Search Results (15 total)

We report the first unambiguous demonstration of near-field imaging of optical diffraction radiation (ODR). The source of the ODR was an aluminum metal reflective surface with a 7-GeV electron beam passing nearby its single edge. Because of the high Lorentz factor γ involved, appreciable ODR is emitted at visible wavelengths even for impact parameters of 1 to 2 mm, so standard imaging techniques were employed. The experimental results are compared to a simple near-field model. We show that the ODR signals are sensitive to both beam size and position. Applications to multi-GeV beams in transport lines in the major synchrotron radiation facilities, x-ray free-electron lasers, energy recovering linacs, and the International Linear Collider are possible.

We report the first measurements of z-dependent coherent optical transition radiation (COTR) due to electron-beam microbunching at high gains ( >10⁴) including saturation of a self-amplified spontaneous emission free-electron laser (FEL). In these experiments the fundamental wavelength was near 530 nm, and the COTR spectra exhibit the transition from simple spectra to complex spectra ( 5% spectral width) after saturation. The COTR intensity growth and angular distribution data are reported as well as the evidence for transverse spectral dependencies and an “effective” core of the beam being involved in microbunching.

Optical transition radiation (OTR) has proven to be a versatile and effective diagnostic for measuring the profile, divergence, and emittance of relativistic electron beams with a wide range of parameters. Diagnosis of the divergence of modern high brightness beams is especially well suited to OTR interference (OTRI) techniques, where multiple dielectric or metal foils are used to generate a spatially coherent interference pattern. Theoretical analysis of measured OTR and OTRI patterns allows precise measurement of electron beam emittance characteristics. Here we describe an extension of this technique to allow mapping of divergence characteristics as a function of transverse coordinates within a measured beam. We present the first experimental analysis of the transverse phase space of an electron beam using all optical techniques. Comparing an optically masked portion of the beam to the entire beam, we measure different angular spread and average direction of the particles. Direct measurement of the phase-space ellipse tilt angle has been demonstrated using this optical masking technique.

We present quantifiable images of the angular distributions (AD’s) of parametric x radiation (PXR), and vacuum-ultraviolet transition radiation (vuv TR) from 230 MeV electrons interacting with a silicon crystal. Both AD’s are highly polarized. The vuv TR and optical TR data provide measurements of the beam energy and effective divergence angle. Using these quantities and separately known values of the electronic susceptibility ‖χ₀‖, we show that the measured PXR AD is in good agreement with the predictions of single crystal theory. Our analysis suggests a method to measure ‖χ₀‖ using PXR AD’s.

We have observed up to 8 orders (n) in the spectra of parametric x-radiation (PXR) in the range 5–40 keV, produced by the interaction of a 90 MeV electron beam with mosaic graphite and single silicon crystals. The measured yields and intensity ratios, I(n≥2)/I(n=1), in graphite are not in agreement with the theory of PXR for mosaic crystals. In comparison, the ratios of intensities in silicon are close to the predictions of PXR theory for perfect crystals. The bandwidths of spectral lines measured in both silicon and graphite are in good agreement with theoretical predictions.

The authors present a new formulation of the beam-density effect on energy loss by charged particles passing through matter, which exhibits an increased loss with a beam-shape dependence. This arises from a long-range dipolelike term contained in the two-particle vicinage function for cooperative energy loss by a pair of nonrelativistic particles. A new analytic expression for the vicinage function, which exhibits the long-range term, is also presented.

A general expression for the energy loss per unit length of a relativistic cluster of particles is derived. The collective nature of the energy loss is examined using a classical dielectric description of the medium to which the particles lose energy in distant collisions. A special case, in which the particles in the cluster are assumed to have equal velocities, is also given using an extension of Fermi's method for deriving the density effect. In the nonrelativistic limit of the general result, new expressions for collective energy loss are obtained for clusters of particles moving with different velocities, as well as for particles having identical velocities. The simple dielectric function used in the nonrelativistic results can represent the frequency-dependent response function for a gas, a solid, or a plasma medium.

