With the first tokamak designed for full nuclear operation now well into final assembly (ITER), and a major new research tokamak starting commissioning (JT60SA), nuclear fusion is becoming a mainstream potential energy source for the future. A critical part of the viability of magnetic confinement for fusion is superconductor technology. The experience gained and lessons learned in the application of this technology to ITER and JT60SA, together with new and improved superconducting materials, is opening multiple routes to commercial fusion reactors. The objective of this roadmap is, through a series of short articles, to outline some of these routes and the materials/technologies that go with them.

Superconductors for fusion: A roadmap

The EuroCirCol project is part of the Future Circular Collider (FCC) study under the European Union leadership in the framework of a H2020 project. In particular, the Italian Institute for Nuclear Physics, in collaboration with CERN and other European laboratories, is developing the design of a cos theta ${{\rm Nb}_{3}{\rm Sn}}$ dipole magnet which will be part of the Conceptual Design Report of the FCC studies in 2019. The magnet, with an aperture diameter of 50 mm and a bore field of 16 T, will be able to bend the beams at final energies and within collider size constraints. Here we present an update of the electromagnetic design of the ${{\rm Nb}_{3}{\rm Sn}}$ cos theta dipole in double-aperture configuration, the 2-D mechanical analysis, and also the 3-D coil-ends study.

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Baseline Design of a 16 T $\cos \theta$ Bending Dipole for the Future Circular Collider

Three different 16 T dipole magnet options for the Future Circular Collider (FCC) have been designed within the H2020 EuroCirCol collaboration, namely a cosθ-, a block-, and a common-coil -type of magnet. All magnets were designed using the same constraints related to magnetic field, cable parameters, mechanics, and quench protection. The quench protection analysis during the magnet design was centralized and done for all the options using the same tools and methods. This paper summarizes the final conceptual protection schemes. They are based either on the novel CLIQ-technology (Coupling Loss Induced Quench) or on traditional quench heaters. We compare the performance of the protection systems and show that while both can protect the magnet at nominal operation current, CLIQ is more efficient in reducing peak temperatures than heaters, and the system operation is simpler due to smaller amount of protection units. Therefore, CLIQ has been chosen as the baseline protection option for the FCC dipole magnets. The future development of heaters is considered as a back-up option.

Quench Protection of the 16 T Nb3Sn Dipole Magnets Designed for the Future Circular Collider

The most effective way to achieve very high collision energies in a circular particle accelerator is to maximize the field strength of the main bending dipoles. In dipole magnets using Nb-Ti superconductor the practical field limit is considered to be 8-9 T. When Nb 3 Sn superconductor material is utilized, a field level of 15-16 T can be achieved. To further push the magnetic field beyond the Nb 3 Sn limits, High Temperature Superconductors (HTS) need to be considered in the magnet design. The most promising HTS materials for particle accelerator magnets are Bi2212 and REBCO. However, their outstanding performance comes with a significantly higher cost. Therefore, an economically viable option towards 20 T dipole magnets could consist in an “hybrid” solution, where both HTS and Nb 3 Sn materials are used. We discuss in this paper preliminary conceptual designs of various 20 T hybrid magnet concepts. After the definition of the overall design criteria, the coil dimensions and parameters are investigated with finite element models based on simple sector coils. Preliminary 2D cross-section computation results are then presented and three main layouts compared: cos-theta, block, and common-coil. Both traditional designs and more advanced stress-management options are considered.

Towards 20 T Hybrid Accelerator Dipole Magnets

The most effective way to achieve very high collision energies in a circular particle accelerator is to maximize the field strength of the main bending dipoles. In dipole magnets using Nb-Ti superconductor the practical field limit is considered to be 8-9 T. When Nb3Sn superconductor material is utilized, a field level of 15-16 T can be achieved. To further push the magnetic field beyond the Nb3Sn limits, High Temperature Superconductors (HTS) need to be considered in the magnet design. The most promising HTS materials for particle accelerator magnets are Bi2212 and REBCO. However, their outstanding performance comes with a significantly higher cost. Therefore, an economically viable option towards 20 T dipole magnets could consist in an “hybrid” solution, where both HTS and Nb3Sn materials are used. We discuss in this paper preliminary conceptual designs of various 20 T hybrid magnet concepts. After the definition of the overall design criteria, the coil dimensions and parameters are investigated with finite element models based on simple sector coils. Preliminary 2D cross-section computation results are then presented and three main layouts compared: cos-theta, block, and common-coil. Both traditional designs and more advanced stress-management options are considered.

A future circular collider (FCC) with a center-of-mass energy of 100 TeV and a circumference of around 100 km, or an energy upgrade of the LHC (HE-LHC) to 27 TeV require bending magnets providing 16 T in a 50-mm aperture. Several development programs for these magnets, based on Nb3Sn technology, are being pursued in Europe and in the U.S. In these programs, cos–theta, block-type, common-coil, and canted–cos–theta magnets are explored; first model magnets are under manufacture; limits on conductor stress levels are studied; and a conductor with enhanced characteristics is developed. This paper summarizes and discusses the status, plans, and preliminary results of these programs.

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Inigo Sancho Fernandez

Papers

The 16 T Dipole Development Program for FCC and HE-LHC