Bridging the gap of storage ring light sources and linac-driven free-electron lasers

High-gain free-electron lasers (FELs) are driven by short, high-charge density electron beams as only produced at dedicated single pass or recirculating linear accelerators. We describe new conceptual, technical, and modeling solutions to produce subpicosecond, up to 100 \u3bcJ energy extreme ultra-violet and soft x-ray FEL pulses at high- and tunable repetition rates, from diffraction-limited storage ring light source. In contrast to previously proposed schemes, we show that lasing can be simultaneous to the standard multibunch radiation emission from short insertion devices, and that it can be obtained with limited impact on the storage ring infrastructure. By virtue of the high-average power but moderate pulse energy, the storage ring-driven high-gain FEL would open the door to unprecedented accuracy in time-resolved spectroscopic analysis of matter in the linear response regime, in addition to inelastic scattering experiments

Wavelength control in high-gain harmonic generation seeded free-electron lasers

The basic theory of free-electron lasers (FELs) indicates that the output wavelength of a seeded FEL operated in the high-gain harmonic generation (HGHG) configuration is determined by the wavelength of the seed laser and light is emitted when the undulators are tuned to an exact harmonics of the seed laser. In a realistic case, when taking into account the electron beam imperfections and the finite bandwidths of the seed and of the amplification process, the output wavelength is influenced by these factors and can deviate from the exact harmonic resonance. These effects are responsible for the small wavelength fluctuations of the FEL pulses but can also be exploited for an accurate FEL wavelength tuning. In this work, we show how the dispersive section, the curvature of the electron beam longitudinal phase-space and frequency pulling can influence the FEL wavelength and can be, in principle, used to control it. Furthermore, we show how one can reconstruct the electron beam longitudinal phase-space from the analysis of the FEL wavelength sensitivity to the seed laser delay with respect to the beam arrival time

Interference of two-photon transitions induced by XUV light

International audienceThe relative phase of first ( ω 1 ) and third harmonics ( ω 3 ) extreme ultraviolet light pulses was varied to control the population of the 2 s 2 state in helium through the interference of ω 1 + ω 1 and ω 3 − ω 1 two-photon excitation paths. The population was monitored by observing the total electron yield due to the 2 s 2 autoionization decay. Maximum yield occurs when the relative phase of the two harmonics matches the phase difference of complex atomic amplitudes governing the two excitation paths. The calculated trend of atomic phase differences agrees well with the measured data in the spectral region of the resonance, provided that time-reversed − ω 1 + ω 3 path is also taken into account. These results open the way to accessing phase differences of two-photon ionization paths involving energetically distant intermediate states and to perform interferometry in the extreme ultraviolet range by monitoring final state populations

Nonlinear Harmonics of a Seeded Free-Electron Laser as a Coherent and Ultrafast Probe to Investigate Matter at the Water Window and Beyond

The advent of free-electron lasers (FELs) in the soft- and hard-x-ray spectral regions has introduced the possibility to probe electronic, magnetic, and structural dynamics, in both diluted and condensed matter samples, with femtosecond time resolution. In particular, FELs have strongly enhanced the capabilities of several analytical techniques, which have taken advantage of the high degree of transverse coherence provided. Free-electron lasers based on the harmonic up-conversion of an external coherent source (seed) are characterized also by a high degree of longitudinal coherence, since electrons inherit the coherence properties of the seed. For the state of the art, the shortest wavelength delivered to user experiments by an externally seeded FEL light source is about 4 nm. In this paper we demonstrate that pulses with a high longitudinal degree of coherence (first and second order) covering the water window and with photon energy extending up to 790 eV can be generated by exploiting the so-called nonlinear harmonic regime, which allows generation of radiation at harmonics of the resonant FEL wavelength. In order to show the suitability of the nonlinear harmonics generated by a seeded FEL for research in the water window and beyond, we report the results of two proof-of-principle experiments: one measuring the oxygen $K$-edge absorption in water (∼$530$ eV) and the other analyzing the spin dynamics of Fe and Co through magnetic small-angle x-ray scattering at their $L$ edges (707 and 780 eV, respectively)