Novel alignment solutions: A Structured Laser Beam at the CLEAR accelerator facility

Martin Dušek ^1,3, Antonio Gilardi ², Jean-Christophe Gayde ¹, Miroslav Sulc ^3,4

¹ CERN BE-GM; ² CERN BE-ABP, ³ TUL – Technical University of Liberec (CZ), ⁴ IPP – Institute of Plasma Physics (CZ)

At CERN’s CLEAR (CERN Linear Electron Accelerator for Research) facility, researchers are continuously striving to advance accelerator technology and pioneer new methods that enhance both research and find other applications. CLEAR provides a unique experimental platform where novel accelerator concepts can be tested, refined, and validated under real operating conditions. Its versatility has made it an essential hub not only for developing innovative beam technologies but also for exploring their impact in diverse fields—from next-generation particle accelerators to medical and industrial applications. Within this spirit of innovation, the introduction of Structured Laser Beams (SLBs)—https://kt.cern/technologies/structured-laser-beam—represents a significant step forward in precision alignment, addressing long-standing challenges in accelerator beamline referencing and setting new standards for experimental accuracy.

Laser beams are often used as alignment references because they provide a stable, straight, and precisely measurable light path that can serve as an accurate spatial benchmark over long distances. Traditionally, Gaussian laser beams are used to establish an alignment reference. While effective, Gaussian beams converge to a narrow focus known as a beam waist. When propagating beyond this waist, Gaussian beams gradually diverge, causing the beam’s energy to distribute over a wider area, reducing its intensity and increasing its spatial dimensions. This limits their usefulness, for as a Gaussian beam propagates, its transversal profile expands and loses definition, resulting in a weak, broad laser spot by the time it reaches distant experimental stations. This reduced clarity and increased size makes their use for precise alignment over long distances difficult.

Structured Laser Beam technology addresses this with a fundamentally different approach. Unlike a Gaussian beam, the SLB maintains a stable, narrow inner core while it propagates. The divergence of the inner core can remain below ten microradians, which offers a substantial improvement over a Gaussian beam. Moreover, the SLB design allows a tunable control of both the size of the inner core and the configuration of the surrounding rings. The feature of rings around the central core has proven useful for visually locating the beam when starting the alignment process, as the rings provide a clear indication of the laser beam position.

Structured Laser Beam technology uses a simple, compact generation method that can be optimized to allow the laser beam to propagate inside a particle beam pipe. This met the requirements of the CLEAR facility, where the SLB has proven useful in several experiments. Its advantages have been demonstrated in a range of contexts:

– In plasma lens experiments, the SLB’s compact inner core is essential for the alignment of sapphire capillaries filled with plasma. Alignment was essential both early in the accelerator line and at its end, where a Gaussian beam would already have diverged beyond use. The SLB, however, was able to provide the alignment reference along the whole trajectory of the particle beam.

– In medical irradiation studies, the SLB mimicked the electron beam trajectory with high precision. Its narrow core remained small enough to precisely match that of the beam throughout the entire CLEAR injection line, serving as a robust reference for robotic systems that position biological samples.

– For advanced particle beam diagnostics, specifically electro-optical sampling (EOS), a carefully tailored SLB was employed to replicate the electron beam’s size and position. The tuneablility of the SLB helped provide a laser beam reference that allowed a precise positioning of the electro-optical crystal, lowering the risk of damaging critical components when the crystal was then used with the particle beam.

The Structured Laser Beam system is now used as an operational alignment device in the CLEAR facility. This success opens up the possibility for further use of SLBs throughout the CERN complex. The ability to maintain a precise alignment reference is interesting for alignment applications over very long distances, such as for the Future Circular Collider. The operational use of a Structured Laser Beam in CLEAR has set a new benchmark for accelerator alignment solutions, delivering a robust, tunable, and precise optical alignment reference that overcomes the traditional limitations of Gaussian beams. Beyond accelerators, Structured Laser Beam technology could have an impact in fields such as free-space optical communication or industrial laser processing, where beams with a small, intense inner core propagating over long-distances are essential.