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A model is described for tomographic phase-space mapping and emittance measurement in beams with space-charge. The tomographic results of an experiment with a 10 keV space-charge dominated electron beam are presented. The data are compared against a direct experimental sampling of the phase-space using a pinhole scan. It is found that the accuracy of the tomographic method is within 10% for beams with space-charge intensity less than 90%. With the aid of numerical simulation the observed similarities and differences between the tomography method and pinhole scan are discussed. [ABSTRACT FROM AUTHOR]

The study of modulated electron beams is important because they can be used to produce coherent radiation, but the modulations can cause unwanted instabilities in some devices. Specifically, in a free electron laser, proper prebunching at the desired emission frequency can enhance performance, while bunching resulting from instabilities and bunch compression schemes can degrade performance. In a photoinjector accelerator, tailoring the shape of the drive laser pulse could be used as a technique to either enhance or mitigate the effect of these modulations. This work explores the possibility of creating deeply modulated electron beams at the photocathode by using a modified drive laser designed to produce multiple subpicosecond pulses repeated at terahertz frequencies. Longitudinal space charge forces can strongly influence the evolution of modulations by converting density modulations to energy modulations. Experiments at the Source Development Laboratory electron accelerator at Brookhaven National Laboratory and PARMELA simulations are employed to explore the dynamics of electron beams with varying charge and with varying initial modulation. Finally, terahertz light generated by a transition radiator is used to confirm the structure of the electron beam. [ABSTRACT FROM AUTHOR]

Tomographic phase-space mapping in an intense particle beam is reviewed. The diagnostic is extended to beams with space-charge by assuming linear forces and is implemented using either solenoidal or quadrupole focusing lattices. The technique is benchmarked against self-consistent simulation and against a direct experimental sampling of phase-space using a pinhole scan. It is demonstrated that tomography can work for time-resolved phase-space mapping and slice emittance measurement. The technique is applied to a series of proof-of-principle tests conducted at the University of Maryland. [ABSTRACT FROM AUTHOR]

When considered as non-neutral plasmas, space-charge dominated charged particle beams have significantly smaller Debye lengths than their beam sizes. Therefore, collective effects due to space-charge forces are very important to the beam dynamics. As an example, density perturbations generated in a space-charge dominated charge particle beam will stimulate longitudinally space-charge waves. To study the modification of transverse beam distributions by longitudinal beam dynamics, experimental studies of low-energy electron beams, with and without longitudinal density perturbations, have been undertaken at the University of Maryland’s electron ring and the Long Solenoid Experiment (LSE). We have taken time-resolved beam images on these machines using two different diagnostics: Optical transition radiation, produced from an intercepting aluminized silicon screen, and a fast (<3 ns decay time) phosphor screen. Results from both techniques show that both the transverse size and transverse particle distribution of a space-charge dominated beam are affected by the longitudinal dynamics of the beam. In addition to the fast imaging measurements, longitudinal mean energy profiles of different beams have also been measured at different locations in the LSE system. [ABSTRACT FROM AUTHOR]

