Let's talk tokamaks
Let’s shine the spotlight on one of the ventures endeavouring to solve the practicalities of actually making it happen. James heard from Hannah Willett at Tokamak Energy in Oxfordshire. They’re using the same technology as the STEP 2040 prototype plant will…
Hannah - Our approach is called magnetic confinement fusion, and essentially imagine trying to hold a jelly in a net except our jelly is our plasma fusion fuel - the hydrogen - and the net is made of magnetic field lines. So we have a machine called a tokamak which is a vacuum chamber in a kind of a cored apple shape. Inside this vacuum chamber, we suck as much air as possible out of it, put our fusion fuels inside it, heat it up, strike up a plasma as it's called and we use our magnetic net. So we have lots of field coils around the outsides of the chamber to hold the plasma inside, keep it away from the walls, make it stay nice and hot for the fusion reactions to happen.
James - A really nice minute long introduction to how it works. I wonder if we can unpack that slightly. Let's start with the inputs of the process. You need those hydrogen isotopes to smash into each other. Now where do these come from?
Hannah - The hydrogen isotope that is most common is hydrogen (1H), which is one proton and one electron. What we'll need to use for fusion on earth is deuterium and tritium, which are slightly heavier isotopes. So for deuterium, you add one neutron and for tritium, you add a second neutron. Now, deuterium is still naturally occurring. It's about one in every 6,700 hydrogen atoms, which doesn't sound like much, but when you consider how much hydrogen there is on earth, mostly in the form of water, H2O, in every cubic metre of seawater there's 33 grams of deuterium atoms. Tritium is a little bit more awkward, so it's slightly radioactive. It doesn't hang around forever. That will need to be produced, which is one of the, the big technological challenges of fusion. But it can be produced from lithium. So designing your power plant to be able to manufacture tritium as well to feed back in, that is part of the development of technology that needs to happen.
James - But one of the other major challenges is that enormous, extreme amount of heat you need to generate to get the deuterium and the tritium into a plasma and to keep it in that state. It needs to be millions of degrees, even hotter than the sun. How are you doing that? Where are you doing that? That's where the tokamak comes in, I assume.
Hannah - Exactly, yes. So the reaction rate does depend on the temperature. So the reason we have to reach these temperatures several times hotter than the centre of the sun, these millions of degrees, is because the reaction is most efficient at those temperatures. Fusing two nuclei together, they're both positively charged. So there's an electrostatic repulsion between them. They have to have enough energy to be able to be pushed close enough. Together we have neutral beam heating, which means that we create high energy beams of neutral hydrogen or deuterium, give them loads of energy, fire them into the plasma, and then once they're inside the plasma they have to be neutral to be able to get inside that magnetic net. But once they're in, they interact with the plasma particles that are already there and transfer that energy and actually allow that fusion reaction to happen.
Hannah - So the temperature is a big part of it. The other thing is density. Basically the denser your plasma is the more particles you have flying around in there and the more likely they are to interact. So there's kind of a careful balance between temperature, your density, and then the third element of N for density, T for temperature and Tau for confinement time. So how long we can hold the energy inside the plasma in the tokamak, because if you put the energy in and it all falls out again straight away, it's not super helpful. So you have to increase the confinement time to get that going, make ignition happen and keep your plasma burning.
James - You following us so far? Still with us? Well, don't worry if the answer is no because after her explanation, Hannah suggested I consolidate my knowledge by roleplaying as a tokamak engineer on earth in the far future. Keen to take up the challenge and prove what I'd learned, I tried my hand at the mobile game which thrusts the player into the hot seat of a futuristic power plant...
James - <Sci-fi music fades in> 'The energy shortage of the 21st century has been overcome. The solution: fusion energy. Fuel is heated to its plasma state at 200 million degrees at which it fuses and releases vast amounts of energy. Powerful magnets hold the plasma inside the power plant. You are the operator of a typical fusion power plant in 2103. As operator, you must drive the machine to the ideal fusion conditions. You control strong magnets to cage the plasma in the steel vessel, a powerful microwave heating system enabling the plasma particles to fuse, and a microwave cannon to blast magnetic islands. You've got to look after the temperature gauge, which goes up to 200 million degrees, and the confinement gauge where you adjust the magnetic power to keep the confinement at the right level.'
James - Right. Okay, I think I'm ready to give this a go. So I want to raise the magnet power. Oh, the confinement is way too high. Sorry. Okay, we'll increase the temperature, but that in turn brings the confinement quite low. Up comes the magnet. Okay, well done, core temperature is rising again. Phew. So the trick is to raise the magnet power and the heating power sort of evenly. We're up to 70 million degrees in my fusion reactor now things are going pretty well. Got to zap those magnetic islands with my microwave cannon. We're going to whack the heating power onto full now and raise the magnet power in turn. Ticking over 190 million degrees, 197, 200. 'Well done. You succeeded in driving the machine to the ideal fusion conditions.' Well,
James - Pretty pleased with myself there. Now before I let Hannah go, I wanted to ask her about Ian's query regarding how these obscene temperatures can be maintained while at the same time being able to extract energy from the fusion reaction.
Hannah - We come to another of the technological challenges, which is capturing the energy that comes out. So you want to make sure that, in the walls of your tokamak, you have the right kind of elements that can capture these neutrons, slow them down, extract the energy, and then that also feeds back into tritium breeding because you can use those neutrons then to react with the lithium to produce tritium. So it kind of makes a nice, neat puzzle. But I mean these are still big technological challenges. The engineering of how that needs to be done is a work in progress.
James - To be able to withstand these extreme conditions, what materials are tokamaks made of?
Hannah - So another technical challenge: material science. A big point of research and not just at tokamak energy but organisations globally looking at different options for how to create these plasma facing components, like the first wall inside the tokamak. So there are different options. Heavy metals with very high melting points, things like molybdenum, tungsten, they can withstand the highest heat loads if a plasma touches the wall. But the problem with them is that if they do get into the core of the plasma, because they're so big, they can end up radiating away a lot of the energy which, if we want to keep the confinement time up, is not great again. Other options are lighter elements right at the other end of the periodic table; boron, lithium we've already mentioned. But because they're light, if they get inside the plasma, they don't do as much damage as it were to the heating, to the maintaining of that temperature.
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