Longitudinal electron beam and free electron laser microbunch measurements using off-phase rf acceleration

Magnetic dispersion followed by off-phase rf acceleration and an energy spectrometer were used to measure the longitudinal density profile of bunched relativistic electron beams with a resolution approaching 100 fs. Various electron bunch shapes were measured by this method and compared to bunch length measurements derived from the spectra of coherent transition radiation produced as the bunches traversed a thin foil. A reasonable agreement was found between the two bunch length methods. The off-phase rf acceleration method was then used to observe the modulated profile of a microbunched electron beam at the exit of a far-infrared free electron laser oscillator. In a preliminary study of the relationship between optical cavity power and the microbunch distribution, the density modulation and the energy spread of the electron beam were seen to increase as the optical field strength increased.