3 February 2016: The first hydrogen plasma in Wendelstein 7-X. It reached a temperature of 80 million degrees Celsius. (Photo: MPI for Plasma Physics/ Wigner RCP).
The world’s most powerful stellarator, Wendelstein 7-X, has set the record in a key parameter of fusion physics: the triple product.
On the path toward a fusion power plant, stellarators are among the most promising concepts. In the future, they could generate usable energy by fusing light atomic nuclei. This reaction must take place in a plasma — a hot gas of ionised particles heated to many tens of millions of degrees Celsius.
Birger Buttenschön, of the Max Planck Institute for Plasma Physics, explains the concepts behind stellarators, the ‘opposing’ tokamak technology and Wendelstein 7-X’s place in the race to create real-world nuclear fusion plants.
ScienceVega: What are the key differences then between stellarators and tokamaks?
BB: For the nuclear fusion we are talking about, we always discuss magnetic confinement nuclear fusion, meaning we’re creating a plasma that is confined in a strong magnetic field that has certain properties that actually confines the plasma. Also, at these very, very high temperatures, you cannot have the plasma touch the walls at 150 million°C as your walls would melt and you would destroy your device instantly. You have to keep the plasma away from the wall and to do that you need a very strong magnetic field in the region of a few Tesla.
The technical difference between the stellarator and the tokamak is that the stellarator produces a confining field with all its twists and shapes through external coils and the field is there as soon as you switch on the coils. The tokamak is different in that it’s geometrically simpler in that it has a doughnut shape. Building the device is simple, but to create the confining properties of the magnetic field you actually need a plasma inside the tokamak in which you can drive a current, which will then produce an additional component of the field that will then only make the field confining. Without that plasma the magnetic field is not confining the hot plasma. And that is a drawback of the tokamak because if you’re driving this current, and we’re talking about approximately 1,000,000 amperes, and you lose it, you instantly lose your confining properties. This disruption in the tokamak instantly loses all your energy to the wall and, in the worst-case scenario, destroys your machine.
Schematic view of the Wendelstein 7-X superconducting magnets system with non-planar coils (blue) and planar coils (brown). Credit: M Nagel, CP Dhard, H Bau, H-S Bosch, U Meyer, S Raatz, K Risse and T Rummel, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons.
The main difference between the two, therefore, is a tokamak needs the plasma to have a confining field. The stellarator does not.
A second difference is to be able to create the current in the tokamak plasma, you essentially build a transformer to induce the current in the plasma. You do that by using the plasma essentially as the second winding of a transformer a bit like a normal transformer that you put into your wall plug. A transformer is an alternating current device so you have to ramp up a voltage in the transformer to induce the current and when you’re done with ramping up, you have to switch the thing off. You then ramp it up again, then switch it back on, and so on. A tokamak is inherently not steady state capable; it is a pulsed machine.
The stellarator is not because as you have the confining properties from the external coils, you can switch on the magnetic field and you can run the device as long as you want.
ScienceVega: What is so special about the Wendelstein 7-X stellarator?
BB: Stellarator research in Project Matterhorn was relatively quickly abandoned because it was pretty obvious that they couldn’t reach the necessary performance parameters of the plasma. The losses were significant compared to tokamaks. But Wendelstein 7-X is the first stellarator in the world that was numerically optimised to suppress a huge load of these losses.
The beginnings of W7-X were in the mid-1980s when what we now call super computers became fast enough and strong enough to calculate the system of a stellarator plasma with all the then known 3D effects of drifts and other phenomena. We could simulate that and optimise the shape of the magnetic field to suppress these losses. That is one of the main differences of W7-X as compared to all the other stellarators that have ever been: It is an optimised stellarator which means, amongst other things, optimised against these losses.
And that is exactly what we see. Our performance parameters and the numbers we get are now comparable with the tokamaks so we can show that this optimisation works and the stellarator and the tokamaks are at similar levels now.
View into the Wendelstein 7-X experimental hall at the Max Planck Institute for Plasma Physics in Greifswald, Germany, November 2021. Credit: MPI for Plasma Physics, Jan Michael Hosan.
ScienceVega: W7-X is designed to demonstrate that stellarators can in reality achieve the outstanding properties predicted in theory. Has it achieved this goal?
BB: Yes. There were a few design criteria we needed to consider to build W7-X and one of them was what is called neoclassical transport. It is to do with particle-based transport perpendicular to the magnetic field. We have measured that and we know that we have reached exactly this optimisation criteria. That is done. We know how to do it, and we have done it. The device is ready.
There were a few other design criteria, half of them of a technical nature, like the coils would have to be built and the entire device would have to be in a geometry that it can actually be realised. All that has been done.
