possible.

Filamentary instability of a relativistic electron beam

Abstract: The transverse electromagnetic or filamentary instability is reconsidered in light of recent experiments which were unable to suppress the instability by increasing the beam transverse temperature. It is found that only the applied magnetic field can stabilize the mode. While the beam temperature can cause stabilization in a collisionless, $nu$=0, plasma, for finite $nu$ the beam is unstable to filamentation if $omega$/subp//subb/$sup 2$$beta$$sup 2$$gamma$>$Omega$/subc//sube/$sup 2$ regardless of its temperature. (AIP)

Multi-target X-ray source

Abstract: An X-ray source provides for selective variations of the wavelength spectra or other characteristics of the X-ray output by including means for directing an electron beam to any selected one of a plurality of target-anodes. Beam energy may also be selectively adjusted. Beam position stabilization means respond to any change of beam energy by making compensating changes in the deflection signals so that the electron beam remains directed at a selected point on a selected target-anode in the presence of energy changes including both deliberate energy changes and variations resulting from random voltage fluctuations in the beam accelerating system. The beam position stabilization means may include a function generator to accommodate to a non-linear relationship between beam energy changes and the deflection signal changes required to maintain a selected beam trajectory.

Low‐power cw generation of coherent anti‐Stokes Raman radiation in CH4 gas

Abstract: Coherent continuous wave anti‐Stokes Raman scattering has been observed using a fixed‐frequency pump beam (at the argon laser wavelength 514.53 nm) and a tunable‐frequency Stokes beam (provided by a dye laser at 605.41 nm) focused in a cell containing 1 atm of CH4 gas. This four‐frequency mixing experiment utilized the ν1 symmetric vibrational mode in CH4 at 2916.7 cm−1, resulting in a coherent anti‐Stokes signal at 447.37 nm. The power measured in the 514.53‐nm pump beam was 0.46 W and the effective power in the Stokes beam was calculated to be about 36 mW. The calculated coherent anti‐Stokes power of 2.18×10−13 W agrees very well with the experimentally observed anti‐Stokes power.

Ignition and maintenance of subsonic plasma waves in atmospheric pressure air by cw CO2 laser radiation and their effect on laser beam propagation

Abstract: Laser supported combustion (LSC) waves, subsonically propagating plasma, are ignited from solid targets and maintained in atmospheric pressure air using focused high−power, as high as 15 kW, cw CO2 laser radiation. The intensity necessary to ignite LSC waves, Ii, is measured for steel and aluminum targets as a function of beam focal spot diameter and wind velocity transverse to the beam axis as is the intensity needed to maintain such waves in atmospheric pressure air, Im. In the absence of a transverse wind, the quantities Ii and Im are essentially identical for diameters smaller than 0.1 cm, but Ii is substantially larger at larger diameters. The reason for this is discussed in terms of the ignition mechanism itself. In the presence of a 20−m/sec transverse wind, Im and Ii are essentially identical at 0.2 cm, the largest diameter for which Ii was determined. The measurements of Im extend to 0.5 cm beam diameter and indicate, in the absence of the transverse wind, that while thermal conduction determines...

Multiply-excited states in beam-foil spectroscopy

Abstract: The phenomenon of the excitation of multiply-excited states in beam-foil spectroscopy is reviewed: 1. Introduction, early B.F.S. work. 2. (a) Earlier non-beam-foil observations of multiply-excited states; (b) Theoretical development of multiply-excited systems; (c) Experiments and theory since 1961. 3. B.F.S. photon measurements: (a) He I isoelectronic sequence. (b) Li I isoelectronic sequence. (c) Be I and heavier isoelectronic sequences. (d) Transition probabilities, comparison of theory and experiment. 4. B.F.S. electron measurements. 5. The beam-foil interaction process. 6. Present problems and future aspects.

Interactions of relativistic electron beams with high atomic‐number plasmas

Abstract: A relativistically correct Fokker–Planck analysis is used to develop fluid equations which model the interaction of relativistic electron beams with high atomic‐number plasmas. The derived collision terms can be used to describe scattering and energy loss in materials ranging from the solid to high‐temperature plasma forms. The full set of equations discussed can be used to study electron‐beam‐initiated pellet fusion in a completely self‐consistent fashion. Specifically, the model is applicable to study of beam pinching in plasma‐filled diodes, the interaction of focused beams with target plasmas, and the transport of high ν/γ beams in high atomic‐number plasmas. Although the general equations are amenable to solutions only by computational techniques, analytic solutions describing the time‐dependent, collisional interaction of a beam in an infinite plasma and the one‐dimensional equilibrium of a beam in a plasma with applied electric field have been obtained.

Velocity distribution and angular distribution of molecular beams from multichannel arrays

Abstract: Three stainless‐steel multichannel arrays with channels of 16, 50, and 140 μm diameter have been investigated experimentally. The whole flow pattern is measured, including the angular dependence of the velocity distribution. All measurements concern the opaque mode, with λ⩽L and λ⩾α, where λ is the mean free path in the source and α and L are the radius and length of the channel, respectively. The results are compared with the predictions for free molecular flow in the same channel, as a function of the reduced source density n*=L/λ. The peaking factor is 30% lower than predicted by the Giordmaine‐Wang model. At angles ϑ larger than the half‐width–half‐maximum ϑ1/2 the angular distribution is not perturbed by the opaque conditions; for ϑ<ϑ1/2 it levels off to a lower peaking factor due to operation in the opaque mode. The center‐line beam shows an increasing loss of slow molecules for increasing n*. At n*=10 the gain in mean translational energy is 15%, much larger than predicted by the model of Olander. The deformation of the velocity distribution decreases with increasing angle, and for ϑ≳ϑ1/2 the molecules again have an unperturbed Maxwell‐Boltzmann distribution.