The largest programs that kept stellarators alive during the tokamak era were the Wendelstein program in Germany and the large helical device (LHD) in Japan. How stellarators look nowadays is a far cry from the first primitive devices. A schematic of the magnet coil structure in a classical stellarator is shown in Fig. 10.2. The circular coils generate the toroidal field, and the helical coils add the poloidal field. The number of conductors on a minor circumference determines the periodicity of the helical field. The magnetic island structure is fixed externally and not by the internal plasma current. Note that the plasma is no longer circular; it follows the helical structure of the coils.

Now imagine that the circular and helical coils are combined into individual coils that produce the same magnetic fields. We then have the structure shown in Fig. 10.3, which is a diagram of Wendelstein 7-X, the newest stellarator being con­structed in Greifswald, Germany (formerly East Germany). These coils are easier

to assemble, since the poloidal-field coils do not have to be threaded through the toroidal-field coils. To conserve magnetic-field volume (which is costly), the coils are also shaped to conform to the shape of the plasma.

Design of the coils can be done with computers, but these unusual coils have actually been constructed in special jigs. The coils are of superconducting NbTi cooled with liquid helium. Not all the coils are different, of course. There will be seven different types, 10 of each, and 70 coils overall. Figure 10.4 shows two of these being lifted. The vacuum chamber will also conform to the plasma shape; a section of it is shown in Fig. 10.5. This will link the coils together and have to be assembled with them.

Wendelstein 7-X is a very large and complicated machine to be finished in 2014. Plans are to reach 30-min pulses with 40 MW of heating power, reaching condi­tions approaching those in ITER but only with aneutronic fuels.