An X-ray free-electron laser with a highly configurable undulator and integrated chicanes for tailored pulse properties

Abstract

X-ray free-electron lasers (FELs) are state-of-the-art scientific tools capable to study matter on the scale of atomic processes. Since the initial operation of X-ray FELs more than a decade ago, several facilities with upgraded performance have been put in operation. Here we present the first lasing results of Athos, the soft X-ray FEL beamline of SwissFEL at the Paul Scherrer Institute in Switzerland. Athos features an undulator layout based on short APPLE-X modules providing full polarisation control, interleaved with small magnetic chicanes. This versatile configuration allows for many operational modes, giving control over many FEL properties. We show, for example, a 35% reduction of the required undulator length to achieve FEL saturation with respect to standard undulator configurations. We also demonstrate the generation of more powerful pulses than the ones obtained in typical undulators. Athos represents a fundamental step forward in the design of FEL facilities, creating opportunities in FEL-based sciences.

Fig. 1. Athos sketch and images.

Top: SwissFEL schematic (not to scale), adapted from Fig. 1 in Abela, R. et al. The SwissFEL soft x-ray free-electron laser beamline: Athos. J. Synchrotron Rad. 26, 1073–1084 (2019). Bottom: view of the undulator (left), picture of an undulator module (centre), and picture of an intra-undulator chicane installed in the tunnel (right).

Fig. 2. Measured FEL gain curves.

The error bars represent one standard deviation of uncertainty. a Close to maximum achieved pulse energy for photon energies of 1.2 keV and 360 eV. b Comparison between helical and planar undulator configuration with and without OK for a photon energy of 545 eV. Circles indicate the measured values, solid lines connect the measurement points, dashed lines are exponential fits to the first undulator modules, diamonds correspond to saturation points.

Fig. 3. Measurement of variable polarisation for soft X-rays at 700 eV.

The plots show the recoil ion momentum distribution of He⁺ ions projected onto the plane of the X-ray propagation direction with (a) linear horizontal (LH), (b) linear vertical (LV), and (c) circular polarised (CP) radiation. The arrows indicate the polarisation direction. The radius of the distribution corresponds to the absolute momentum of 7 a.u. (atomic units) imparted by the photoelectron onto the recoil ion.

Fig. 4. FEL power profile reconstruction for four different configurations.

a standard pulses, where the whole bunch is lasing, (b) short pulses with standard power, (c) two-colour short pulses, and (d) high-power short pulses. Top plots: single-shot images for lasing-on conditions. Middle plots: single-shot images for lasing-off conditions. In the images, E indicates the energy of the electrons in arbitrary units, and t the time in femtoseconds (with t > 0 corresponding to the head of the bunch and t < 0 to the tail). Bottom plots: FEL power profiles reconstructed by comparing lasing-on and -off conditions (the bold profiles show average profiles, the rest correspond to single-shot power profiles). See text and Methods for more details.

Fig. 5. Measurements of spectral correlation functions for different conditions.

a without external optical laser and with intra-undulator chicanes disabled, (b) without external optical laser and with matching mode-locked lasing condition, (c) with external optical laser and with intra-undulator chicanes disabled, (d) with external optical laser and with matching mode-locked lasing condition. See text for more details.