Echo-enabled harmonic generation studies for the FERMI free-electron laser

Studying ultrafast processes on the nanoscale with element specificity requires a powerful femtosecond source of tunable extreme-ultraviolet (XUV) or x-ray radiation, such as a free-electron laser (FEL). Current efforts in FEL development are aimed at improving the wavelength tunability and multicolor operation, which will potentially lead to the development of new characterization techniques offering a higher chemical sensitivity and improved spatial resolution. One of the most promising approaches is the echo-enabled harmonic generation (EEHG), where two external seed lasers are used to precisely control the spectro-temporal properties of the FEL pulse. Here, we study the expected performance of EEHG at the FERMI FEL, using numerical simulations. We show that, by employing the existing FERMI layout with minor modifications, the EEHG scheme will be able to produce gigawatt peak-power pulses at wavelengths as short as 5 nm. We discuss some possible detrimental effects that may affect the performance of EEHG and compare the results to the existing double-stage FEL cascade, currently in operation at FERMI. Finally, our simulations show that, after substantial machine upgrades, EEHG has the potential to deliver coherent multicolor pulses reaching wavelengths as short as 3 nm, enabling x-ray pump-x-ray probe experiments in the water window

Microbunching instability characterization via temporally modulated laser pulses

22High-brightness electron bunches, such as those generated and accelerated in free-electron lasers (FELs), can develop small-scale structure in the longitudinal phase space. This causes variations in the slice energy spread and current profile of the bunch which then undergo amplification, in an effect known as the microbunching instability. By imposing energy spread modulations on the bunch in the low-energy section of an accelerator, using an undulator and a modulated laser pulse in the center of a dispersive chicane, it is possible to manipulate the bunch longitudinal phase space. This allows for the control and study of the instability in unprecedented detail. We report measurements and analysis of such modulated electron bunches in the 2D spectrotemporal domain at the Fermi FEL, for three different bunch compression schemes. We also perform corresponding simulations of these experiments and show that the codes are indeed able to reproduce the measurements across a wide spectral range. This detailed experimental verification of the ability of codes to capture the essential beam dynamics of the microbunching instability will benefit the design and performance of future FELs.openopenBrynes A.D.; Akkermans I.; Allaria E.; Badano L.; Brussaard S.; Danailov M.; Demidovich A.; De Ninno G.; Giannessi L.; Mirian N.S.; Penco G.; Perosa G.; Ribic P.R.; Roussel E.; Setija I.; Smorenburg P.; Spampinati S.; Spezzani C.; Trovo M.; Williams P.H.; Wolski A.; Di Mitri S.Brynes, A. D.; Akkermans, I.; Allaria, E.; Badano, L.; Brussaard, S.; Danailov, M.; Demidovich, A.; De Ninno, G.; Giannessi, L.; Mirian, N. S.; Penco, G.; Perosa, G.; Ribic, P. R.; Roussel, E.; Setija, I.; Smorenburg, P.; Spampinati, S.; Spezzani, C.; Trovo, M.; Williams, P. H.; Wolski, A.; Di Mitri, S

Femtosecond Polarization Shaping of Free-Electron Laser Pulses

We demonstrate the generation of extreme-ultraviolet (XUV) free-electron laser (FEL) pulses with time-dependent polarization. To achieve polarization modulation on a femtosecond timescale, we combine two mutually delayed counterrotating circularly polarized subpulses from two cross-polarized undulators. The polarization profile of the pulses is probed by angle-resolved photoemission and above-threshold ionization of helium; the results agree with solutions of the time-dependent Schrödinger equation. The stability limit of the scheme is mainly set by electron-beam energy fluctuations, however, at a level that will not compromise experiments in the XUV. Our results demonstrate the potential to improve the resolution and element selectivity of methods based on polarization shaping and may lead to the development of new coherent control schemes for probing and manipulating core electrons in matter

High-brightness self-seeded X-ray free-electron laser covering the 3.5 keV to 14.6 keV range

A self-seeded X-ray free-electron laser (XFEL) is a promising approach to realize bright, fully coherent free-electron laser (FEL) sources in the hard X-ray domain that have been a long-standing issue with longitudinal coherence remaining challenging. At the Pohang Accelerator Laboratory XFEL, we have demonstrated a hard X-ray self-seeded XFEL with a peak brightness of 3.2 �� 1035 photons s?1 mm?2 mrad?2 0.1% bandwidth (BW)?1 at 9.7 keV. The bandwidth (0.19 eV) is about 1/70 times as wide (close to the Fourier transform limit) and the peak spectral brightness is 40 times higher than in self-amplified spontaneous emission (SASE), with substantial improvements in the stability of self-seeding and noticeably suppressed pedestal effects. We could reach an excellent self-seeding performance at a photon energy of 3.5 keV (lowest) and 14.6 keV (highest) with the same stability as the 9.7 keV self-seeding. The bandwidth of the 14.6 keV seeded FEL was 0.32 eV, and the peak brightness was 1.3 �� 1035 photons s?1 mm?1 mrad?1 0.1%BW?1. We show that the use of seeded FEL pulses with higher reproducibility and a cleaner spectrum results in serial femtosecond crystallography data of superior quality compared with data collected using SASE mode. ? 2021, The Author(s), under exclusive licence to Springer Nature Limited part of Springer Nature.11Nsciescopu