Direct measurement of electron bunch shapes and coherent undulator radiation produced by 100 femtosecond structure

Abstract

At the Stanford FEL Center, magnetic longitudinal dispersion has been used to improve the well-known technique of measuring electron bunch time-profiles with off-phase acceleration and an energy spectrometer. This unambiguous direct method allowed us to measure, without extrapolation or the mathematical reconstructions of other high-resolution techniques, a variety of bunch profiles at approximately 100 fs resolution – including asymmetric bunches with a single extremely narrow peak. The sharply peaked bunches generated far-infrared undulator radiation two orders of magnitude brighter than bunches without such structure. A Fabry–Perot-type scan of the undulator radiation indicated nearly 100% coherence of the radiation from the brightest bunches, and provided optical evidence consistent with the bunch shape measurements.