Division of accelerators, cryogenics and magnetism

Thanks to the expertise developed by the CEA during the SPIRAL2 and IFMIF projects, in 2014 the CEA signed a contract with the Soreq Nuclear Research Centre (SNRC, Israel) to build a superconducting linear accelerator called SARAF (Soreq Applied Research Accelerator Facility). The aim is to build an accelerator capable of delivering proton and deuteron beams with energies ranging from 5 to 40 MeV and intensities of up to 5 mA. This equipment will be an important tool for fundamental research, particularly in nuclear physics, the characterisation of materials using neutrons and the production of medical radioisotopes. The CEA is responsible for the design, manufacturing and qualification of 5 sub-assemblies: the MEBT and the 4 cryomodules of the linear accelerator. In 2023, the CEA qualified the MEBT, which was delivered in 2020, and delivered the first cryomodule to Soreq.

The PIP-II (Proton Improvement Plan-II) project is the first particle accelerator to be built in the United States with major in-kind contributions from international partners. CEA/IRFU provides its expertise in the field of superconducting radio-frequency cryomodules and associated technologies. CEA/IRFU has been involved in this project since 2018, with a major contribution to the LB650 superconducting accelerator section, including the design studies, manufacture, and qualification of 10 cryomodules (each of them housing 4 superconducting accelerating cavities), i.e. 1 pre-production cryomodule and the 9 production cryomodules of the accelerator. A major milestone in the CEA/IRFU project was reached in April 2023, with the final design review of the LB650 cryomodule, validating 5 years of design activities. The project is now entering the pre-production cryomodule construction phase.

The recent update of the European Strategy for Particle Physics recommended a feasibility study for the future generation of collider. In this context, the Laboratory Directors Group, of which IRFU is a member, has been mandated by the CERN Council to oversee the development of an accelerator R&D roadmap. One of the objectives of this roadmap is the development of technologies for the manufacture of high-field superconducting magnets, essential for future colliders: this is the HFM (High Field Magnets) project.

The LEAS (Laboratoire d'Etude des Aimants Supraconducteurs) at CEA Paris-Saclay has entirely manufactured a coil based on the superconductor Nb3Sn (niobium-tin), of the SMC (Short Model Coil) type. This coil is a short model intended to be assembled in a magnet structure, then to be tested at cryogenic temperature. Nb₃Sn is being considered for future accelerator magnets generating magnetic fields up to 16 T (teslas), which would double the performance of the best magnets currently in use. However, this requires a great deal of technological development. This type of short coil has been developed by Cern, in collaboration with the CEA, to allow the testing of new technologies and new manufacturing processes under conditions representative of future high-field magnets. The fabrication of the SMC-CEA coil took place at LEAS from May to October 2021, then the coil was delivered to Cern to be assembled in a structure, and finally tested in a liquid and superfluid helium bath, under high current, in a dedicated station. The tests delivered encouraging results, demonstrating that LEAS is one of the few European laboratories that now has all the capabilities to manufacture Nb3Sn superconducting coils. This proof of feasibility validates the first step of the development program of high field magnets for future accelerators.

A year and a half after the delivery of the prototype cryomodule (CM00) to ESS, the first production medium beta cryomodule (CM01) has now arrived at the ESS site. It left CEA on September 22, 2020 for a two-day trip to Lund, Sweden. The Irfu teams had previously validated the RF and cryogenic performances of this cryomodule. It will be tested again on the ESS test bench before being integrated in its final position in the accelerator tunnel. This is a first step. Starting next year, ESS will receive an average of one cryomodule per month for 3 years.

The EUPRAXIA project has just completed its design study phase with the delivery of the Conceptual Design Report (CDR) at the end of 2019. The strong involvement of IRFU, particularly in the field of particle beam physics, has made it possible to show that solutions exist for the realization of a plasma wakefield accelerator, with a beam quality approaching that of conventional accelerators.
Detailed studies of the physical mechanisms involved have efficiently guided the numerical simulations, each lasting more than 10 hours on 2048 computing nodes, to demonstrate that all the objectives on the output beam can be achieved with a plasma of 30 cm long, 1.10¹⁷ cm^-3 electronic density and a laser of 400 terawatts power, 50 joules energy. Innovative methods have been developed for accelerating and driving the beam through the two plasma stages to the end user without degrading the beam. A first analysis of error tolerances allowed to identify the most sensitive components to which particular care should be taken during the fabrication and implementation.

After more than 5 years of development, including 6 months of integration work of the 12,000 separate components to a complete cryomodule, the CEA-Irfu has just validated the technology of this complex system that reached the nominal ESS accelerating field in the 4 superconducting accelerating cavities.
At the limits of technology, this is the first time that such an intense accelerating field, maintained over such long pulse durations and with such high RF power, has been measured in superconducting cavities installed in a complete cryomodule.
This key step makes it possible to start the production phase of the 30 cryomodules that France is to deliver to the ESS research infrastructure, which will be operational in Sweden in 2023. This serial integration will begin in January 2019 under the supervision of Irfu with the contribution of the company B&S France and should be completed in 2022.