Coherent transition radiation in the far-infrared region

Experimental Characterization of Electron-Beam-Driven Wakefield Modes in a Dielectric-Woodpile Cartesian Symmetric Structure

Photonic structures operating in the terahertz (THz) spectral region enable the essential characteristics of confinement, modal control, and electric field shielding for very high gradient accelerators based on wakefields in dielectrics. We report here an experimental investigation of THz wakefield modes in a three-dimensional photonic woodpile structure. Selective control in exciting or suppressing of wakefield modes with a nonzero transverse wave vector is demonstrated by using drive beams of varying transverse ellipticity. Additionally, we show that the wakefield spectrum is insensitive to the offset position of strongly elliptical beams. These results are consistent with analytic theory and three-dimensional simulations and illustrate a key advantage of wakefield systems with Cartesian symmetry: the suppression of transverse wakes by elliptical beams.

Single-shot coherent diffraction imaging of microbunched relativistic electron beams for free-electron laser applications

With the advent of coherent x rays provided by the x-ray free-electron laser (FEL), strong interest has been kindled in sophisticated diffraction imaging techniques. In this Letter, we exploit such techniques for the diagnosis of the density distribution of the intense electron beams typically utilized in an x-ray FEL itself. We have implemented this method by analyzing the far-field coherent transition radiation emitted by an inverse-FEL microbunched electron beam. This analysis utilizes an oversampling phase retrieval method on the transition radiation angular spectrum to reconstruct the transverse spatial distribution of the electron beam. This application of diffraction imaging represents a significant advance in electron beam physics, having critical applications to the diagnosis of high-brightness beams, as well as the collective microbunching instabilities afflicting these systems.

Radio-frequency measurements of coherent transition and cherenkov radiation: implications for high-energy neutrino detection

We report on measurements of (11-18)-cm wavelength radio emission from interactions of 15.2 MeV pulsed electron bunches at the Argonne Wakefield Accelerator. The electrons were observed both in a configuration where they produced primarily transition radiation from an aluminum foil, and in a configuration designed for the electrons to produce Cherenkov radiation in a silica sand target. Our aim was to emulate the large electron excess expected to develop during an electromagnetic cascade initiated by an ultrahigh-energy particle. Such charge asymmetries are predicted to produce strong coherent radio pulses, which are the basis for several experiments to detect high-energy neutrinos from the showers they induce in Antarctic ice and in the lunar regolith. We detected coherent emission which we attribute both to transition and possibly Cherenkov radiation at different levels depending on the experimental conditions. We discuss implications for experiments relying on radio emission for detection of electromagnetic cascades produced by ultrahigh-energy neutrinos.

Observation of coherent cerenkov radiation from a solid dielectric with short bunches of electrons

Short bunches of 150-MeV electrons of a linear accelerator passed along the surface of a crystal quartz or a teflon and coherent Cerenkov radiation from the solid dielectrics has been observed in the wavelength range from 0.5 to 4 mm. Properties of the radiation have been experimentally investigated. The angular distribution of the observed radiation showed a maximum peak in the direction of the Cerenkov angle with several satellite peaks. The intensity increased linearly with increasing the length of the medium and was proportional to the square of the number of electrons in the bunch. The spectral intensity was enhanced by almost five orders of magnitude in comparison with the theoretical calculation of incoherent radiation.

Grating transition radiation: a source of quasimonochromatic radiation

We have observed transition radiation at optical wavelengths, emitted by 3-13-MeV electrons interacting with a diffraction grating. Near the direction of specular reflection we observed broadband radiation with the same properties as optical transition radiation from a flat surface. In addition, at large angles with respect to this direction and for low angles of incidence of the electrons on the grating surface, we observed the emission of quasimonochromatic radiation. This grating transition radiation may offer an alternative method for production of quasimonochromatic radiation in the far-infrared to mm wavelength range.