Electron beams with a sinusoidal energy modulation have the potential to emit subfemtosecond x-ray pulses in a free-electron laser. An energy modulation can be generated by overlapping a powerful infrared laser with an electron beam in a magnetic wiggler. We report on a new infrared source for this modulation, coherent radiation from the electron beam itself. In this self-modulation process, the current spike on the tail of the electron beam radiates coherently at the resonant wavelength of the wiggler, producing a six-period carrier-envelope-phase (CEP)-stable infrared field with gigawatt power. This field creates a few MeV, phase-stable modulation in the electron-beam core. The modulated electron beam is immediately useful for generating subfemtosecond x-ray pulses at any machine repetition rate, and the CEP-stable infrared field may find application as an experimental pump or timing diagnostic.

• Received 17 May 2019
• Revised 26 September 2019

DOI:https://doi.org/10.1103/PhysRevLett.123.214801