KAOS performance and diagnostics

 KAOS performance is assessed through a multi-technique approach that combines in-situ Hartmann wavefront sensing, PMMA single-shot ablation imprints, scintillator/phosphor screens, mirror metrology and physical-optics simulations with WISE/WISER.

Best demonstrated focusing

After the mechanical upgrade and systematic wavefront-based optimization, KAOS reached focal spots in the few-micrometre range on both FEL1 and FEL2. A representative benchmark is the FEL2 result at 4.14 nm, where the reconstructed spot size was about 1.8 µm x 2.4 µm FWHM. This value was confirmed by PMMA ablation imprints and was in good agreement with WISER simulations.

At longer wavelengths, independent diagnostics also showed micrometre-scale spots, with good agreement among wavefront reconstruction, PMMA imprint analysis and simulations. This agreement is important because no single diagnostic alone provides the full picture of the focal distribution under all operating conditions.

Performance highlights

Validated results
  • Few-µm focusing on FERMI FEL1 and FEL2.
  • FEL2 benchmark: 1.8 x 2.4 µm2 at 4.14 nm.
  • Good agreement between WFS back-propagation, PMMA imprints and WISER simulations.
  • Routine optimization of mirror curvature, pitch and roll.

KAOS focal spot comparison between diagnostics and simulations

Diagnostics workflow

Hartmann wavefront sensor

A Hartmann wavefront sensor is mounted downstream of the nominal focus. The measured spot displacement and intensity pattern provide amplitude and phase information. By back-propagating the electric field, the focal spot can be reconstructed and the residual aberrations can be quantified.

PMMA ablation imprints

PMMA samples placed at or near the focal plane provide single-shot imprints of the FEL beam. The imprint analysis gives a direct real-space probe of the focal intensity distribution and is particularly valuable for detecting tails and low-intensity structures.

WISE/WISER simulations

WISE and WISER propagate the FEL wavefront using physical optics and the measured surface metrology of the actual mirrors. This makes it possible to estimate the expected focal spot, understand the impact of figure errors and guide the experimental optimization.

KAOS optimization workflow

Typical KAOS optimization workflow: measured wavefront, adjustment of mirror curvature and back-propagation to the focal plane.

Why several diagnostics are needed

FERMI delivers highly coherent, intense and tunable pulses. Under these conditions the focal spot may be affected by residual aberrations, diffraction, source-position changes, alignment errors and mirror-surface imperfections. KAOS performance is therefore evaluated by comparing independent techniques. Wavefront sensing gives online information and enables rapid optimization, while PMMA imprints provide a direct single-shot measurement at the interaction region. The comparison with WISER simulations closes the loop with the measured metrology of the optical elements.

KAOS FEL2 performance result

Last Updated on Friday, 03 July 2026 12:26