We have also measured a few things: we know that the magnetic field looks very similar on a level of one to approximately 100,000 as predicted. The coil system works even though it’s weirdly 3D. Everything we wanted to happen happened and was successfully integrated into W7-X.
Segment of the plasma vessel for Wendelstein 7-X during production. (Photo: MPI for Plasma Physics, Wolfgang Filser).
ScienceVega: What is the triple product for long plasma durations?
BB: The triple product is the one and only number in energy-producing nuclear fusion. It’s essentially a threshold which if reached, if you have a device that can even overcome this number, then you will have a burning plasma, meaning you would not have to heat the plasma anymore; the particles and the energy produced by the fusion reactions would sustain the plasma itself. It’s a hard threshold. Below the threshold you have to heat the plasma and you have net energy loss. Above this threshold, you have net energy gain. It’s pretty binary.
This is a number that is constructed from the particle density in the reactor, the particle temperature in the reactor and the energy confinement time, which can be understood as something like the thermal insulation of the plasma. The product of these three numbers has to exceed a specific number to have a burning reactor.
Going back to the difference between stellarators and tokamaks: Because a tokamak is a pulsed device, there are no long plasma durations. At some point you cannot drive the plasma current anymore because your transformer is at its end and you have to switch it off again. The typical tokamak plasma has a length of between five and ten seconds, and that’s it. Within these few seconds you can actually reach pretty high triple products, but there are no points where you have a high value of the triple product and long plasmas. That’s not easily doable in a tokamak.
This is where the stellarator, and specifically W7-X, comes into play. From the optimisation that we have done on W7-X, we now know we can also reach these high triple products. For short discharges the tokamaks are still better than we are, but for long discharges, once we have created a plasma with a high triple product, we can in principle prolong it until it’s say 30 minutes long. There is no physical boundary that would prohibit us from letting the plasma run, and that is not possible in a tokamak.
We did think we had exceeded the best value of the tokamak, but unbeknownst to us, there was unpublished data from the JET tokamak in the UK, which finished operations in December 2023, where they had discharges with the triple product and plasma duration close to the one we had achieved in our last campaign. But since JET has now shut down, that’s the last data there is for the JET tokamak. In our next campaign, from both the technical point of view and from the plasma physics point of view, we know how to produce high triple products and we know how to produce long discharges. We just need to put those two together. That is a simple technical thing making the discharges longer, and this will without any significant effort on our side, lift us above the tokamak values.
ScienceVega: If the hottest part of the Sun, which is a nuclear fusion reactor, reaches temperatures of 15 million°C, why do we need to reach 30-40 million°C with Wendelstein 7-X?
BB: We actually need to be at roughly 150 million°C. That is the target temperature, regardless of whether you’re using a stellarator or a tokamak. Magnetic confinement fusion with these properties needs a temperature roughly ten times hotter than the core of the sun. Why? That is a very good question, but in short, the Sun is extremely inefficient at nuclear fusion. It’s doing a very complex chain of fusion reactions where if you take a cubic metre of the material in the core of the sun, that cubic metre will produce about 20 watts of power. That is ridiculously low.
There are other fusion reactions which are 26 orders of magnitude more efficient than the Sun. The drawback of that is you have to go to 150 million°C to be in the peak of efficiency for fusion reactions. This fusion reaction will show a power density of around 500kW/m3. Compared to the 20W of the Sun, that is something we can work with. It gives us plasma volumes of a few hundred cubic metres scale to give us a few gigawatts of electrical output power. That’s how it works; we need a different reaction to be more efficient and for this efficiency, we need higher temperatures.
View inside the vacuum vessel of the stellarator Wendelstein 7-X at the Max Planck Institute for Plasma Physics in Greifswald, Germany. Credit: MPI for Plasma Physics, Jan Hosan.
ScienceVega: How far are we from having nuclear fusion as a practical real-world technology?
BB: That is a very good question and I would like to know that myself! It’s difficult to say because the landscape is changing very quickly. Currently there are a number of private companies that have been founded in the last five to ten years, working on nuclear fusion and perhaps a handful are pursuing concepts that may work. Some are working on tokamaks, some are working with stellarators and some say they have their first prototype running within the next ten years or so. It’s difficult to say how realistic that is.
It’s not that far away. Our federal government asked for an assessment a couple of years ago and the key soundbite to come from that was ’20 years, €20bn’. Put simply, with the publicly funded fusion research we are doing here, we are saying that with a budget of approximately €20bn (~£17bn) over the next 20 years, we would be able to build a nuclear fusion power plant. It is still a while to go, but the world has come a long way. We are actually getting there.
I’m 43 now and my personal goal is to still be alive when the first nuclear fusion power plant actually becomes active on the grid!
