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Description
The extension of the FERMI free-electron laser toward shorter wavelengths (FERMI 2.0) challenges the performance of the Photon Analysis Delivery and Reduction System (PADReS), originally optimized for operation above a few nanometers.
At λ < 5 nm, photon transport efficiency and diagnostic capability are limited by reduced mirror reflectivity, gas transparency in intensity monitors, and the spectral reach of existing energy spectrometers. Nitrogen-based intensity diagnostics become ineffective below ~8–10 nm, while current spectrometer geometries constrain operation to ≳2 nm. In addition, coating selection and grazing-incidence configurations impose critical trade-offs between transmission and optical performance.
Ongoing developments address these limitations through: (i) krypton-based intensity monitors with differential pumping; (ii) spectrometer upgrades enabling operation down to ~1.5 nm; and (iii) optimization of transport via multi-coating optics (e.g., Ni, SiC). Further actions include revised filtering strategies for harmonic suppression and mitigation of radiation damage.
Active focusing systems (KAOS) are being evaluated for improved flexibility and compatibility with multi-coating operation. In parallel, more extensive upgrades of the transport layout and spectrometers are considered to extend low-wavelength capabilities to all beamlines.
This work outlines the main challenges and presents a staged upgrade strategy to preserve full diagnostic capability and efficient photon delivery in the FERMI 2.0 regime.