Subpicosecond electro-optic measurement of relativistic electron pulses

Time-resolved measurements of the transverse electric field associated with relativistic electron bunches are presented. Using an ultrafast electro-op tic sensor close to the electron beam, the longitudinal profile of the elec tric field was measured with subpicosecond time resolution and without time -reversal ambiguity. Results are shown for two cases: inside the vacuum bea m line in the presence of wake fields, and in air behind a beryllium window , effectively probing the near-field transition radiation. Especially in th e latter case, reconstruction of the longitudinal electron bunch shape is s traightforward.