by Dr. Panos Charitos and Dr. Laurent Jean Tavian

The Large Hadron Collider (LHC) features one of the largest cryogenic systems in the world. Cryogenics plays an indispensable role in applied superconductivity, and the development of cryogenics for accelerators has allowed researchers to efficiently cool both the high-field magnets presently used at the LHC and also those being developed by researchers working on the Future Circular Collider (FCC) study.

One of the scenarios explored under the FCC study is currently designing a future hadron-hadron collider (FCC-hh) expected to reach energies of 100 TeV c.m. in a 90-100 km tunnel. An efficient cryogenics system is a major consideration for the collider as it will be necessary to cool a range of systems including SRF cavities and new 16-T superconducting magnets.

The use of LTS superconducting devices cooled with an efficient cryogenic system will lower construction and operational cost, but designing such a system requires overcoming many challenges. For one, the FCC tunnel will be much deeper than its predecessor at depths of 300 to 400 meters. In order to save expensive underground space and to provide easy access to the equipment, the refrigeration system has to be put as much as possible at the ground level. The team, therefore, decided to apply a cut-off temperature of 40 K. All the refrigeration below 40 K will take place underground while the refrigeration equipment needed to reach temperatures of 40 K will be installed at ground level.

The team will also need to develop a complex system to cool the new FCC 16-T superconducting magnets. When the LHC was designed, it was foreseen to install it in the existing 27-km LEP tunnel. In order to reach a nominal energy of 14 TeV c.m., 9-T superconducting magnets were required. This posed a clear challenge for the development of a cryogenic system that could cool the magnets close to absolute zero.

“The use of superconducting magnets seemed to be the only way forward, though one had to choose whether the LHC would work at a temperature of 4.5 K or 1.9 K,” says Dr. Laurent Tavian, a cryogenics expert at CERN in charge of the FCC cryogenics study.

CERN ultimately chose to operate the LHC at 1.9 K. Choosing 4.5 K would have eased the design of the cryogenics team but would have also required using niobium tin (Nb3Sn) for the magnet coil production, a material that is much more difficult to process and handle due to its mechanical properties. Choosing 1.9 K posed a bigger challenge for the cryogenics team but allowed using niobium titanium (NbTi) that is easier to handle and which could ensure the reliability and manufacturability of magnet production.

The use of new superconducting materials is needed in order to attain the goals of High Luminosity LHC and FCC and to build more powerful superconducting magnets. NbTi doesn’t go higher than 10 T at 1.9 K and thus the use of Nb3Sn proves to be an attractive option. The Nb3Sn coils can sustain the required current densities to create magnetic fields of up to 16 T. Therefore it could fulfill the requirements of the HL-LHC upgrade and also be suited to a future circular hadron collider like those explored by the FCC study.

Sources of Heat
There are three main sources of heat to take into account when designing a cooling system for an accelerator infrastructure. Static heat in-leaks are the first. This heat is generated by the environment at ambient temperature and enters the cryogenic system by conduction or by radiation. It is deposited on the thermal shields cooled at 40-60 K, on the magnet cold masses cooled at 1.9 K and is induced even when there are no particle beams travelling in the accelerator.
A second major source is the so-called resistive heating generated by the “Joule” effect inherent to the non-superconducting splices interconnecting the different coil layers inside the magnet cold masses and the cryo-magnets that are electrically fed in series. Resistive heating also occurs in the non-superconducting sections of the current leads.

The third source of heat is the beam-induced heat loads deposited in the magnets through several processes that take place in the circulating and colliding beams themselves. The beam-induced heat depends strongly on the energy, the bunch intensity, number and length of the circulating bunches as well as on the luminosity of collisions.

These sources of heat were well studied for the LHC and are also taken into account in the design of the challenging cryogenic system for a future circular collider under the FCC study. For FCC, the superconducting magnets will be larger and heavier; consequently, higher specific static heat in-leaks have to be considered with respect to LHC (+ 50 percent at 1.9 K and + 25 percent at 40-60 K).

Concerning the resistive heating, the current in the main magnets will be larger (+33 percent) and the Nb3Sn splice resistance will be higher (factor 1.5); consequently, the specific resistive heating contribution will increase with respect to LHC by a factor of three.

