Stellarators like the Wendelsteins are large machines with large aspect ratios R/a, where R is the major radius of the ring and a is the radius of the cross section. There is a movement to build smaller, more economical machines by shrinking R to get aspect ratios of 3-5 instead of 10 or more. Proposed compact stellarators have been designed with different magnetic-field configurations to see which would work better. This freedom of design is not available for tokamaks, but it also means that it is harder to converge on the optimal design. The National Compact Stellarator Experiment (NCSX) was funded and under construction at the Princeton Plasma Physics Laboratory, but the project was canceled during the 2009 worldwide eco­nomic depression. Figure 10.8 shows the NCSX and its coil structure. There are only 18 coils of three different shapes. Although this machine was well designed and would have complemented the Wendelstein 7-X nicely, its discontinuation was reasonable. Tokamaks are far ahead in development, and to get a fusion reactor working the fastest way is to give them the highest priority.

Stellarators are second-generation confinement devices. They are probably better suited for reactors than tokamaks, but we need much more experience with how they run. An obvious question is: Where do you put the blankets in a DT stellarator

reactor? The problem is that the magnet coils are not circular but have small twists and bends. The coils have to be close to the plasma for these fine features to be felt; too far, and the details will be smeared out. That’s why the vacuum chamber has to be shaped to fit the coils. In a reactor, one still has to leave room for the tritium­breeding blanket, and the only way to do this is to scale the whole machine larger. There have been several reactor studies from Germany, Japan, and the USA. The ARIES-CS design is shown in Fig. 10.9 and the overall view in Fig. 10.10. It was found possible to place the blanket modules between the plasma and the vacuum wall and superconducting coils.