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.
The world of fusion is currently dominated by a single project: The International Thermonuclear Experimental Reactor. It is by far the biggest project ever undertaken in the field of fusion, aiming to create a plant capable of producing sustained bursts of 500MW. Unfortunately due to the nature of fusion and the numerous nations involved in the project it’s already a decade behind where it was supposed to be with conservative estimates having it come online sometime in 2027. Now this isn’t an area I’d usually considered ripe for private industry investment (it’s extremely risky and capital intensive) but it appears that a few start-ups are actually working in this area and the designs they’re coming up with are quite incredible.
There’s 2 main schools of thinking in the world of fusion today: inertial confinement and magnetic confinement. The former attempts to achieve fusion by using incredible amounts of pressure, enough so that the resulting reaction plasma is 100 times more dense than lead. It was this type of fusion that reached a criticla milestone late last year with the NIF producing more energy in the reaction than they put into it. The latter is what will eventually power ITER which, whilst it has yet to provide a real (non-extrapolated) Q value of greater than 1 it still has had much of the basic science validated on it, thus providing the best basis from which to proceed with. What these startups are working on though is something in between these two schools of thinking which, potentially, could see fusion become commercially viable sooner rather than later.
The picture above is General Fusion’s Magnetized Target Fusion reactor a new prototype that combines magnetic confinement with aspects of its inertial brethren. In the middle is a giant core of molten lead that’s spinning fast enough to produce a hollowed out center (imagine it like an apple with the core removed). The initial plasma is generated outside this sphere and contained using a magnetic field after which it’s injected into the core of the molten lead sphere. Then pistons on the outside of the molten sphere compress it down rapidly, within a few millionths of a second, causing the internal plasma to rapidly undergo fusion reactions. The resulting heat from the reaction can then be used in traditional power generators, much like it would in other nuclear reactors.
The design has a lot of benefits like the fact that the molten lead ball that’s being used for containment doesn’t suffer from the same neutron degradation that other designs typically suffer from. From what I can tell though the design does have some rather hefty requirements when it comes to precision as the compression of the molten lead sphere needs to happen fast and symmetrically. The previous prototypes I read about used explosives to do this, something which isn’t exactly sustainable (well, at least from my point of view anyway). Still the experiments thus far haven’t disproved the theory so it’s definitely a good area for research to continue in.
Whether these plucky upstarts in fusion will be able to deliver the dream faster than ITER though is something I’m not entirely sure about. Fusion has been just decades away for the better part of a century now and whilst there’s always the possibility these designs solve all the issues that the other’s have it could just as easily go the other way. Still it’s really exciting to see innovation in this space as I honestly thought the 2 leading schools of thought were basically it. So this is one of those occasions when I’m extraordinarily happy to be proven wrong and I hope they can dash my current skepticism again in the not too distant future.