Electron pulses generated at the Stanford SUNSHINE facility with (Z-4.6) x 10' electrons per microbunch have been measured as short as 50fs (rms). The bunch length is determined by optical autocorrelation via a far-infrared Michelson interferometer using coherent transition radiation emitted at wavelengths longer than and equal to the bunch length. This frequency-resolved autocorrelation method demonstrates a better sub-picosecond resolving power than any existing time-resolved method. The experimental setup and the results will be described.

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