Generation of large-bandwidth x-ray free-electron-laser pulses

X-ray free-electron lasers (XFELs) are modern research tools in disciplines such as biology, material science, chemistry, and physics. Besides the standard operation that aims at minimizing the bandwidth of the produced XFEL radiation, there is a strong scientific demand to produce large-bandwidth XFEL pulses for several applications such as nanocrystallography, stimulated Raman spectroscopy, and multiwavelength anomalous diffraction. We present a self-consistent method that maximizes the XFEL pulse bandwidth by systematically maximizing the energy chirp of the electron beam at the undulator entrance. This is achieved by optimizing the compression scheme and the electron distribution at the source in an iterative back-and-forward tracking. Start-to-end numerical simulations show that a relative bandwidth of 3.25% full-width can be achieved for the hard x-ray pulses in the SwissFEL case.

Figure 1

SwissFEL layout including the $S$-band photocathode gun and boosters, the $X$-band linearizer cavity, the $C$-band linacs and the two bunch compressors. Two transverse deflector cavities, one situated after the first bunch compressor and the other at the end of the linac, will be used for longitudinal diagnostics. The lattice variables that are used in the optimization of the compression scheme are indicated in red, under their corresponding component. The different codes used to perform start-to-end tracking simulations, their range of usage and tracking direction (forward/backward) are indicated with the arrows below the lattice layout.

Figure 2

Product of the point-charge wake function with the full length of the $S$-band boosters (dotted red), the $X$-band linearizer (dashed green), and $C$-band linacs (dashed dotted blue) of SwissFEL.

Figure 3

Scheme of the procedure to generate electron beams with a large energy chirp at the entrance of the undulators, by optimizing the compression scheme of the linac and the electron distribution at the source.

Figure 4

Longitudinal phase space and current profile of the electron beam during the forward optimization of the compression scheme. The use of the semianalytic method already enables an overcompressed distribution with a large energy chirp (red). The numerical optimization has been applied to the output of the semianalytic method to fine-tune the current profile (blue).

Figure 5

Left: Comparison of the longitudinal phase space of the electron beam and their current profiles at the position of the laser heater. In blue the initial distribution (A) with a standard flat current profile and in red the modified source distribution (A”) with a ramped current profile, resulting from the backward optimization. Center: Comparison of the longitudinal phase space of the electron beam and their current profiles at the entrance of the undulators. Although the current profiles and bunch lengths are similar, the energy chirp is 50% larger for the input distribution with a ramped profile (red). Right: XFEL radiation from the Aramis undulators; the pulse bandwidth (FWHM) is 50% larger for the input distribution with a ramped profile (red).