The natural time scale of valence electronic motion in molecular systems is on the order of hundreds of attoseconds. Consequently, the time-resolved study of electronic dynamics requires a source of sub-femtosecond pulses. Pulses in the soft x-ray domain can access core-level electrons, enabling the study of site-specific electron dynamics through attosecond pump/probe experiments. As time-resolved pump/probe experiments are nonlinear processes, these experiments require high brightness attosecond x-ray pulses. The X-ray Laser-Enhanced Attosecond Pulses (XLEAP) collaboration is an ongoing project for the development of attosecond x-ray modes at the Linac Coherent Light Source (LCLS). Here we report development of a high power attosecond mode via cascaded amplification of the x-ray pulse. We experimentally demonstrate generation of sub-femtosecond duration soft x-ray free electron laser pulses with hundreds of microjoules of energy. In conjunction with the upcoming high repetition rate at LCLS-II, these tunable, high intensity attosecond capabilities enable new nonlinear spectroscopic techniques and advanced imaging methods. This work was supported by US Department of Energy Contracts No. DE-AC02-76SF00 and the Basic Energy Sciences Accelerator and Detector Research Program.

European XFEL offers a unique combination of high electron energy and soft X-ray SASE3 undulator resonant to low photon energies with high K parameter. The long undulator allows us to employ split-undulator scheme to deliver pump-probe radiation to users. Pulse energies depend on photon energy and range from 100uJ at 2200eV to 900uJ at 600eV per pulse respectively. We plan to install optical delay line in the chicane to reliably scan through zero delay. Strongly compressed 16.5GeV electron beam combined with maximized orbit dispersion allowed us to generate 1keV- and 1mJ-order radiation with predominantly less than 2 spectral spikes. This operation mode was also employed to generate 100uJ-order pump-probe pulses.

This contribution presents the experimental demonstration of improved performance of an X-ray free-electron-laser (FEL) using the optical klystron mechanism and helical undulator configuration in comparison to a standard planar undulator without optical klystron. The demonstration has been carried out at Athos, the soft X-ray beamline of SwissFEL. Athos has variable-polarization undulators and small magnetic chicanes placed between every two undulator modules to fully exploit the optical klystron. It is shown that, for wavelengths between 1 and 3 nm, the required length to achieve FEL saturation is reduced by about a factor of two when using both the optical klystron and helical undulators, with each effect accounting for about half of the improvements. Moreover, it is shown that a helical undulator configuration provides a 20% or higher saturation power than planar undulators. This work represents an important step towards more compact and high-power FELs, rendering this key technology more efficient, affordable, and accessible to the scientific community.

Phase shifters at undulator line are usually used for optimizing FEL intensity by setting 'in-phase' by matching the FEL pulse and electrons phases. π-offset so called ‘out-phase’ may suppress FEL intensity at the resonant frequency, therefore the 'out-phase' condition is an unwanted state. However, this 'out-phase' setting can arise side band spectrums. This phenomena can be explained by the theory of the spontaneous radiation or low-gain FEL, and it expects that these side band spectrums have two main spectrums with the spectrum difference determined by the number of undulator period. This poster shows amplification of the two-colored spectrum seeded by the spontaneous spectrum feature. Results of two colored FEL is studied by simulations and experiments are performed at PAL-XFEL showing it’s intensity grows exponentially along the number of undulator segments and reaches the saturation resulting in hundreds μJ energy.