The implementation of a helical afterburner undulator at DESY's VUV-FEL source is part of the current FLASH2020+ upgrade program. The device shall be installed downstream of the present FLASH2 SASE undulators and will provide radiation with variable polarization from 1.33 nm to 1.77 nm (890-700eV) and thus also cover the L-edges of the 3d transition metals Fe, Co, and Ni. Despite a moderate energy upgrade of the machine to 1.35 GeV, the required wavelengths and tunability range can only be reached by a high magnetic performance of the undulator. We report on design and development of an APPLE-III undulator with 17.5 mm period length operating at a minimum magnetic gap of 8 mm which will make use of a magnetic force compensation scheme. A short prototype has been built to verify and iterate both the mechanical and magnetic concept. Details on keeper design, prototype results and the tuning concept will also be discussed. The full length device is presently under construction and shall also verify this concept for the future seeding undulators at FLASH1.

The XFELs with an anomalously high peak brilliance are opening the way to a number of novel X-ray photon research paths. At SPring-8 Angstrom Compact Free-Electron Laser (SACLA) [1], the XFEL pulses with high stability and short pulse duration (6-7 fs) have been regularly provided thanks to the unique electron gun, accelerator, and undulator systems [2]. By focusing these XFELs to 1um-100nm, the peak intensity has been dramatically increased and new phenomena in hard X-ray nonlinear optics have been explored, such as observation of saturable absorption [3], two-photon absorption [4], and the atomic inner-shell laser emission [5]. To further promote the study in the ultra-intense X-ray laser field, we have developed a focusing system that achieves sub-10nm spot size and 10²² W/cm² intensity. For the sub-10 nm focusing optics, an advanced Kirkpatrick-Baez (AKB) mirror system based on Wolter-type III geometry [6] has been adopted. The AKB consisting of one-dimensional Wolter mirrors can satisfy Abbe’s sine condition, which leads to a reduced coma aberration and a high tolerance to the incident angle error. We have designed and developed the AKB mirror system for SACLA BL3-EH4c at a photon energy of 9.1 keV. One of the remarkable challenges for the development was the fabrication of the mirrors with 1-nm accuracy. We applied an X-ray wavefront correction scheme [7] for the precise fabrication, and achieved wavefront accuracy of λ/15 rms which satisfies Maréchal’s criterion. Ptychographic probe measurements revealed the focusing spot size of 6.6 nm (horizontal) × 7.1 nm (vertical), indicating eventually attained focused intensity of 1.21 × 10²² W/cm². References: [1] T. Ishikawa et al., Nat. Photon. 6 (2012). [2] For example, I. Inoue et al., Phys. Rev. Lett. 127 (2021). & T. Osaka et al., Phys. Rev. Research 4 (2022). [3] H. Yoneda et al., Nat. Commun. 5 (2014). [4] K. Tamasaku et al. Nat. Photon. 8 (2014). & K. Tamasaku et al., Phys. Rev. Lett 121 (2018). [5] H. Yoneda et al., Nature 524 (2015). [6] J. Yamada et al., Opt. Express 3 (2019). [7] S. Matsuyama et al., Sci. Rep. 8 (2018).

Cavity-Based X-ray Free-Electron Lasers (CBXFELs) employ an X-ray cavity formed by crystal mirrors such that X-ray pulses receive periodic FEL-amplification and Bragg-monochromatization. CBXFELs enable improved longitudinal coherence and spectral brightness over single-pass self-amplification of spontaneous radiation (SASE) FELs [1,2] for high-repetition rate FELs. Construction and alignment of a stable low-loss cavity of Bragg-reflecting mirrors has been considered a daunting challenge and has not seen previous experimental implementation of large X-ray cavities in the hard X-ray regime. In this work, we demonstrate stable operation of a low loss 14-m-roundtrip rectangular cavity of four Bragg-reflecting diamond (400) mirrors. 9.831 keV X-rays from the Linac Coherent Light Source (LCLS) were in-coupled into the cavity via a thin diamond transmission grating. X-ray ring-down was characterized using fast photodiodes and a nanosecond-gated camera. Intra-cavity focusing was introduced to further stabilize the cavity, enabling observation of X-ray storage at >50 round trips. This experiment demonstrates feasibility of a stable low-loss hard X-ray cavity that will support future CBXFEL tests and operation [3].

Free-electron lasers (FELs) are currently the most advanced class of light sources, by virtue of their unique capability to lase high-brightness and ultrashort pulses characterized by wavelengths spanning the Extreme-Ultraviolet (EUV), the Soft (SXR) and Hard (HXR) X-Ray spectral domains, alongside with temporal duration lying in the femtosecond (fs) timescale [1]. Specifically, the advent of FELs light sources has recently allowed to perform, in a time-resolved fashion approach, both established spectroscopies, daily employed at synchrotron light sources, and novel non-linear optical methods, mostly combining FELs and laser pulses. Nonetheless, the next step to push the ultrafast X-Ray science standards is widely recognized to be linked to go beyond the current time-resolved schemes, so performing experiments engaging exclusively EUV, SXR and HXR pulses. Indeed, exciting (and probing) matter at its (or nearby) electronic resonance is largely speculated to be one of the key for discriminating and revealing the microscopic mechanisms hiding behind some of the most exotic phases of physical, chemical, and biological systems. Such a challenge calls the design of optical devices capable to both split and delay (in time) FELs pulses, without impacting on their coherence properties, and fully user-friendly in terms of preserving the perfect overlap of the resulting focal spots, even in the few microns spatial domain, a well-known trademark for focusing EUV, SXR and HXR pulses at FELs light sources [2]. At the seeded FERMI FEL (Trieste, Italy) this goal is committed by the novel optical device known as AC/DC, which stands for the Auto Correlator/Delay Creator. AC/DC is purposely designed to double the incoming FEL photon beam into two exact pulse replicas, splitting it by inserting a grazing incidence flat mirror, and further delaying in time, in a controlled way, one of the two pulses, with an intrinsic temporal resolution of approximately 360 attoseconds. A detailed description of AC/DC is highlighted here. Specifically, strong emphasis is dedicated to the opto-mechanical design and the laser-based feedback system, purposely designed and implemented to compensate in real-time any potential drift and pointing mismatch affecting the FEL optical trajectory, ascribable to both mechanical imperfections and residual paraxial errors appearing during a temporal delay scan [3]. [1] Bostedt C., Boutet S., Fritz D.M., Huang Z., Lee H.J., Lemke H.T., Robert A., Schlotter W.F., Turner J.J., Williams G.J., Linac Coherent Light Source: The first five years. Rev. Mod. Phys.88, 015007 (2016) [2] Manfredda M., Fava C., Gobessi R., Mahne N., Raimondi L., Simoncig A., Zangrando M., The evolution of KAOS, a multipurpose active optics system for EUV/Soft X-rays, Synchrotron Radiation News, 0, 0, (2022) DOI: 10.1080/08940886.2022.2066432 . [3] Simoncig A., Manfredda M., Gaio G., Mahne N., Raimondi M., Fava C., Gerusina S., Gobessi R., Abrami A., Capotondi F., De Angelis D., Menk R., Pancaldi M., Pedersoli E., Zangrando M., AC/DC: The FERMI FEL Split and Delay Optical Device for Ultrafast X-rays Science, Photonics, 9(5), 314, 2022