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

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.