The Future Circular Collider (FCC) is an ambitious international proposal for a next-generation particle accelerator complex, building upon the successes of CERN's Large Hadron Collider. The FCC feasibility study has reached a successful mid-term review on its path to completion in 2025. The FCC envisions two complementary machines: the FCC-ee, an electron-positron collider offering unprecedented precision for electroweak and Higgs physics; and the FCC-hh, a hadron-hadron collider pushing the energy frontier to 100 TeV. Together, these machines would offer a remarkably broad and in-depth physics program.
The FCC-ee has undergone significant upgrades since its conceptual design, including layout changes, a new high-energy linac-based pre-injector, and refinements to its RF system. Plans for superconducting RF cavities, beam management, and supporting systems are well-defined. While conventional electromagnets form the baseline, research into high-temperature superconducting magnets offers potential benefits. The FCC-ee's success would pave the way for the ambitious FCC-hh, with refinements maximizing efficiency and a strong focus on the R&D of advanced magnet technologies.
Since the FCC Conceptual Design Report in 2018, the FCC-ee optics (the series of magnets that guide beams around the ring and bring them into collision) has been adapted to a new ring layout with four-fold periodicity, allowing either two or four collision points. Boosting performance, the FCC-ee pre-injector now uses a high-energy linac rather than the SPS to accelerate electrons, while positrons would be generated at a primary electron-beam energy of 6 GeV as opposed to 4.5 GeV. A novel type of positron source is also being developed, a prototype of which will be constructed and later tested with beam at the SwissFEL at PSI.
The FCC-ee main-ring optics are complete, and RF configurations have been refined to simplify operation across the broad FCC-ee energy range. For example, at collision energies of 240 and 365 GeV, the two beams share a common RF system to minimise the number of cavities and associated cost. Due to the large span in beam current and RF voltage of the four FCC-ee operating modes, three different types of superconducting RF cavities operating at 400 or 800 MHz are considered. The baseline FCC-ee operating sequence starts at the Z pole and then increases in energy; an alternative sequence beginning at the ZH energy is being studied. The total SRF system for the highest-energy tt– stage contains 366 cryomodules and 1464 cavities, of which a quarter are based on Nb/Cu technology and the rest on bulk niobium.
With unavoidable radiative Bhabha scattering at the collision points and other effects limiting the baseline FCC-ee beam lifetime to around 15 minutes, novel schemes for beam-based alignment and optics correction are being adopted from modern light sources. Several possible upgrades – including modified SRF systems and configurations, injector variants and modifications to the collider optics – are under consideration that may lead to further performance or cost improvements.
In addition to a well-defined SRF R&D programme, plans for the warm and cold magnets, vacuum system, controls, beam-intercepting devices, beam transfer systems, beam diagnostics, power converters, survey and alignment, machine protection, availability, robotics, and engineering software for FCC-ee have been formulated.
For the magnets, the baseline is conventional iron-dominated electromagnets. As an alternative for the arc sections, promising electricity saving and better optics flexibility, teams at PSI and CERN are jointly developing nested sextupole and quadrupole coils made from high-temperature superconductors (HTS) operating at around 40 K.
In contrast to LEP, where it was essentially an afterthought that famously required the phase of the Moon and passing trains to be taken into account, measuring the collision energy is a central consideration in the design and operation strategy of FCC-ee. A unique attribute of circular e+e– colliders is that the beams can naturally acquire transverse polarisation, the precession frequency of which is proportional to the beam energy. This allows the collision energy to be determined with very high precision via the resonant depolarisation technique, as used at LEP. At FCC-ee this could enable uncertainties of around 100 and 20 keV on the Z mass and width, compared to 1.7 and 1.2 MeV at LEP. It would also be key to an additional FCC-ee run currently under consideration to study direct Higgs production at a centre-of-mass energy of 125 GeV.
Unleash the Power: Towards FCC-hh
The successful mid-term review not only confirmed the maturity of the FCC-ee design but also highlighted its potential as a springboard for a future, even higher-energy collider – the FCC-hh. The FCC-ee will provide significant infrastructure and crucial knowledge about lower-energy physics, laying the groundwork for the FCC-hh.
Following recent refinements to the FCC-ee (electron-positron collider) placement, the overall layout of the FCC-hh (hadron-hadron collider) has undergone significant improvements since the initial Conceptual Design Report (CDR). These advancements offer a more streamlined and efficient project.
The optimized size of experiment caverns allows for the potential sharing of detector components between the FCC-ee and FCC-hh. This fosters collaboration and resource utilization for both colliders. Furthermore, the number of surface sites required has been reduced from 12 to 8, significantly simplifying construction logistics and minimizing environmental impact. Finally, by shortening the tunnel connecting the injector to the collider ring, the project has enhanced its operational efficiency.
A key deliverable of the feasibility study is a summary of R&D plans for Nb3Sn magnets, high-temperature superconducting (HTS) and hybrid magnets. FCC-hh magnet R&D is a challenging task offering opportunities for innovation, synergies with other fields and positive societal impact. The significant time allowed for FCC-hh magnet R&D in the FCC integrated programme allows the highest performance at optimised capital and operational expenditure to be sought. While Nb3Sn magnets are considered relatively low-risk as the technology is reaching maturity, HTS technology would enable the most aspirational goals to be reached. Due to the sizable gap in technology readiness between the two options, however, the feasibility study advises against an early decision. Instead, an adapted “phase-gate” process is proposed with regular review, steering and decision points every five years, and coordinated with the CERN high-field magnet programme. As the FCC-hh magnet development phases progress, a growing involvement of industry is envisioned to engineer magnet concepts and scale-up magnets to collider-relevant lengths, as was the case for the LHC.
The bulk of magnet R&D for FCC-hh is expected to occur in the roughly 15 year-long canvassing, scoping and feasibility phases. Experience from the magnet development programmes for the HL-LHC suggests that this timescale is realistic, albeit ambitious. The subsequent prototyping, pre-series, series manufacturing and commissioning phases resemble the LHC magnet-development programme, which started from a higher technology-readiness level (TRL) and took 24 years to complete. The midterm report also notes that during the early days of LHC conception, Nb3Sn was rejected due to a lack of technology readiness and the associated risk of trying to compress the development programme due to competition with the Superconducting Super Collider in the United States. In contrast, states the report, the FCC-hh timeline in an integrated programme permits the study of low-TRL, high-opportunity magnet technologies, i.e. based on HTS.
Both the e+e- and pp colliders hold immense individual merit. However, their true power lies in their synergy. Operating together, they offer the most extensive physics reach of any proposed future collider.
In summary, the successful midterm review of the FCC project represents a significant milestone, demonstrating its viability and setting the stage for further exploration into the mysteries beyond the Standard Model. The ultimate goal of the FCC project is to operate the new research infrastructure until the end of the 21st century, offering an unparalleled depth and scope for exploring particle physics. Through its commitment to precision and energy reach, the FCC project aspires to revolutionize our understanding of particle physics and our place in the universe.
Read more about the outcome of the FCC Feasibility Study in the latest issue of the CERN Courier magazine.