Generation of subterawatt-attosecond pulses in a soft x-ray free-electron laser

We propose a novel scheme to generate attosecond soft x rays in a self-seeded free-electron laser (FEL) suitable for enabling attosecond spectroscopic investigations. A time-energy chirped electron bunch with additional sinusoidal energy modulation is adopted to produce a short seed pulse through a self-seeding monochromator. This short seed pulse, together with high electron current spikes and a cascaded delay setup, enables a high-efficiency FEL with a fresh bunch scheme. Simulations show that using the Linac Coherent Light Source (LCLS) parameters, soft x-ray pulses with a FWHM of 260 attoseconds and a peak power of 0.5 TW can be obtained. This scheme also has the feature of providing a stable central wavelength determined by the self-seeding monochromator.

Figure 1

Layout of the proposed scheme based on a self-seeded FEL configuration (not to scale). A time-energy chirped electron beam (a) is used. A modulator comprising an infrared optical laser and a short wiggler (W) is added to modulate the chirped electron beam (b). The seeded FEL stage consists of three undulator sections (U2A, U2B, and U2C) separated by two delay chicanes. (c), (d), (e), and (f) are sketches of electron current (blue curves) and FEL power (red curves) profiles after U1 and before U2A, U2B, and U2C, respectively.

Figure 2

Electron beam longitudinal phase space before the SASE undulator (a) and after the bypass chicane (b). The Wigner transformation of the SASE field generated in U1 is shown in (c), and the electron current profile after the bypass chicane is show in (d). The electron bunch head is to the right.

Figure 3

Simulation results of rms electron energy spread (upper) and FEL power profile (lower) right after the bypass chicane (a) and U2A (b), U2B (c), and U2C (d) undulators. The electron bunch head is to the right. The three high energy spread regions in (a) correspond to the three currents spikes from the center to the right in Fig. 2.

Figure 4

Multiple-shot simulation results of FEL power profile (left) and spectrum (right) at the end of U2C undulator. The solid curves in the plots are the average over all shots.

Figure 5

An example of FEL power profiles at U2A entrance (a), U2A exit (b), U2B exit (c), and U2C exit (d) with a misalignment of 8 fs between the modulation laser and the electron bunch. Multi-spikes appear at the first amplifier stage (U2A), but the cascaded delay helps select the dominant spike.