COMPARATIVE-ANALYSIS OF OPTICAL-TRANSITION-RADIATION-BASED ELECTRON-BEAM EMITTANCE MEASUREMENTS FOR THE LOS-ALAMOS FREE-ELECTRON LASER

A series of measurements have been performed to determine the emittance of the electron beam of the LANL FEL. These measurements were made both before and after installation of a photoelectric injector on the LANL linac. An optical-transition-radiation (OTR) interferometer was used to determine the beam divergence while simultaneously using the beam spot imaged in OTR to determine the spatial profile at a beam waist. We have developed two methods to analyze the OTR interference patterns in order to determine the beam divergence from the observed interference-fringe visibility. The first method relies on a simple analytical model in which a Gaussian beam divergence is convolved with the OTR interference pattern for a single particle. The second method uses a numerical convolution of the phase-space distribution in beam angle and energy with the single-particle OTR interference pattern. The six-dimensional phase space used to simulate the experimental OTR patterns was produced by the particle code PARMELA which was run for the beam parameters appropriate for the two sets of experiments. We present a comparison of these two methods with each other and with the experimental observations. The effects of both the beam divergence and the beam energy spread on the emittance measurement is illustrated. © 1990.