We experimentally demonstrate the generation of sub-femtosecond hard XFEL pulses by recompressing the electron bunch in front of the undulators, which encourages time-resolved spectroscopy toward the attosecond regime. Space-charge fields and wakefields of accelerating structures induce a reverse-energy chirp, in which high-energy electrons locate in the bunch head, and it becomes pronounced for high-peak current bunches. At SACLA, we use a Double Bend Achromat lattice of the BL2 dogleg to compress the reverse-chirped bunches. R56 is tunable from negative to positive values by changing the bending angle, and thus the electron beam orbit and transverse envelope downstream are unchanged.Thanks to this simple tuning knob of the bunch recompression, normal SASE operation is quickly switched to sub-femtosecond operation mode in a highly reproducible manner. This scheme is also applicable to the other facilities having a similar setup. In this presentation, we overview the recompression scheme of the electron bunch with a reverse-energy chirp at SACLA, and highlight the experimental results, which show the XFEL pulse duration is below 1 fs, and pulse energy reaches around 60 μJ.

We present a novel unified approach to the expression and correction of electron beam chromatic aberrations, which comprehensively covers optical systems of both a single-pass beam transport line and a circular accelerator satisfying periodic boundary conditions. In an electron circular accelerator, linear chromaticity has been utilized to express the first-order difference in beam focusing caused by the electron energy deviation, which can be computed by a well-known formula using a beam envelope function (Twiss parameters). However, this conventional formulation is incompatible with non-periodic beam transport lines, such as XFELs driven by linacs and the final focus system of linear colliders. We address this discrepancy by deriving a generalized formula that seamlessly connects the two different accelerator topologies. Our representation not only reproduces the well-known ring chromaticity but also provides a consistent and practical definition of chromatic aberrations for a single-pass transport line and their correction methods. This presentation outlines one approach to resolving this apparent contradiction and the universal formula to express chromatic effects.