In a recently published deliverable, THRILL presents the design of an achromatic reflective beam transport telescope for the Apollon F2 ‘1 PW beamline’, together with preliminary design studies for future implementations at the European XFEL and FAIR laser facilities. The primary objective is to improve the near-field spatial quality of high-energy laser beams before compression and focusing, thereby increasing the achievable intensity on target while preserving beam quality.

To eliminate chromatic aberrations associated with refractive optics, the selected solution is based on a 1:1 telescope composed of two large off-axis parabolic (OAP) mirrors. This reflective configuration provides diffraction-limited imaging over a long propagation distance while maintaining excellent spatial and temporal beam quality. The telescope is designed to image the beam from the delay line directly to the entrance of the compressor, minimizing diffraction effects and preserving the super-Gaussian beam profile required for high-intensity laser operation.

The optical design was optimized under several key constraints, including:

· maintaining average fluence below 1 J/cm² on all transport optics;

· preserving a 1:1 magnification over a propagation distance of approximately 10 m;

· ensuring compatibility with the existing Apollon layout and synchronization requirements;

· minimizing chromatic, diffraction and nonlinear propagation effects.

A complete optical and mechanical design was developed, including the specification of 200 mm diameter off-axis parabolic mirrors (4.4 m apparent focal length), transport mirrors, vacuum windows, and precision mounts. Beam propagation simulations confirmed that all optical components operate well below their laser-induced damage thresholds while maintaining the required beam diameter throughout the transport line. Detailed technical specifications were established for procurement, including surface quality, wavefront error, reflectivity, dispersion, and laser damage resistance.

The introduction of the AF09 telescope modifies the optical path length of the F2 beamline. A synchronization study demonstrated that adding a simple optical chicane to the F1 beamline restores synchronization while preserving the required ±5 ns timing adjustment capability, and even extends the accessible synchronization range.

A dedicated Class 1 visible alignment source was designed and built to facilitate installation and maintenance of the telescope. Combined with a dedicated alignment procedure based on autocollimation, focal spot optimization, and reference pinholes, this approach enables accurate alignment of the complete afocal system while ensuring that the compressor input axis and beam quality are preserved.

Finally, the experience gained from the Apollon design has been extended to develop blueprints for long-distance reflective beam transport systems for the European XFEL and FAIR laser facilities. These preliminary studies demonstrate that the same reflective telescope concept can be scaled to transport very large (200–400 mm) CPA beams over distances of several tens of metres while maintaining excellent spatial quality and avoiding chromatic aberrations.

Overall, this work establishes a practical and scalable solution for high-power laser beam transport based on “off-axis parabolic telescopes”. The proposed architecture significantly improves near-field beam quality before compression, reduces diffraction-induced degradation, and provides a robust foundation for future petawatt-class laser facilities requiring long-distance, high-fidelity beam transport.

The work is described in THRILL Deliverable “D5.2 Blueprint of an achromatic, reflective beam transport for ELI-BL, EuXFEL and FAIR laser facilities”: https://www.thrill-project.eu/wp-content/uploads/2026/09/THRILL_Deliverables_13_final.pdf