CERN Accelerating science

A novel optics scheme to meet the HL-LHC targeted performance
by Stephane Fartoukh and Panos Charitos

Beam sizes [mm] along a quarter of the LHC ring. The interaction point (IP) of the ATLAS experiment is exactly in the centre. In the end of the Tele-squeeze the spot size is no larger than 5 microns at the IP, thanks to the ATS scheme transforming 7km of machine into a giant final focus system with natural embedded chromatic correction sections in Arc81 and Arc12. (Image: Stephane Fartoukh)

Upgrading the luminosity of a circular collider means increasing the number of interactions between the two counter-rotating particle beams. The goal is to maximise the potential of observation (or discovery) of rare (or unexpected) events, and to improve the measurement accuracy of already known phenomena.

An important ingredient to reach this goal is to increase the density of particles colliding at the interaction point (IP), in particular decrease the transverse beam spot size at the IP, that is quantified in accelerator science with the so-called beta* parameter. Like light rays, however, a strong focusing assumes a certain lever arm, that is also and mainly a certain distance, between the incoming beam and its focus point. When this distance is fixed, increasing the number of lenses (the role of the quadrupole magnets in particle accelerators) and increasing their strength is the only way forward. But this presents obvious limits in terms of integration and maximum possible field for new magnets.

This is the case for the long straight sections of the LHC, which host the machine experiments. Here the geometry is fixed by the existing LEP tunnel, and no modifications are expected for the LHC luminosity upgrade project (HL-LHC) or its possible energy upgrade (HE-LHC), one of the scenarios explored under the FCC design study.

In order to address this challenge, Stephane Fartoukh, who works in CERN’s beam department came up with the idea of a novel optics scheme inspired by the classical principle of a telescope in light optical systems. The LHC is a ring with a 27 km long circumference where 8 long-straight sections, or “insertions”, are evenly distributed along the ring. Half of the insertions are for special services (such as collimation) and the others host high-energy physics experiments, in particular the two high luminosity insertions ATLAS and CMS.

In the new scheme designed by Stéphane, the beam focusing is achieved in two stages. The first stage, called “Pre-squeeze”, is confined to the high luminosity insertions proper (see Fig. 1 left) until limits of strength are reached in the matching quadrupoles. This is the common approach followed by modern colliders. In order to gain an additional beta* squeezing factor, which is of vital importance for the HL-LHC program, the second stage, called “tele-squeeze”, involves quadrupole magnets which are located in the two insertions 3.5 km downstream and upstream on either side (see Fig.1 right). The two stages are part of a scheme called ATS, short for “Achromatic Telescopic Squeeze”.

The term “Achromatic” reflects the second novelty of the technique. As is the case in light optics, particles with slightly different energies are focused differently, inducing chromatic aberrations, in particular chromatic distortions of the beam spot at the IP, which increase violently for very strong focusing, rendering it inefficient after a certain point. Special magnets called sextupoles are located in the LHC arcs between two consecutive insertions in order to compensate for this effect. Stephane explains: “Contrary to conventional optics squeezing techniques, these magnets are run at constant strength and therefore cannot exceed any field limits in the second, telescopic, part of the ATS, because they are made more efficient when the beam becomes bigger in the arcs.” He adds, “Otherwise it would have been quite a copious effort to build and replace more than 500 lattice sextupoles for the HL-LHC”.

This concept required fully new optics deployed in all parts of the LHC ring, from injection to collision energy. A detailed study program has been setup with dedicated beam time allocated in the LHC schedule. The first stage of the ATS has been successfully tested, and first collisions with nominal specific luminosity have been established.

“The heart of the HL-LHC is already beating in the LHC. We are now ready to enter in what one could call the telescopic era of the LHC, and prove with beam the reliability of the overall scheme, with the High Luminosity LHC and High Energy LHC as medium and long term objectives for the ATS scheme,” Stephane concluded.

For more information on the ATS scheme, please see a detailed report from June 2016 (submitted to the ICFA Beam Dynamics Newsletter of Vol. 70).