Two and three dimension simulations of Smith-Purcell terahertz radiation using a particle-in-cell code

Although several 2-D simulations of Smith-Purcell (SP) radiation using particle-in-cell (PIC) codes have been performed, only recently have 3-D simulations been reported. To some extent this is caused by the requirements of computer memory and time, which are much greater in the latter. We present our new 3-D results concerning the simulation of the MIT experiment that used a pre-bunched 15 MeV beam to generate terahertz SP radiation. In particular, we compare the new results with our older 2-D simulations, in order to see which aspects survive in 3-D simulations. We also address the question of power radiated according to the two simulations.     

First observation of coherent Smith-Purcell radiation in the highly relativistic regime

Coherent Smith-Purcell (SP) radiation has already been applied as a technique to measure the longitudinal bunch profile of charged particle beams in the low to intermediate energy range. However, with the advent of the International Linear Collider and the need to develop a non-invasive method of measuring the bunch profile, it has become necessary to carry out experiments at the highest possible energies. The paper summarizes some recent work at intermediate (45 MeV) energy and presents the first observations of SP radiation from a 28.5 GeV beam at SLAC. The experimental challenges and future possibilities of coherent Smith-Purcell radiation as a longitudinal bunch profile diagnostic tool are also discussed.     

The features of synchrotron radiation from a relativistic particle rotating inside a spherical cavity

We have studied the radiation from a relativistic charged particle in a uniform rotation along the equatorial orbit inside a spherical cavity immersed in continuous loss-free dielectric, the permittivity of which in the frequency range under consideration is ε > 1. A formula for calculation of radiation intensity at large distances from the cavity is derived. It is shown that with a special (resonant) choice of a non-dimensional parameter ξ the intensity of synchrotron radiation in the presence of a cavity may be either amplified or reduced almost by times compared with particle rotation in empty space. The resonance value of ξ is determined by the number of harmonics and is independent of other parameters. A visual explanation of this phenomenon is given and its possible application is discussed.     

Transition radiation in the pre-wave zone for an oblique incidence of a particle on the flat target

Backward transition radiation of an electron with an arbitrary energy is studied when an ideally-conducting target is flat and its normal is not collinear with the particle velocity (oblique incidence). A model for radiation registered with a small flat detector placed at a finite distance (including the radiation in the pre-wave zone) in the vicinity of specular reflection direction is developed. Characteristics of the radiation in the far-field zone are in complete agreement with well-known results. The calculations for pre-wave zone show that the angular distribution of the radiation intensity is distorted compared to the far-field case, and the radiation asymmetry (having a place in the far-field for moderately relativistic energies) is also preserved in the pre-wave zone. Some numerical estimations of the radiation asymmetry at a finite distance are also given. The technique developed may be used for estimations of coherent transition radiation intensity in the mm-wavelength range.     

Imaging of optical diffraction radiation in pre-wave zone and spatial resolution of non-invasive beam size diagnostics

A simple model to evaluate the imaging shape of an optical diffraction radiation (ODR) source focused by a lens on a detector, taking into account the pre-wave zone effect has been developed. The characteristic size of an ODR image does not depend on the Lorentz-factor and is defined by the impact-parameter (minimal distance between a particle trajectory and ODR target edge) only. Using the ODR intensity component polarized parallel to the target edge it is possible to significantly improve the spatial resolution of an ODR beam profile monitor.     

Coherent X-radiation of relativistic electrons in a single crystal under asymmetric reflection conditions

Coherent X-radiation of a relativistic electron crossing a single crystal plate with constant speed is considered in the two-wave approximation of the dynamic diffraction theory [Z. Pinsker, Dynamical Scattering of X-rays in Crystals, Springer, Berlin, 1984] in a Laue geometry. Analytical expressions describing the spectral-angular distribution of parametric X-radiation (PXR) and diffracted transition radiation (DTR) formed on a system of parallel atomic planes situated at an arbitrary angle δ to the surface of the crystal plate (asymmetric reflection) are derived. The dependences of the PXR and DTR spectral-angular density and their interference with angle δ are studied.     

