KAOS

 KAOS, the Kirkpatrick-Baez Active Optics System, is the flagship EUV/soft X-ray active focusing system developed in-house by the PADReS group at FERMI, Elettra Sincrotrone Trieste. It combines bendable KB mirrors, in-situ wavefront sensing, optical metrology and physical-optics simulations to deliver controlled, high-fluence focal spots to FERMI experimental endstations.

Welcome to KAOS @ PADReS

KAOS was conceived within PADReS to solve one of the key challenges of FEL photon transport: focusing highly coherent EUV/soft X-ray radiation over a broad spectral range while preserving the wavefront quality requested by user experiments. Each KAOS unit is based on two long, optically coated plane mirrors arranged in Kirkpatrick-Baez geometry. The mirrors are mechanically bent in a controlled way to approximate the required elliptical profile, independently focusing the vertical and horizontal beam components.

The active design makes KAOS highly versatile. The same system can adapt to the different effective source positions of FEL1 and FEL2, to different photon energies and to different experimental geometries. The curvature, pitch and roll of the mirrors are optimized with the support of Hartmann wavefront sensors, PMMA ablation imprints and physical-optics simulations. This approach has made KAOS one of the enabling technologies for routine FERMI user operation.

Key numbers

KAOS in brief
  • 100-2 nm: EUV/SXR operative range addressed by the KAOS concept.
  • >80x: demagnification achieved in the most demanding optical geometries.
  • 1.8 x 2.4 µm2: FEL2 focal spot inferred at 4.14 nm after the mechanical upgrade and wavefront-based optimization.
  • 5 beamlines: DiProI, LDM and MagneDyn at FERMI, plus FL23 and FL24 at FLASH.

KAOS mechanical concept and wavefront sensing

Why KAOS?

Active focusing

Unlike a fixed-shape refocusing mirror, KAOS can change the curvature of the two mirrors. This allows the system to compensate for source-position changes, optimize the focal plane, correct aberrations and tailor the beam according to the experimental request.

Wavefront-driven operation

KAOS operation is strongly linked to Hartmann wavefront sensing. The wavefront sensor, mounted downstream of the experimental station, provides feedback on the actual beam delivered to the experiment. The measured wavefront is used to optimize bending, pitch and roll and to reconstruct the focal spot.

From FERMI to FLASH

KAOS was developed by the PADReS group for FERMI and later transferred to other FEL environments. Two KAOS systems serve the FLASH2 beamlines FL23 and FL24 at DESY, demonstrating that the concept is not limited to FERMI but can be exported to other EUV/soft X-ray FEL facilities.

Explore the KAOS section

Design and evolution

Mechanical concept, active mirror bending, FEL1/FEL2 source-position compensation, refurbishment and system evolution.

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Performance and diagnostics

Focal spot optimization, WFS reconstruction, PMMA ablation imprints, WISE/WISER simulations and experimental benchmarks.

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Applications and community use

KAOS use at DiProI, LDM, MagneDyn and FLASH, and its impact on external user experiments.

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Publications and resources

A curated bibliography of KAOS, PADReS, active optics, focal spot characterization and beam-shaping papers.

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PADReS and KAOS layout

KAOS in the PADReS photon transport and refocusing architecture.

For further information on KAOS, its operation and possible optical configurations for user experiments, please contact the PADReS team.

Ultima modifica il Venerdì, 03 Luglio 2026 14:31