Recent experimental results for p-⁴He elastic scattering at 1 GeV are in fair agreement with the previous Saclay data, and also with the effective channel approach calculation, while a recent multiple diffraction analysis has found the N^* effect important near the first diffraction minimum. In view of these, we comment on the role of the N^* and other rearrangement channels within the effective channel approach, and show that the effective channel approach contains these effects collectively. The nonorthogonality and double-counting problems are discussed.

NUCLEAR REACTIONS Effective channels for proton-⁴He scattering, rearrangement channels, N^* effects, nonorthogonality.

The first Born approximation (FBA) is applied to the calculation of single-electron-loss cross sections for various ions and atoms containing from one to seven electrons. Screened hydrogenic wave functions are used for the states of the electron ejected from the projectile, and Hartree-Fock elastic and incoherent scattering factors are used to describe the target. The effect of the target atom on the scaling of projectile ionization cross sections with repect to the projectile nuclear charge is explored in the case of hydrogenlike ions. Also examined is the scaling of the cross section with respect to the target nuclear charge for electron loss by Fe⁺²⁵ in collision with neutral atoms ranging from H to Fe. These results are compared to those of the binary-encounter approximation (BEA) and to the FBA for the case of ionization by completely stripped target ions. Electron-loss cross sections are also calculated for the ions O⁺ⁱ (i=3-7) and N⁺ⁱ (i=0-6) in collision with He targets in the energy range of ∼0.1 to 100 MeV/nucleon. These results are found to be in excellent agreement with the available data near the peak of the ionization cross section.

Elastic scattering of protons by helium targets are analyzed using the effective-channel approach at incident laboratory energies of 590 and 720 MeV. The helium nuclear form factor obtained from the electron scattering is used in the direct elastic channel to improve the large angle behavior of the cross section. Lack of reliable p-p and p-n data at these energy ranges severely limits the accurate determination of the input parameters, and makes the present analysis less definite than in the 1 GeV case studied earlier. However, the qualitative features of the result of the calculation show that the effect of the inelastic channels specifically associated with the excitations of the target system during the collision is large at large angles, and that the calculated cross sections are somewhat lower than the experimental data. The Coulomb effect is also studied at the 590 MeV case and found to be small.

NUCLEAR REACTIONS ⁴He(p ,p) elastic cross sections at 0.59, 0.72, and 1.00 GeV, calculated by the effective-channel approach.

The effective channel formulation of high-energy nuclear reactions is applied to the proton-helium system and the result is compared with experimental data. This formulation provides a consistent procedure for the evaluation of the ground state to effective channel coupling potential, average excitation energy, and the average potential in the effective channel. The effect of coupling to the effective channel as well as the effect of each quantity in that channel are demonstrated. The calculated cross sections were compared to the recent data from Saclay as well to the earlier Brookhaven data for 1 GeV p-⁴He elastic scattering. The possibility of using proton-nucleus scattering experiments to obtain a refined estimate of the ratio of the real part to the imaginary part of the proton-nucleon elastic scattering amplitude was investigated. The effect on the calculated cross section due to the uncertainty in the input parameters was carefully analyzed and found to be large enough to obscure the nature of any possible dynamical correlation effects and any possible off-shell effect in the two-body amplitude.

NUCLEAR REACTIONS ⁴He(p, p), E=1 GeV; calculated elastic σ(θ); compared to experiment. Effective channel theory.

The elastic scattering of high-energy particles by composite target nuclei is formulated in terms of coupled equations in which the effect of inelastic processes is represented by an average inelastic channel. The average fluctuation energy and fluctuation potentials which specify this channel are explicitly constructed and shown to be related to the two- and three-particle correlation functions. The theory incorporates approximately the effects of nonlocality, energy-dependence, rescattering, and absorption of all the inelastic channels. Systematic improvements of the method as well as several extensions of the formalism to target excitations and particle exchanges are discussed.

NUCLEAR REACTIONS Scattering theory of protons and pions by nuclei at high energies. Construction of the effective inelastic channels.

The proton-helium elastic scattering data at 1 GeV are analyzed using the effective-channel approach. The calculated spin-averaged cross section is in good agreement with the experiment from Saclay up to 35 deg, effectively with no free adjustable parameters. An estimate for the average excitation energy and for the ratio Ref / Imf of the nucleon amplitude is obtained.