KAOS design and evolution

What is inside a KAOS unit?

Each KAOS unit consists of a pair of plane mirrors, usually referred to as the vertical and horizontal KB mirrors. The mirrors are optically coated fused-silica substrates and are bent tangentially by mechanical benders until their surface approximates the plane-elliptical shape required to focus the beam. Each mirror holder is mounted in vacuum and provides angular steering, including pitch and roll.

This geometry decouples the vertical and horizontal focusing directions. It also allows the same optical concept to handle different source positions and different focal-plane requirements, which is essential for an FEL facility such as FERMI.

Design drivers

• Independent horizontal and vertical focusing: the KB geometry separates the two transverse components of the beam.
• Compatibility with FEL1 and FEL2: active curvature compensates for the different effective source positions of the two FERMI FEL lines.
• High demagnification: KAOS addresses geometries where the source-to-mirror distance is very long and the mirror-to-sample distance is short.
• Wavefront correction: the active mirrors can compensate for defocus and astigmatism introduced by the beam transport optics and by the actual source conditions.

Early development

The KAOS concept grew from the PADReS need to focus and manipulate the FERMI beam while preserving the unique coherence properties of the seeded FEL. Early commissioning at DiProI and LDM demonstrated micrometre-scale focusing and established the diagnostic workflow based on wavefront sensing, scintillator/phosphor screens and damage/ablation imprints.

Mechanical refurbishment

After the first operation period, the KAOS mechanics were modified to improve stability, repeatability and reliability. The upgraded system made it possible to approach the opto-mechanical focusing limit more reproducibly, especially on the FEL2 low-wavelength line.

Wavefront sensing as the pilot

Wavefront sensing was initially used to assess the focusing capabilities of KAOS, but it became much more than a diagnostic. It is now an operational tool used to minimize aberrations, optimize mirror curvature, adjust to changing source positions, shape the beam and provide additional information to users.

Minimum spot size

The most common high-fluence operation mode consists of optimizing mirror curvatures and angular positions to reduce aberrations and reach the smallest possible spot compatible with the optical configuration and beam conditions.

Controlled spot size

Some experiments require a larger or more reproducible footprint rather than the smallest possible focus. KAOS can be deliberately detuned or shaped to obtain a controlled spot size at the sample.

Adaptation to special layouts

Because FERMI can operate in special modes, including multi-colour or polarization-dependent configurations, the effective source may vary. KAOS can accommodate such changes by re-optimizing the active curvatures.