In an IFEL electrons interact resonantly with a high-power laser in an undulator, and gain or lose energy according to their position in the resonant phase bucket. In general, the electron bunch length is much longer than the laser wavelength, so in a single stage IFEL, some of the electrons are accelerated while the rest are decelerated. This leads to a large energy spread in the final beam. 

If the beam is first microbunched at the laser wavelength, however, a larger fraction of electrons can sit at the accelerating phase and be boosted to high energy. In this two stage IFEL scheme, the final energy spread is reduced and the efficiency of the IFEL is increased. At NLCTA, we recently demonstrated this principle in the optical regime. An 800 nm weak laser pulse was used to generate a small energy modulation the 120 MeV beam in the first undulator. After traversing the subsequent chicane, the beam became microbunched (or density modulated) at 800 nm. A second, more powerful 800 nm laser then accelerated or decelerated the electrons in a second undulator, changing the energy of the microbunches by 200 keV. This is proof-of-principle demonstration of a cascaded IFEL in the optical regime suggests a path toward ultra-high gradient IFELs to generate GeV caliber e-beams with TW level commercial laser sources. 

For more details, see the paper published in Physical Review Letters:PhysRevLett.110.244801.pdfPhysRevLett.110.244801.pdf​​​
or the article in Symmetry Magazine​