Nondestructive diagnostics for relativistic picosecond bunched electron beams

The duration and form of relativistic picosecond electron bunches in the Duke University Mark III free-electron laser have been nondestructively measured by monitoring the submillimeter radiation produced by the bunches as they pass by or through a rectangular waveguide. Unlike other methods, our technique produces negligible electron bunch perturbation and allows real-time beam diagnostics to be performed simultaneously with free-electron laser (FEL) operation. We have measured 2.1-ps full width at half maximum duration electron bunches, studied the effect of electron gun and FEL modifications on bunch duration, and observed electron bunch variations during bunch trains.