Evidence for transverse dependencies in COTR and microbunching in a SASE FEL

Abstract

Using coherent optical transition radiation (COTR) techniques, we have observed transverse dependencies, which in some aspects relate to the electron beam microbunching in a visible wavelength (540 nm) self-amplified spontaneous emission (SASE) free-electron laser (FEL). The experimental COTR observations include the zdependent e-beam sizes, the z-dependent angular distributions, and the z-dependent spectra (which show an x-dependence). A 30-40% narrowing of the observed beam size using COTR is explainable by the mechanism's dependence on the square of the number of microbunched particles. However, additional effects are needed to explain beam size reductions by factors of 2-3 at different z locations. Localized e-beam structure in the gun or induced in the bunch compression process may result in microbunching transverse dependence, and hence the observed COTR effects.

Tapered helical undulator system for high efficiency energy extraction from a high brightness electron beam

“…The resulting energy modulation amplitude (up to 2.8 MeV peak-to-peak), is then converted into density modulation by a tunable R56 (10-20 µm) chicane prior injecting the beam at the right phase with respect to the seed pulse into the TESSA-266 gap-tapered undulator. A microbunching diagnostic chamber [32,33,34] is to be installed in the section between the prebuncher and the main undulator to optimize the bunching factor. The microbunching diagnostics is based on the generation of coherent optical transition radiation (COTR) which would be enhanced by almost seven orders of magnitude over incoherent OTR with only a 10% microbunching fraction.…”

Transverse profile imager for ultrabright electron beams

“…Recent developments toward the highest brilliance X-rays and toward sub-femtosecond pulse length are leading to more stringent requirements on this instrumentation. Besides the increased demands on resolution, a new effect has been observed in recent years: highly compressed electron beams emit coherent optical transition radiation (COTR) at visible wavelengths [4][5][6][7][8][9], which impairs the measurement of profiles if it enters the imaging system.…”