Figure 9.21 shows what a niobium-tin cable looks like inside. There are over 1,000 strands in six bundles. At the center is a helix making room for the pipe that carries the liquid helium. The outer casing is a stainless steel jacket 37.5 mm (1.5 in.) in diameter. This cable, designed for the toroidal field coils of ITER, can carry 80 kA at 9.7 T. Each strand is about 0.8 mm in diameter and consists of a Nb3Sn filament sheathed with chromium and covered with about as much copper as Nb3Sn. The copper is necessary to mitigate quenches. A quench occurs when part of the superconductor goes normal, losing its superconductivity because of over­heating or over-current. Huge voltages would build up as the current tries to force

its way through a normal conductor with resistance, and there could be an explosion. Copper can make this a gentler accident. The complexity of superconducting cables is bad enough, but to wind them into magnetic coils means that each cable has to be over 1.5 km (a mile) long.

A tokamak has many different kinds of magnet coils, and each requires a different design. Some of these can be seen in Fig. 9.22. The toroidal field (TF) coils are the large D-shaped coils. They operate up to 6 T and are the heaviest ones. Transporting them to the ITER site requires special barges, trucks, and roads. The large, horizon­tal ones encircling the machine are the poloidal field (PF) coils, which give the field lines their twist and shape the plasma. Because of their size, they cannot be trans­ported and must be wound on site. The coil winding building at the ITER will be 253 m long, 46 m wide, and 19 m high.3 A critical component is the central solenoid (CS), seen inside the hole in the torus. There is very little space there, and most of it is taken up by the interior blanket modules. This coil is the other half of the PF system that shapes the plasma and drives the tokamak current. The CS is 13 m tall and 4.3 m in diameter, weighing 1,000 tons. It also produces the highest field of 13.5 T. Figure 9.23 shows a test section of it that has been made.

There are smaller coils besides these main coils, but the difficult part is to join the superconductors to their feeds. Current is fed into the coils from normal­conducting cables, and then a superconducting switch is turned on so that the

current flows only in the superconductors and the feed cables can be disconnected. These junctions are very complicated, especially since the current has to go through the wall of the cryostat from room temperature to 4 K. Almost all the nations supporting ITER participate in designing and producing the magnet system. Some make the NbTi and Nb3Sn materials. Some make it into strands and cables. Some wind the cables into coils. And some make the feed cables and the junctions. The technology has already been developed for smaller tokamaks, and the steps to ITER, DEMO, and reactor are only matters of scale.