Accelerator physicists in the US have proposed an alternative approach to the design of the proposed Future Circular electron-positron Collider (FCC-ee), generating lively discussions in the community on the eve of the update of the European strategy for particle physics. A 360-page long conceptual design report for the 100 km FCC-ee, a possible successor to the high-luminosity LHC at CERN, was published in January following a five-year study by the international FCC collaboration. A key consideration of the baseline design was to minimise energy consumption — a challenge addressed by the novel US proposal based on technology recently explored for future electron-ion and electron-proton colliders.
The modified acceleration scheme, laid out in a preprint published recently by Vladimir Litvinenko (Stony Brook) and Thomas Roser and Maria Chamizo-Llatas (Brookhaven National Laboratory), uses Energy Recovery Linacs (ERLs) to purportedly reduce synchrotron radiation by a factor of ten compared to the FCC-ee baseline design. “In addition to the potential power saving, the ERL version of the FCC-ee could extend the centre-of-mass energy reach up to 600 GeV while providing very high luminosities,” says Chamizo-Llatas. The maximum energy discussed in the conceptual design report for the FCC-ee baseline is 365 GeV, as required for top-antitop production.
First proposed by Maury Tigner in 1965, ERLs recoup the kinetic energy of particle bunches by manipulating their arrival time in the radio-frequency (RF) cavities. Previously accelerated bunches encounter a decelerating electric field, and the regained energy, stored once again in the cavity’s field, may be recycled to accelerate subsequent bunches. Though an old idea, ERLs are only now becoming feasible due to the high quality of modern superconducting RF cavities.
In June the Cornell–Brookhaven ERL Test Accelerator (CBETA) facility, which was envisaged as an ERL demonstrator for the Electron-Ion Collider (EIC) proposed in the US, achieved full energy recovery for a single pass. Prior to this, the concept was demonstrated at Jefferson Laboratory in the US and at Daresbury Laboratory in the UK. Further R&D with cavity technology compatible with FCC-ee proposal is planned for the Powerful Energy-Recovery Linac for Experiments (PERLE) project at Orsay, which was conceived as a test facility for electron-proton colliders.
The basic feasibility of the proposed concept must still be demonstratedFrank Zimmermann
The US trio’s alternative FCC-ee proposal, which was inspired by past design work for the EIC, maintains high beam quality by decelerating the beams after every collision at one of the interaction points, and “cooling” them in dedicated rings. The use of ERLs allows the beams to be decelerated, cooled and re-accelerated with minimal energy expended, potentially yielding much lower emittances than found in conventional circular machines. “The electric power consumption of a future FCC-ee will be a limiting factor for luminosity and centre-of-mass energy,” says Roser. “During our design studies for the EIC we realised that using an ERL for the electrons could produce significantly more luminosity for a given electron beam current,” he explains, though the team admits that their concept would require extensive studies similar to what the FCC-ee design team did for the storage-ring design.
The BNL proposal is certainly tantalising, agrees FCC deputy study leader Frank Zimmermann of CERN. “Presently, Energy Recovery Linacs are a topic of great worldwide interest, with efforts ongoing, for example, at Cornell, Jefferson Lab, KEK, Mainz, and Orsay,” he says. “However, the basic feasibility of the proposed concept must still be demonstrated and the potentially high investment cost understood, before this approach could be considered as a highest-energy option for a future circular lepton collider.”