Giant Superconducting Magnets for Fusion Reactors

Max Quinn
March 19, 2012

Submitted as coursework for PH241, Stanford University, Winter 2012


Strong international cooperation has resulted in plans to build the International Thermonuclear Experimental Reactor (ITER). This project will hopefully result in the creation of the largest experimental tokomak nuclear fusion reactor in the world. A tokomak utilizes a magnetic field to confine a plasma in the shape of a torus. This reactor will be located at Catarache in the southern part of France. The European Union will pay 45% of the costs while India, Japan, China, Russia, South Korea, and the United States will each pay 9% of the remaining costs. [1] This project is an attempt to bridge the gap between the study of plasma physics and the practicality of full-scale electricity-producing fusion power plants. The projections for the power output are 500 MW out per 50 MW in, a ten-fold increase in power. This is a long-term project with construction that began in 2007. With a timeline in place, the first plasma is expected to be achieved in 2019. This project is certainly going to be a challenge. With supporters and cynics on both sides, it will be interesting to see if the project upholds its promises. [1]

Very intense magnetic fields are required for the magnetic confinement fusion of thermonuclear plasmas. Thus, one of the important goals of the ITER is the incorporation of giant superconducting magnets into the design. In particular, the design uses three giant superconducting systems: a toroidal magnetic field system (TF) consisting of 18 TF coils, a central solenoid (CS), and a poloidal magnetic field system (PF) consisting of 6 PF coils. [2] The functionality of these systems is crucial. It is very important to optimize the superconducting configuration, materials, and design. 31% of the machine's costs are from the cryogenic refrigerator and magnetic system, capturing the significance of this aspect of the ITER. [2]

Plans for the ITER use Nb3Sn as a superconducting material to achieve peak magnetic fields of 13 T. [3] Here, we look at two promising superconducting materials that may be able to yield improvements including fields approaching 20 T in the future.

Nb3Al Superconducting Materials

Large current superconductors that can be operated at high magnetic fields are important when designing fusion magnets. It was mentioned that Nb3Sn was chosen for use in the ITER. A large reason for this was that this conductor was relatively well-developed and researched, minimizing R&D and processing costs. However, this material is far from ideal. [2]

Nb3Al offers potential improvements in future design. First, it has a higher critical magnetic field. It has been established that a material cannot exist in a superconducting state at magnetic fields above this critical value. Therefore, Nb3Al can be operated at fields of 16 T, an improvement of 3 T over Nb3Sn superconductors. [3]

Second, Nb3Al offers superior strain tolerance. This can simplify the coil fabrication process, especially for the TF coil, by winding the conductor after heat treatment as opposed to before heat treatment. Effective heat treatment and processing methods including a reliable jelly-roll processing method pioneered by JAERI have made use of this material more practical. Despite this progress, production challenges still remain. For example, the inclusion of copper is required for conductors in fusion reactors. Copper's melting temperature of 1,080°C makes this difficult when high-temperature heat treatment is done at 1,800°C to raise the critical magnetic field value. [4,5] This problem will hopefully be solved in the future by research including stabilization using segregated copper wires. If production techniques can be improved, Nb3Al would be a very promising next-generation superconducting material in fusion design. [5]

High-Temperature Superconductors

High-temperature superconductors (HTS) exhibit superconducting behavior at abnormally high temperatures. The first HTS material was discovered by 2 IBM researchers in 1986. [6] Originally, it was believed that only certain compounds of copper and oxygen displayed this behavior. But recently, some iron-based compounds have been shown to be superconductors at elevated temperatures. [6]

These materials are promising because some have very high critical current densities exceeding 1 kA/mm2 at 4 K in magnetic fields as high as 20 T. [5] In addition, HTS current leads can be significantly cheaper than conventional copper leads because of their low thermal conductance. This would reduce heat dissipation and energy losses as current is carried to the magnet. Thus, magnet stability would increase and refrigerant costs could be reduced. [5]

While HTS current leads have been gradually implemented into various designs, large-scale incorporation of HTS materials has not yet been realized. This is because of heat treatment challenges including the difficulty of treating these materials in oxygen-rich environments. Another problem is that the heat-reaction temperature must be controlled with great precision. This is difficult to manipulate at greater scales. [5] Therefore, work must be done before HTS materials can be integrated into superconducting magnet designs for fusion reactors.

© Max Quinn. The author grants permission to copy, distribute, and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] J. Amos, "Key Component Contract for Iter Fusion Reactor," BBC News, 14 Oct 10.

[2] J.-L. Duchateau, "Superconducting Magnets for Fusion," CLEFS CEA, Commissariat à l'Energie Atomique, No. 56 (Winter 2007-2008),p. 12.

[3] N. Koizumi, T. Takeuchi and K. Okuno, "Development of Advanced Nb3Al Superconductors For a Fusion Demo Plant," Nucl. Fusion, 45, 431 (2005).

[4] W. D. Callister Jr., Fundamentals of Materials Science and Engineering: An Integrated Approach (Wiley, 2004).

[5] K. Okuno, A. Shikov and N. Koizumi, "Superconducting Magnet System in a Fusion Reactor," J. Nucl. Mat. 329-333, Part A, 141 (2004).

[6] P. J. Ford, The Rise of the Superconductors (CRC Press, 2004).