Efficient harmonic microbunching in a 7th-order inverse-free-electron laser interaction

We have shown that a seventh-order inverse-free-electron laser (IFEL) interaction, where the radiation frequency is the seventh harmonic of the fundamental resonant frequency, can microbunch a beam of relativistic electrons inside an undulator. Using coherent transition radiation (CTR) emitted by the bunched 12.3 MeV beam as a diagnostic, strong microbunching of the beam is inferred from the observation of CTR at the first, second, and third harmonics of the seed $10\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ radiation. Three-dimensional IFEL simulations show that the observed harmonic ratios can be explained only if transverse spatial distribution of the steepened bunched beam is taken into account.