Mode Locking in a Free-Electron Laser Amplifier

A technique is proposed to generate attosecond pulse trains of radiation from a free-electron laser amplifier. The optics-free technique synthesizes a comb of longitudinal modes by applying a series of spatiotemporal shifts between the copropagating radiation and electron bunch in the free-electron laser. The modes may be phase locked by modulating the electron beam energy at the mode spacing frequency. Three-dimensional simulations demonstrate the generation of a train of 400 as pulses at gigawatt power levels evenly spaced by 2.5 fs at a wavelength of 124 Å. In the x-ray at wavelength 1.5 Å, trains of 23 as pulses evenly spaced by 150 as and of peak power up to 6 GW are predicted.

Figure 1

Schematic of three regimes of FEL interaction: (a) SASE regime (b) Mode-coupled SASE regime and (c) Mode-locked SASE regime. The inset shows a detail of the electron delay.

Figure 2

The radiation power as a function of time $t$ with radiation power spectrum as a function of wavelength in nm (inset) for the three cases of (a) SASE, (b) mode coupled, and (c) mode locked.

Figure 3

A mode-locked 1.5 Å x-ray FEL example: the radiation power as a function of time $t$ with radiation power spectrum as a function of wavelength in nm.