The dominant features of a tokamak or any other magnetic bottle are the heavy coils that generate the large magnetic field used to confine the plasma. Until recently, all tokamaks had magnet coils made of copper, which conducts electricity better than any other metal except silver. Even so, it takes a lot of energy to drive megamperes of current through copper coils, and fusion reactors will have to use superconducting coils. Superconductors have zero resistivity, and once the current has been started in them, it will keep going almost forever. The hitch is that superconductors have to be cooled below 4.2 K with liquid helium. A cryogenic plant has to be built to supply the liquid helium, and the magnet coils (and hence the whole machine) have to be enclosed in a cryostat to insulate them from room temperature. The good news is that this technology is well developed and is not one of the serious obstacles to fusion power. In 1986, the world’s largest superconducting magnet, the MFTF (mirror fusion test facility), was completed at the Lawrence Livermore Laboratory in California. It was a different type of magnetic bottle that we will describe in Chap. 10. However, the program was almost immediately canceled by the Reagan administration in favor of the tokamak because the USA could not afford to follow two expensive paths to fusion. The MFTF was so large that for a while it became a museum that one could walk through. Currently, three superconducting tokamaks are in operation: the Tore Supra in France, the EAST (Experimental Advanced Superconducting Tokamak) in Hefei, China, and K-STAR, in Daejon, Korea. Soon to join them is an upgrade to Japan’s JT-60U (Fig. 8.6) called JT-60SA. In addition, the Large Helical Device, a superconducting stellarator-type machine, has been operating for two decades in Japan. ITER will, of course, have superconducting magnets.

Two superconducting materials are available on a large scale: niobium-titanium (NbTi) and niobium-tin (Nb3Sn). NbTi is cheaper and easier to make, but it loses its superconductivity above 8 T. A tesla is a large unit of magnetic field equal to 10,000 G, the old unit. Common magnets rarely go above 0.1 T, but some magnetic resonance imaging (MRI) machines in medicine can go up to 1.5 T. The earth’s magnetic field is only about 0.5 G or 0.00005 T. In ITER, fields up to 13.5 T are needed, so some coils are made of Nb3Sn, and others (for lower fields) are made of NbTi. The dividing line is around 5 T [14]. Superconducting cables are complicated to make because they have to be made of a thousand thin strands. This is because the current in superconductors flows only on the surface, and thin strands have large surface areas compared to their volumes. Also, the cables have to be bendable.