Optical Free-Electron Lasing Driven by a Flying Focus*

Optical free-electron lasers (OFELs) replace magnetic undulators with the electromagnetic field of a laser pulse. By decreasing the undulator period to half an optical wavelength, OFELs lower the electron energy needed to produce the same radiation frequency and shorten the interaction length needed for the same gain. Appreciable amplification in an OFEL requires sustaining a highly uniform, high-intensity laser field over the interaction length. Here we evaluate the application of flying focus pulses to OFELs. Flying-focus pulses feature an intensity peak that travels at a tunable velocity over many Rayleigh ranges while maintaining a near-constant profile. An intensity peak that travels backward with respect to the phase fronts of the pulse enables an OFEL configuration in which an electron beam collides head-on with the phase fronts and experiences a near-constant undulator strength over the entire interaction length.