(By Max-Planck Institut für Plasmaphysik – Max-Planck Institut für Plasmaphysik, CC.)

Part 3 of 5 Parts (Please read Parts 1 and 2 first)
Stellarator
      Stellarators resemble tokomaks because they confine plasma streams in a circular reactor vessel with magnetic coils. However, instead of a symmetrical donut shape, a stellarator sends its plasma around in irregular circles that twist and turn within an incredibly complex series of magnetic coils. It may seem counterintuitive, but this arrangement of coils actually produces more stability in the plasma because of the differences in the internal current.
     The stellarator was invented by Lyman Spitzer at Princeton University in 1951. Much of its early development was carried out by Spitzer’s team which became the Princeton Plasma Physics Laboratory (PPPL). Spitzer’s Model A Stellarator began operating in 1953, demonstrating plasma confinement. By the early 1960s, it was clear that existing stellarators had serious confinement problems. By the mid-1960s, Spitzer was convinced that stellarators would never be practical fusion reactors for the production of energy.
     For the next two decades, tokamaks got most of the attention and funding in fusion research. However, in the 1990s, stellarators have seen renewed interest. Major stellarators include the Wendelstein 7-X in Germany, the Helically Symmetric Experiment in the U.S, and the Large Helical Device in Japan,
     Matthew Hole, a nuclear fusion expert and research fellow at Australian National University, said, “In toroidal magnetic confinement, you need the current to twist. Tokamaks do this with large internal current that causes the field to twist and rotate as it goes around the bend. In a stellarator, you deliberately twist the whole cross section of the bend. With twisted coils, you twist the magnetic current. This means you don’t need a large internal current to generate the twist. So in some sense, you’re translating a physics problem into an engineering problem.”
     Hole notes that such a design creates huge mathematical problems when trying to describe the twisting torus. With respect to mathematical and engineering challenges, the Wendelstein 7-X has plenty of both. It is the largest stellarator in the world and could not have been designed without the use of supercomputers. Stellarators are built to continuously contain super-hot plasma for over thirty minutes at a time. The Wendelstein 7-X was turned on for the first time in 2015 and has been moving incrementally toward it design goal ever since.
     It originally confined helium plasma but, by 2016, it was confining hydrogen plasma. By 2018, sub-second-long flashes had been extended to longer than one hundred seconds which was a record for stellarator designs. The temperature of the plasma was over thirty-six million degrees Fahrenheit. The Wendelstein 7-X also achieved huge energy yields during these tests.
      Stellarators don’t require the large internal current found in tokamaks and they offer more stability during operations than the tokamaks. They may be better suited to providing power to the grid than tokamaks. This is assuming, of course, that the incredibly complex infrastructure can be constructed in a way that is not prohibitively expensive.
     Hole said, “A stellarator has more intrinsic appeal, perhaps, than a tokamak, in the long term. But to an engineer, a stellarator is a bit of a nightmare. So that’s why both are worth exploring.”
Please read Part 4 next