When you read news about fusion it’s likely to be about a tokamak type reactor. These large doughnut shaped devices have dominated fusion research for the past 3 decades mostly because of their relative ease of construction when compared to other designs. That’s not to say they’re not without their drawbacks, as the much delayed ITER project can attest to, however we owe much of the recent progress in this field to the tokamak design. However there are other contenders that, if they manage to perform at similar levels to tokamaks, could take over as the default design for future fusion reactors. One such design is called the stellarator and its latest incarnation could be the first reactor to achieve the dream: steady state fusion.
Compared to a tokamak, which has an uniform shape, the stellarator’s containment vessel appears buckled and twisted. This is because of the fundamental design difference between the two reactor types. You see in order to contain the hot plasma, which reaches temperatures of 100 million degrees celsius, fusion reactors need to contain it with a magnetic field. Typically there are two types of fields, one that provides the pinch or compressing effect (poloidal field) and another field to keep the plasma from wobbling about and hitting the containment vessel (toroidal field). In a tokamak the poloidal field comes from within the plasma itself by running a large current through the plasma and the poloidal field from the large magnets that run the length of the vessel. A stellarator however provides both the toroidal and poloidal fields externally requiring no plasma current but necessitating a wild magnet and vessel design (pictured above). Those requirements are what have hindered stellarator design for some time however with the advent of computer aided design and construction they’re starting to become feasible.
The Wendelstein 7-X, the successor to the 7-AS, is a stellarator that’s been a long time in the making, originally scheduled to have been fully constructed by 2006. However due to the complexity and precision required of the stellarator design, which was only completed with the aid of supercomputer simulations, construction only completed last year. The device itself is a marvel of modern engineering with the vast majority of the construction being completed by robots, totalling some 1.1 million hours. The last year has seen it pass several critical validation tests, including containment vessel pressure tests and magnetic field verification. Where it really gets interesting though is where their future plans lead; to steady state power generation.
The initial experiment will be focused on short duration plasmas with the current microwave generators able to produce 10MW in 10 second bursts or 1MW for 50 seconds. This is dubbed Operational Phase 1 and will serve to validate the stellarator’s design and operating parameters. Then, after the completion of some additional construction work to include a water cooling transfer system, Operational Phase 2 will begin which will allow the microwave system to operate in a true steady state configuration, up to 30 minutes. Should Wendelstein 7-X be able to accomplish this it will be a tremendous leap forward for fusion research and could very well pave the way for the first generation of commercial reactors based on this design.
Of course we’re still a long way away from reaching that goal but this, coupled with the work being done at ITER, means that we’re far closer than we ever were to achieving the fusion dream. It might still be another 20 years away, as it always is, but never before have we had so many reactor designs in play at the scales we have today. We’ll soon have two (hopefully) validated designs done at scale that can achieve steady state plasma operations. Then it simply becomes a matter of economics and engineering, problems that are far easier to overcome. No matter how you look at it the clean, near limitless energy future we’ve long dream of is fast approaching us and that should give us all great hope for the future.