In this article we present a mammography unit design using a parametric x-radiation (PXR) source. We show that PXR can provide a fanned quasimonochromatic x-ray beam that can be used to obtain mammography images of higher contrast and lower dose than those obtained from a conventional x-ray system. Changing the Bragg angle of the PXR crystal with respect to the electron beam changes the photon energy, improves image quality, and minimizes dose. Monte Carlo computer simulations are given that show that the PXR source with a 5% bandwidth gives a figure of merit close to that of the ideal monoenergetic source and significantly higher than that of the filtered-x-ray-tube sources. In order to simultaneously obtain adequate flux and achieve bandwidths below 5%, we utilized an electron-beam energy of 35 MeV and an average current of 300 μA to 1 mA for 3 s (depending upon breast thickness and density). Slits after the PX radiator are used to define both the spatial distribution and the spectral bandwidth of the x-ray beam, which is scanned over the breast in approximately 3 s. A graphite crystal C (002) in the Laue geometry is utilized as the PX radiator. Lower electron-beam currents might be possible as higher efficiency PX or hybrid radiators become available. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Previously, a resonance theory had been devised by us for the acoustic transmission (T) and reflection coefficient (R) of a fluid layer, or elastic plate imbedded in a fluid. This theory, based on the use of a ''resonance approximation,'' was restricted to the case of light fluid loading, or of a large impedance mismatch at the boundaries between the external fluid and the fluid layer or plate. In the present paper, the meromorphic function T (together with R) for a fluid layer is represented by a (Mittag-Leffler type) series expansion which not only directly exhibits resonance properties, but also furnishes the value of each resonance peak correctly, including the effect of the underlying tails of all other resonances. Being exact, this expansion is now applicable to the case of heavy fluid loading, or of a close impedance match between the external fluid and the layer. [ABSTRACT FROM AUTHOR]

We present a resonance theory for the acoustic transmission and reflection coefficients of an elastic plate imbedded in a fluid medium, which includes the effects of plate viscosity. This formulation provides a direct means for determining the material parameters of the plate from the measured acoustic resonances of the Rayleigh and Lamb waves in the plate (i.e., their positions in frequency or angle, their widths, and their heights) which are given in our formalism by explicit analytic expressions that depend on the material parameters. Viscosity is seen to manifest itself in a decrease of the resonance peaks and in a broadening and frequency dependence of the resonance widths, which may be used to determine the frequency-dependent loss factor of the plate. [ABSTRACT FROM AUTHOR]

The vibratory response of a driven viscoelastic rod has been used in earlier investigations to measure the complex elastic modulus of the rod material. The transfer function T, i.e., the ratio of displacements (or accelerations) at the end of the rod divided by those at the driven end, is obtained as a function of frequency and rod length. In terms of these variables, it exhibits resonance peaks. In the present work, following the developments of Fiorito et al. [J. Acoust. Soc. Am. 79, 222 (1986)], the transfer function is represented as a meromorphic function of the frequency with complex poles, in order to isolate the individual resonances. Such a representation allows a clean determination of resonance frequencies and widths, and thus leads to a more sensitive measurement of the complex elastic modulus of the rod material. Comparison of exact theory and this resonance theory formulation of T is presented for various rod materials. [ABSTRACT FROM AUTHOR]

Our preliminary analysis [R. Fiorito, W. Madigosky and H. Überall, J. Acoust. Soc. Am. 62, S83(A) (1977)] of the acoustic properties of fluid-loaded elastic plates employing a resonance theory commonly used in nuclear physics. and recently applied to acoustic scattering [see, e.g., L. Flax, L. R. Dragonette and H. Überall, J. Acoust. Soc. Am. 63, 723 (1978)] has been completed, and numerical results for the resonance positions and widths of Rayleigh and Lamb modes in the acoustic reflection and transmission coefficients of a steel plate immersed in water have been obtained as functions of frequency-thickness or angle of incidence. Using resonance formalism, we have been able to explain analytically the previous empirical findings of Pitts et al. [L. E. Pitts, T. J. Plona, and W. G. Mayer, J. Acoust. Soc. Am. 60, 374 (1976)], regarding the frequency dependence of the positions of the poles and zeros of the reflection coefficient in the complex angle plane. Our resonance formalism has the added advantage of employing purely real equations (even for the case of fluid loading) to calculate the real and imaginary parts of pole and zero positions, hence allowing a great computational simplification. [H. Ü. is also at Catholic University and is supported in part by ONR, 421.] [ABSTRACT FROM AUTHOR]