Focusing of transition radiation from a paraboloidal target

For the first time the focusing effect of coherent transition radiation generated in a paraboloidal target by electrons with energy of 6.1 MeV has been observed experimentally. A comparison of the angular distribution of detected radiation was made for flat and paraboloidal targets under focusing and defocusing conditions. Using paraboloidal targets one can considerably increase the spectral-angular density of the radiation in the paraboloidal focus without any additional optical devices.     

Model for X-ray Cherenkov radiation by MeV electrons at grazing incidence

A possible source of radiation in the X-ray range can be obtained by penetration of a dielectric slab by electrons with sufficient kinetic energy. We present a near-exact solution of Maxwell equations to obtain the intensity of Cherenkov (and transition) radiation from slabs thicker than the absorption length, the only approximation being that the observation angle of the radiation with respect to the electron axis is taken to be small. Contrary to earlier models our model accounts for all relativistic electron energies, dielectric constants and grazing incidence angles. Also scattering of the electrons is incorporated very accurately, including the effect of scattering on the electro-magnetic fields within the slab. We report significant differences with earlier calculation for radiation yields from e.g. Si. A new effect has been found for intermediate angles of incidence (around 45°). At these angles, the cone of Cherenkov radiation exhibits pronounced minima near the direction perpendicular to the plane of incidence. We present arguments based on the special characteristics of Cherenkov radiation why in that direction internal reflection of the radiation occurs at the medium-vacuum interface.     

Resonant diffraction radiation from inclined gratings and bunch length measurements

A simple scheme for the measurement of sub-mm bunch lengths using coherent resonant diffraction radiation (CRDR) from a tilted grating is proposed. The CRDR spectral-angular characteristics have been calculated using an adapted Kirchhoff model, taking into account the pre-wave zone effect. It is shown that the latter leads to a distortion of the CRDR monochromaticity. Choosing the appropriate distance between grating and detector such that the pre-wave zone effect becomes negligible, it is possible to measure the CRDR yield in the sub-THz range by a broadband detector. While changing the grating inclination angle with respect to the beam axis, the CRDR line is shifted and it is possible to obtain information about the bunch length, measuring the signal ratio from two detectors located at fixed observation angles instead of complicated spectral measurements which rely on absolute values of the intensity.     

The Borrmann effect in parametric X-radiation under asymmetric reflection conditions

Parametric X-radiation (PXR) of a relativistic electron traversing a single crystal plate is considered in Laue geometry. The expressions describing spectral-angular distributions of PXR formed on the atomic planes situated under arbitrary angle δ to surface of the plate (asymmetric reflection) obtained on basis of two-wave approximation of dynamic diffraction theory are used for definition of the conditions of the most pronounced manifestation of the Borrmann effect (optimal value of angle δ) are clarified. This effect leads to considerable increase of the intensity of the quasi-monochromatic tuning source of coherent X-radiation built on basis of PXR.     

Dependence of PXR beam performance on the operation of the pulsed electron linac

Advanced applications of parametric X-ray radiation (PXR) such as energy-dispersive X-ray absorption fine structure (DXAFS) analysis and phase-contrast imaging have been developed at the Laboratory for Electron Beam Research and Application (LEBRA) of Nihon University. To improve the electron beam quality and geometrical stability of the target crystal, the cooling-water system for the linac and the PXR target was replaced with a new one capable of more precise control. As a result, the reliability of the experimental data in PXR applications, especially in X-ray imaging, has improved. The effect of the electron beam focusing on the target crystal was also investigated. The results of X-ray imaging with a long propagation distance and measurement of the time-structure of the PXR intensity suggest that the correlation between the electron beam profile and the X-ray coherence is rather complicated. It is possible that incident electrons cause some deformation of the target crystal, becoming the dominant factor restricting the quality of intense PXR.     

