by Dr. Panos Charitos, CERN, panagiotis.charitos@cern.ch

Particle colliders are the most advanced scientific tools used to explore the mysteries of matter. And today CERN’s Large Hadron Collider (LHC) is also the world’s most powerful collider with planned operation through the mid 2030s. Therefore, it is timely to design a future collider that could identify physics beyond the standard model and help us explain the open questions about our universe.

The Future Circular Collider (FCC) study explores options for a post-LHC collider, and by the end of 2018 the team had contributed to decisions from the global particle physics community towards a research infrastructure that will keep it on the path of exploration.

Unprecedented particle energies and the luminosities expected as FCC collider options both call for developments in cryogenics. High-field superconducting magnets—in the case of a hadron machine (FCC-hh)—and the efficient high-power RF system for the lepton machine (FCC-ee) are key challenges, as large-capacity refrigeration down to 1.9 K is required in both cases.

Cryogenics studies also explore corresponding distribution, recovery and inventory storage systems. In addition, the large amount of synchrotron radiation emitted and the multiple dynamic heat loads at different temperatures create new challenges for the transient operation of the FCC cryogenic system. Whereas efficiency can be increased by additional instrumentation in many cases, the reliability often suffers from an increased amount of technical components. Therefore, a good compromise has to be found in order to design a reliable, efficient and affordable cryogenic distribution system.

The most challenging option is the 100 TeV proton collider (FCC-hh) based on 16 T magnets that use superconducting Nb3Sn technology. The successful design—applied in the LHC to integrate both proton beams in one magnet—was adopted for the FCC. Based on technical and economic optimization, the operation temperature of 1.9 K has been chosen as baseline parameter. The extra cost of cooling down to 1.9 K is largely compensated for by savings on the amount of superconducting material. As the proton collider will be divided in ten, 10 km-long sectors, the cooling of the superconducting magnets will require a refrigeration capacity of 15 kW at 1.8 K per sector of cryogenic plant, a capacity almost six times larger than the state-of-the-art cryogenic plants presently in operation at LHC.

A second challenge comes from the much higher values of synchrotron power that will be emitted by the 50 TeV beams of FCC-hh, dissipating at about 5 MW in the cryogenic environment. The power has to be intercepted by beam screens operating from 40 to 60 K before reaching the magnet’s cold masses, thus representing the single largest fraction of the total capacity. In this temperature range, the beam screens can be cooled by circulation of supercritical helium at 50 bar. And as the thermal shielding will be performed at the same temperature level, the total power to be extracted between 40 and 60 K is about 6 MW, i.e., 600 kW per sector of cryogenic plant.

Another major cooling requirement will come from the superconducting current that will power the FCC. With the conservative assumption that HTS technology is similar to the present state-of-the-art—i.e., the upper part of the HTS section of the current leads being cooled between 40 K and 300 K—this refrigeration load scaling proportionaly to the current and to the number of circuits would amount to extracting an equivalent capacity of 3.6 kW at 4.5 K per sector of cryogenic plant.

There will also be a 100 km circumference tunnel where a thermal contraction of 300 m is expected, thus challenging the use of conventional bellow-based compensation units for a cryogenic distribution system. Invar, an iron-nickel alloy that exhibits a very low thermal contraction, is an attractive option that calls for future R&D that will minimize the need for compensation bellows and increase the reliability of the machine.

Overall, the total FCC-hh refrigeration capacity accounts for about 1 MW at 4.5 K distributed around the ring circumference. The FCC baseline design foresees a tenfold sectorization of the FCC cryogenic distribution system. This consists of eight long sectors with a length from 9.3 to 10.6 km each and two short sector pairs with a length of 5.8 km. Ten cryoplants distributed over six locations will provide the sectors with helium. FCC cryogenics can distribute 600 kW at 40-60 K and 15 kW at 1.8 K. The equivalent refrigeration capacity of 4.5 K on each of the 10 sector cryogenic plants is about 100 kW. This unit capacity exceeds that of the LHC cryogenic plants delivering 18 kW at 4.5 K, as well as the present state-of-the-art of 25 kW at 4.5 K represented by the ITER cryogenic plants.

To reach the FCC target, CERN needs to advance R&D efforts on alternative refrigeration cycles, in particular Brayton cycles that use turbo-machinery and helium-neon mixtures as process fluids. Such cycles are being developed for hydrogen liquefaction with the promise of significant improvements in thermodynamic efficiency. The suggested Brayton cycle can have Carnot efficiency higher than 40 percent for refrigeration capacity of about 40 K.

A Brayton cycle can also be used for 40 K precooling of the pure helium cycle. This cycle is required for the production of refrigeration capacity at 1.8 K used for cooling the 16 T superconducting magnets. Cooling below 40 K uses a cold-compressor train and is also based on a pure helium refrigeration cycle with a Carnot efficiency of 28.8 percent.

In the case of a future lepton collider (FCC-ee), the cryogenic design is mainly driven by the need to cool down the superconducting RF cavities. The energy-staged approach of the FCC-ee requires a gradual increase of the number of SRF modules and consequently a staging of the cryogenics system.

Operation at the Z, W and Higgs thresholds requires only 400 MHz STF cavities housed in cryomodules, immersed in a helium bath at the temperature of 4.5 K at 1.3 bar. To reach the top quark threshold of 350 GeV, a series of 800 MHz cavities would be installed in a saturated superfluid-helium bath at 2 K and 30 mbar. These would require four cryogenic plants having a unit refrigeration capacity of 12 kW at 2 K, much larger than the capacity of present state-of-the-art plants.

Similar to the case of an energy-frontier collider, the FCC-ee cryogenic system of the circular lepton collider must cope with thermal load variations and a large dynamic range while intercepting heat loads at higher temperatures. The cryogenic system must be able to cool down and fill the cryomodules while avoiding thermal gradients greater than 50 K in the cryogenic structures. The first major heat intercept, sheltering the cavity cold mass, is foreseen at 50 to 75 K. For the 400 MHz, normal liquid helium will cool them to 4.5 K while the 800 MHz will need superfluid helium cooling down to 2 K. As in the case of FCC-hh, the cryomodules and the cryogenic distribution line apply several low temperature insulation and heat interception techniques that need to be deployed on a large industrial scale.

Overall, the beam-induced heat load in the case of FCC-ee is dominated by RF losses dissipated in the cavity baths at 4.5 K and 2 K. Given the location of the RF cryomodules, the cryogenic plants will be located in two technical sites proving cooling of the RF cryomodule strings.

In Conclusion
The collider explored by the FCC study poses certain quantitative challenges—like higher total refrigeration capacity and larger helium inventory—and also some qualitative challenges, such as better energetic efficiency and lower refrigeration temperature to improve the performance of advanced superconducting materials for high-frequency cavities. Such new projects constitute a unique opportunity for the industry and academia to push technological development.

The FCC study and the related MSCA EASITrain project are set to explore new paths in cryogenics to meet the challenges of large-scale infrastructure. Taking into account the requirements of FCC, the teams came up with a set of parameters that fit the design of the different collider options while minimizing the operation costs and the risks of downtime, therein ensuring the reliability of the next machine. Ongoing developments for new cooling schemes also exist, focusing on the use of nonconventional fluids, innovative refrigeration schemes and the push for new machinery that could additionally boost the performance of a post-LHC collider. ■