Materials characterization can be carried out experimentally by measuring the complex modulus of linear viscoelastic materials from the displacement ratio of a bar, excited at one end, while the other end (which may carry an end mass) is allowed to move freely. This method, devised by Norris and Young, was recently developed [W. M. Madigosky and G. F. Lee, J. Acoust. Soc. Am. 73, 1374 (1983)] to cover a frequency range of 25-20 000 Hz. Standing waves cause resonances in the transfer function, and the positions and widths of these determine the real and imaginary parts of the modulus. The individual resonances are exhibited through a meromorphic representation of the transfer function; the corresponding facilitation in measuring resonance widths then leads to the possibility of experimentally determining the loss function of the material with improved accuracy. [ABSTRACT FROM AUTHOR]

The Shore or IRHD hardness test has been widely used for many years and has become one of the standards for characterizing materials in the rubber and plastics industries. The obtained hardness scale values, ASTM or IRHD, are often related to the static elastic properties of rubber-like materials. The classical Hertz equation has also been used to correlate the hardness scale values to the static elastic properties. Recently, progress has been made on the problem of obtaining dynamic viscoelastic properties through the use of a vibrating spherical indentor and modeling the measured input impedance by the radiation impedance of a sphere radiating into a half-space. In the limiting case where the sphere radius is small, such that ka<1 for longitudinal waves, it is shown that the measured shear modulus and loss factor are in good agreement with that found using other dynamic methods such as the DMTA or the resonance method. The model is most appropriate for relatively soft materials or very thick hard materials where the thickness of the material can be ignored. The problem of very hard materials and finite thickness where the terminating boundary is important will be discussed. [ABSTRACT FROM AUTHOR]

Sound transmission through a fluid layer which is imbedded in lighter density fluid is analyzed in terms of a resonant transmission theory. Expressions for the transmission coefficient (T) have been obtained indicating that T can be decomposed into resonance terms corresponding to symmetric and antisymmetric propagation modes in the layer. A computer progarm is utilized to compute both total T and T for each resonance for several cases of interest. Dispersion curves for the layer are obtained and compared to calculations and experimental data in the literature. The resonance widths are also calculated and an interpretation of total T as a function of angle of incidence and frequency thickness product is made in terms of these individual resonances. A similar but more complicated analysis holds for the elastic plate. [ABSTRACT FROM AUTHOR]

An acoustic transmission matrix program capable of computing transmission (T), reflection (R), and absorption (A) coefficients for a single or multiple layer system of elastic/viscoelastic plates immersed in a fluid for arbitrary angle of incidence and frequency has been developed by extending the formulation of Young for elastic layers. [J. Young, J. Acoust. Soc. Am. 59, 1500 (1976)] The program which utilizes elastic/ viscoelastic material properties is applied to several cases of interest. The single steel plate is studied parametrically for various loss factors; single-layer high-loss plates of neoprene and plexiglass and, finally, two-layered systems of low-loss/ high-loss materials (steel/neoprene, steel/plexiglass) are also examined. Transmission and reflection coefficients are interpreted in terms of propagation and material constants. Modal and half-wave resonances are discussed and used to interpret the behavior of T as a function of incidence angle and frequency thickness product. Comparison is made between theory and experiment. [ABSTRACT FROM AUTHOR]

A finite plane acoustic beam 5 in. in diameter at a frequency of 15 MHz is used to spot-insonify various simple geometrical shapes (i.e., cylinder, sphere, plates) both at normal and oblique incidence in a water tank. Echo levels are measured in the backscattered direction. Since a finite aperture transducer is used to both send and receive the signals, suitable Fresnelzone, absorption, and spreading corrections are made to compare measurements to theoretical predictions for the sonar cross sections. The sonar cross sections for the finite spot insonification cases of interest are developed from the Rayleigh-Kirchhoff integrals by means of the Kirchhoff approximation. The resulting expressions contain contributions from the specular reflection point, the (shadow) boundary of the spot, and interactions of these two. As expected, at high frequencies, the specular reflection contribution is dominant and good agreement between it and the measurements is found. [This work was partially supported by the Naval Sea Systems Command-0342.] [ABSTRACT FROM AUTHOR]