Concerning the dynamic beam-induced heat loads, the specific heat load due to the beam-gas scattering effect, which depends on the lifetime, current and energy of the beam, will increase by a factor of nine with respect to the LHC. Finally, the synchrotron radiation from the bent beams is both “harder” (X-rays) and more intense, leading to a huge increase in the specific heat load by more than two orders of magnitude with respect to LHC. That imposes the use of beam screens to absorb this synchrotron radiation, and to operate them at higher temperature (40-60 K) in order to reduce the entropic load on the refrigeration system.

For FCC, the specific heat load in total at the 1.9 K temperature level will be 1.4 W/m (0.45 W/m for LHC) with a dynamic range of 3 (idem for LHC). The specific heat load on the 40-60 K temperature level will be 73 W/m (7.6 W/m for LHC) with a large dynamic range of 8 (no dynamic range is required for LHC at this temperature level).

To address these challenges, 10 cryoplants, which will produce about 600 kW at 40-60 K and 12 kW at 1.9 K, will be distributed along the 100 km tunnel in six technical sites and will cool
sectors over a distance of five to 10 km.

Other Challenges
Several additional cryogenics challenges for FCC were discussed during 2016 FCC Week, held April 11-17 in Rome, including questions of reliability of the cryogenics system, the use of more efficient refrigeration cycles and alternative techniques for refrigeration.

The FCC team must also study an electron-positron collider (FCC-ee) making use of a large number of SRF cavities operating at 4.5 K with an option of using technologies requiring operating temperature of 1.6 K. One of the FCC-week presentations concerned the use of magnetic refrigeration—a cooling technology based on the magnetocaloric effect—instead of the more conventional approach based on pumping on a saturated helium bath with cold compressors. For an operating temperature of 1.6 K, magnetic refrigeration could be safer, quieter, more compact and could have a higher cooling efficiency. Though higher temperatures are presently foreseen for SRF cavities, the idea posed a challenge for the study team to consider whether magnetic refrigeration would be suitable for FCC-ee. CEA Grenoble is currently studying the idea.

Another important topic discussed during the conference was the cooling of the beam screens for the future circular collider. The synchrotron radiation from proton beams accelerated to an energy of 50 TeV will be about 500 kW per sector; this means that 5 MW in total at 40-60 K should be extracted from the beam-screen cooling circuits. In terms of refrigeration cost, this large heat load equals almost half of the total refrigeration cost of FCC. This realization has led to collaboration with TU Dresden to develop a turbo-Brayton cycle that uses a mixture of helium and neon. Helium is widely considered the best refrigerant fluid. However, the compression of pure helium, a light monoatomic gas, creates a large temperature increase that limits the pressure ratio of oil-free turbo-compressors and degrades its compression efficiency. The use of a mixture of helium (75 percent) and neon (25 percent) results in higher pressure ratios with higher compression efficiency. The expected Carnot efficiency of such a cycle is 42 percent.

Involving industrial partners during the conceptual design phase of the refrigeration system is also important. Air Liquide and Linde Kryotechnik AG (Switzerland), two worldwide helium refrigeration specialists, will be involved in these studies, working closely with the FCC team on efficient and reliable solutions. “I think that it’s an advantage to have industrial partners already thinking about our future needs and of solutions that could be industrialized in order to meet future needs of colliders like those explored under the FCC,” says Tavian.

There is also a collaboration agreement with Wroclaw University in Poland that focuses on the cryogenic distribution system. “For FCC to be more efficient we have to increase operating pressure of some cooling circuits up to 50 bar,” Tavian says. “The distribution system has to cope with higher design pressures and the collaboration is looking at the corresponding impact on the heat in-leaks.”

The university collaborators are running simulations to understand how static heat in-leaks evolve with pressure. In addition, their studies focus on the development of novel solutions to deal with the contraction of the pipes used in the distribution system. In conventional distribution systems based on stainless steel pipes, during cool-down from room temperature to 4 K a contraction of 3 mm/m has to be compensated with expansion bellows which are always a weak point of the system. An option based on Invar pipes is being examined. Invar, an iron-nickel alloy, has a very low contraction coefficient and internal compensation bellows are not required, allowing a more robust design.
The cryogenics team will meet again at FCC Week 2017 in Berlin. The transient operation modes inherent to the beam-energy ramp and to the resistive transitions of superconducting magnets are expected to be addressed, along with the conclusions of the industrial studies and of the Invar alternative for the distribution system.