An analytic formalism for the emission of coherent transition radiation from an oblique finite thin metallic target screen

Long-wavelength coherent transition radiation is commonly used in electron beam diagnostics for the determination of bunch lengths. Typically the spectrum of coherent transition radiation for a short bunch accelerator is settled in the low or sub-THz regime. Hereby, we present a theoretical model based on physical optical techniques in order to calculate emission characteristics for transition radiation, for both the radiating near-field and the far-field. This approach yields analytic solutions for the emitted electromagnetic fields without the need to solve integral equations. The simulated intensity distribution is compared with measurements showing good agreement.     

Angular distributions of diffracted X-ray radiation from channeled electrons in Si and LiF Crystals: Influence of energy levels band structure

It is shown that the band structure of the energy levels of planar channeled electrons qualitatively changes the angular distributions of X-rays emitted at Bragg angles.     

Formation of spectral characteristics of channeling radiation from 800 to 2000 MeV electrons and positrons in a thin silicon crystal

The influence of the tails of particle trajectories on planar channeling radiation (CR) spectra from relativistic (800-2000 MeV) electrons and positrons in a thin silicon crystal is investigated. It is shown that the trajectory tails significantly change the CR spectra from electrons and positrons in specific parts of the spectra compared to calculations which do not take into account this effect.     

Grain boundary segregation in low Cr Fe-Cr alloys: The effect of radiation induced vacancies studied by metropolis Monte Carlo simulations

Radiation damage may modify the segregation state and phase separation conditions in Fe-Cr alloys with compositions and temperatures of technological interest. We use Metropolis Monte Carlo simulations to study segregation effects at the best stable variant of the Σ = 5 (2 1 0) grain boundary, in the 5-10 at.% range of Cr composition. The role of irradiation induced vacancies in segregation is discussed. At a low Cr composition, an oscillatory Cr segregation profile is evidenced in the vicinity of the boundary. Under specific conditions, Cr ordering is observed close to the boundary. The correlations between such ordering and local stress is discussed. The binding energy of vacancies to specific sites of the grain boundary is found to be positive and the interplay between Cr segregation and the clustering of vacancies at the boundary is discussed.     

Comparative study of different initiation methods in the functionalization of low-density polyethylene with diethyl maleate: Gamma radiation and ultrasound

This work unfolds a comparative study of the grafting of a low-density polyethylene with diethyl maleate (DEM) using gamma irradiation and ultrasound as means of initiating the grafting reactions. The grafting degree was determined by FTIR using a reported calibration curve. The efficiency of both functionalization methodologies was calculated and the polymers were characterized by differential scanning calorimetry (DSC) and gel permeation chromatography (GPC). The results showed that the grafting degree increases with the radiation dose when the monomer is inserted by gamma radiation. When the ultrasound methodology was used, an additional initiator such as dicumyl peroxide had to be used in order for the grafting to take place. In this last case, changing the time of exposure to the ultrasound source did not induce significant changes in the obtained grafted degree. The use of ultrasonic radiation plus dicumyl peroxide as grafting initiators promoted more insertion than that when gamma radiation was employed.     

Shielding data for 100-250 MeV proton accelerators: Attenuation of secondary radiation in thick iron and concrete/iron shields

Double differential distributions of neutrons produced by 100, 150, 200 and 250 MeV protons stopped in a thick iron target were calculated with the FLUKA Monte Carlo code at four emission angles: forward, 45°, transverse and 135° backwards. The attenuation in thick iron shields of the dose equivalent due to neutrons, protons, photons and electrons was also calculated. The contribution to the total ambient dose equivalent from photons and protons is limited to a few percent at maximum. Source terms and attenuation lengths are given as a function of energy and emission angle, along with fits to the Monte Carlo data, for shallow depth and deep penetration in the shield. A brief discussion of simulations performed with composite iron/concrete shields is also given, showing the need for further investigations.