Femtosecond and Subfemtosecond X-Ray Pulses from a Self-Amplified Spontaneous-Emission–Based Free-Electron Laser

We propose a novel method to generate femtosecond and subfemtosecond photon pulses in a free-electron laser by selectively spoiling the transverse emittance of the electron beam. Its merits are simplicity and ease of implementation. When the system is applied to the Linac Coherent Light Source, it can provide x-ray pulses the order of 1 fs in duration containing about ${10}^{10}$ transversely coherent photons.

Figure 1

(a) Sketch of electron bunch at center of magnetic bunch-compressor chicane with tilted beam in horizontal, $x$, and longitudinal coordinates, $t$. (b) The slotted foil at chicane center leaves a narrow, unspoiled beam center.

Figure 2

Distribution of electrons at chicane center showing the very strong position-time ($x$-$t$) correlation (tilt).

Figure 3

Peak current (red ●) and FWHM pulse length (blue ◇) of unspoiled electrons vs slot half-width, $\Delta x$. The minimum $e^{-}$ pulse at full current is 8 fs ($\Delta x=125\mu \mathrm{m}$).

Figure 4

Temporal (right) and energy (left) profiles, and phase space (center) at undulator entrance. The central spike in the temporal profile is due to the slotted foil.

Figure 5

Electron current and GENESIS 1.3 simulation of 2-fs FWHM LCLS x-ray pulse at saturation ($z\approx 60\mathrm{m}$).

Figure 6

GENESIS 1.3 simulation showing radiation power for three values of $t$, varying along the undulator length.