AC/DC: The FERMI FEL Split and Delay Optical Device for Ultrafast X-ray Science

Abstract

Free-electron lasers (FELs) are the most advanced class of light-sources, by virtue of their unique capability to lase high-brightness pulses characterized by wavelengths spanning the extreme-ultraviolet, the soft and hard X-ray spectral domains, as well as by temporal lengths lying in the femtosecond (fs) timescale. The next step to push the current standards in ultrafast X-ray science is strongly linked to the possibility of engineering and exploiting time-resolved experiments exclusively for FELs pulses, ideally having different colors tunable at specific electronic resonance of the chemical elements. At the seeded FERMI FEL (Trieste, Italy) this goal is committed to the optical device known as AC/DC, which stands for the auto correlator/delay creator. AC/DC is designed to double the incoming FEL pulse splitting the photon beam by inserting a grazing incidence flat mirror, thus preserving the spectral and temporal properties, and further delaying one of these two pulses in time. It can independently tune the FEL pulses fluence on the two optical paths by means of solid-state filters, too. Here, we present a detailed description about this optical device. Strong emphasis is dedicated to the AC/DC opto-mechanical design and to the laser-based feedback systems implemented to compensate for any mismatch affecting the FEL optical trajectory, ascribable to both mechanical imperfections and paraxial errors rising during a temporal delay scan.

Splitting and delaying fully coherent FELs pulses for advancing ultrafast x-ray science: the AC/DC optical device at the FERMI FEL

“…Such a design is dictated by the need to keep fixed the pointing of the probe beam, i.e., to leave the radiation monitoring the sample's response totally unaffected by any possible vibrations and other systematic sources, which could arise during the temporal scan. The AC/DC is composed of three different ultra-high vacuum (UHV) chambers, constantly kept at a value approximately in the 10 −9 mbar range and installed onto concrete bases to isolate them from any external vibrations [8]. Specifically, M1 and M4 are mounted into two separate bell-shape UHV chambers, while the other mirrors composing the two optical branches are installed into a separate UHV one.…”

“…the Photon Analysis Delivery and Reduction System (PADReS), and designed to seed three (over six) experimental end-stations (DiProI, LDM, EIS-Timex). Such a device is called AC/DC, which stands for Auto Correlator/Delay Creator [8]. The goal of AC/DC is to split the FEL photon beam by inserting a grazing incidence flat mirror to create two exact half-spots replicas of the incoming pulse, and to further propagate them along two different optical paths (see Figure 1).…”