Observation of Very High-order Electron-radiation Coupling in a 7$^{th}$ Harmonic IFEL Experiment

FEL/IFELs based on the resonant interaction between laser and electron beams in an undulator would benefit from using efficient high-order resonances. The high-order harmonic FEL/IFEL interactions were considered theoretically as a technique for reduction of the beam energy without corresponding decrease in the undulator period and the magnetic field strength in a single-pass X-ray FEL. We demonstrate microbunching of the 12.3 MeV electrons in a 7$^{th}$ order IFEL interaction, where the seed radiation frequency is seven times higher then the undulator's fundamental frequency. Strong longitudinal modulation of the beam is inferred from the observation of the first, second and third harmonics of the seed radiation in a Coherent Transition Radiation spectrum. The highest CTR frequency is the 21$^{st}$ harmonic of the fundamental interaction. The level of seed power is comparable to that required for microbunching at the fundamental frequency in this undulator. This work was supported by DOE grant DE-FG03-92ER40727.