The High Luminosity Large Hadron Collider (HL-LHC) is the new flagship project of CERN. First endorsed in 2013 and approved in 2016, HL-LHC is an upgrade of the accelerator aiming to increase by a factor of ten the statistics of the LHC collisions at the horizon of 2035–2040. HL-LHC relies on cutting edge technologies: among them, large aperture superconducting magnets will replace the present hardware to allow a smaller beam size in two interaction points (IPs). The project involves the construction of about 150 magnets of six different types: the quadrupole triplet, two main dipoles and three orbit correctors. The triplet, manufactured at CERN and in the USA, will consist of 30 magnets based on Nb3Sn technology, with an operational peak field of 11.4 T. These will be the first quadrupole Nb3Sn magnets installed in a particle accelerator. The other five types of magnets, all relying on Nb–Ti technology, present non-trivial challenges in the design and construction; they will be manufactured as part of in-kind contribution under the responsibility of institutes in Japan, China, Spain, and Italy. The project is now in the phase of transition between qualification through short models and prototypes and the beginning of the series construction. In this paper we review the magnet requirements, the reasons for selecting the design, the technological challenges with respect to previous projects, and we summarize the steps that have been taken to validate the baseline.

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The High Luminosity LHC interaction region magnets towards series production

To enable the physics research that continues to deepen our understanding of the Universe, future circular colliders will require a critical and unique instrument—magnets that can generate a dipole field of 20 T and above. However, today’s maturing magnet technology for low-temperature superconductors (Nb-Ti and Nb3Sn) can lead to a maximum dipole field of around 16 T. High-temperature superconductors such as REBCO can, in principle, generate higher dipole fields but significant challenges exist for both conductor and magnet technology. To address these challenges, several critical research needs, including direct needs on instrumentation and measurements, are identified to push for the maximum dipole fields a REBCO accelerator magnet can generate. We discuss the research needs by reviewing the current results and outlining the perspectives for future technology development, followed by a brief update on the status of the technology development at Lawrence Berkeley National Laboratory. We present a roadmap for the next decade to develop 20 T-class REBCO accelerator magnets as an enabling instrument for future energy-frontier accelerator complex.

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Dipole Magnets Above 20 Tesla: Research Needs for a Path via High-Temperature Superconducting REBCO Conductors

The US Department of Energy (DOE) Office of Science (SC), is funding a large bore “Cable Test Facility Magnet” for testing advanced cables and inserts in high transverse field. This is a joint effort between the Office of High Energy Physics (HEP) and the Office of Fusion Energy Sciences (FES). The background field magnet for this facility is being developed at Lawrence Berkeley National Laboratory (LBNL) while the cryostat and test facility will be located and operated by Fermi National Accelerator Laboratory (FNAL). The Nb $_{3}$ Sn dipole magnet, which will provide the transverse background field, is designed to generate a field of 15 T in a 100 × 150 mm bore at 1.9 K. The conceptual design of a block-type dipole with flared ends and a structure based on key-and-bladder technology will be introduced. The results of the magnetic and mechanical analysis will be presented.

Magnetic and Mechanical Analysis of a Large Aperture 15 T Cable Test Facility Dipole Magnet

Numerical analysis of the screening current-induced magnetic field in the HTS insert dipole magnet Feather-M2.1-2

In the framework of the Future Circular Collider (FCC) program, and in collaboration with CERN, CEA is developing a 1.5-m-long single aperture Nb
 3
Sn block dipole. The so-called FCC Flared-end Dipole Demonstrator (F2D2) will produce at least 15 T with 14% margin at 1.9 K and will be assembled in a shell-based support structure. Its magnetic design is derived from the block-coil option proposed in the EuroCirCol studies and relying on graded cables for improved efficiency. The design uses identical strand dimensions but a lower critical current density according to the performance of a currently available conductor. The main challenge of this magnet lies in the concepts of joints between the conductor grades, which will be implemented for the first time in a Nb
 3
Sn block dipole. After presenting the overall design of the magnet from a magnetic and mechanical standpoint, chosen fabrication techniques are detailed.

F2D2: A Block-Coil Short-Model Dipole Toward FCC

The Nb$_3$Sn block coil dipole magnet FRESCA2 was developed within the framework of a collaboration between CEA Saclay and CERN, in the continuity of the European project EuCARD and EuCARD2. With an aperture of 100 mm and a target bore field of 13 T at 10.6 kA, the magnet is required for a new FRESCA2 cable test facility at CERN. In 2017, the magnet was pre-loaded to retain the forces while the magnet was powered to achieve 13.3 T in the magnet bore. Results of these tests were published. In 2018, the loading of the magnet has been increased for powering to higher current. In this paper, the updated results of the cold powering tests are discussed in terms of training, memory, and stable operation. The loading of the magnet and the mechanical measurements during cooldown are shown and compared to the earlier loading steps. The protection of the magnet is further reviewed and measured results are compared to the model simulations.

Francoise Rondeaux

Papers

Tests of the FRESCA2 100 mm bore Nb$_3$Sn block-coil magnet to a record field of 14.6 T