Science Interviews


Tue, 2nd Sep 2014

The Tokamak Method

Sarah Medley, Culham Centre for Fusion Energy

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One group of scientists trying to create this elusive positive power output is the team at JET - the Joint European Torus. This machine is the largest facility of its kind in operation today. Its a magnetic confinement plasma physics experiment using a tokamak reactor. Naked Scientist Dave Ansell went to visit JET at the Culham Centre for Fusion Energy in Oxfordshire, to see what its all about. He spoke to Sarah Medley... 

Sarah -   Fusion happens in the sun and that means its very difficult to do it here on Earth because the sun is a huge ball of burning gas and were one little planet, trying to do it here.  So, in fusion research, were trying to recreate that process here on Earth.  Were going to fuse hydrogen together to produce helium and release lots of energy so that one day, we can generate electricity from that fusion reaction.  To do that here on Earth, you need to heat them at such a high temperature that they turn into something called a plasma which is basically you can think of it like a very hot gas where the electrons are separated from the nuclei.

Dave -   So, why do you need it to get it so hot?

Sarah -   It needs to be so hot because of the fact that fusion is very difficult to do.  If you go about to thinking about an atom, an atom has a nucleus in the centre and electrons on the outside.  So electrons are negatively charged and nuclei are positively charged.  So, if you've got one nucleus, and you bring it close to another nucleus, those are both positively charged particles.  And so, because they're both positively charged that you've heard of opposites attract like things repel, well basically, the two positive charges repel each other.  So, to get them close enough in order that they want to actually fuse together, you need to give them a lot of energy, so you need to heat up the whole plasma to actually get the nuclei close enough for fusion to happen.

Dave -   So, you basically got to throw these particles together, hard enough that they can't bounce off and actually stick together.

Sarah -   That's exactly it, yeah.  You want them to not bounce off each other.  Not repel but actually stick together and fuse and release energy.

Dave -   So, how do you go about basically confining a bit of the sun in a large metal box just behind us?

Sarah -   So, when you separate the electrons from the nuclei, it means that this plasma which consists of the electrons and nuclei are going to have to be controlled by magnets because charged particles do react to magnets.  They can actually control them and move them.  So basically, we have the plasma inside the fusion reactor the tokamak and we use huge magnets to control it and keep it away from the metal walls.

Dave -   So, the charged particles essentially get trapped by a magnetic field and will follow that magnetic field.  So, what actually does the JET machine behind this look like inside?

Sarah -   So, at the heart of JET is a doughnut-shaped chamber.  JET actually stands for Joint European Torus and torus is basically the scientific name for doughnut.  Inside the torus is where we put the plasma.  So, we start off just by puffing some gas inside the chamber and then we pass a huge current through it which turns into a plasma and then we apply lots of different heating methods to heat up the plasma to actually 150 million degrees C which is 10 times hotter than the core of the sun.

Dave -   That's really, really hot.  Why do you actually need it to get hotter than the centre of the sun?

Sarah -   Its because of size.  Basically, the sun is so big, if you've got sort of hot plasma inside the sun, the heat can say, contained in the sun.  You think of it like, if you've got, say, a bathtub of hot water and you've got a small cup of hot water, the bathtub will retain its heat for longer than the small cup of hot water.  So, if you think of the sun as being like the bathtub, the heat will stay inside longer whereas here on Earth, the JET Tokamak is like the small cup of water, so it loses its heat quickly, so it has to be hotter to begin with.

Dave -   So, how actually do you heat up the plasma to this incredibly high temperature?

Sarah -   So, there's a few different methods we use.  The first method is called is ohmic heating and what that means basically is, we pass a current through the plasma, in the same way you can pass a current through a copper wire - and everybody knows, if you pass a current through a copper wire, it heats up.  So, the plasma does the same thing.  So, that's the first way we heat the plasma.  Well, that's actually not enough by itself to get the plasma up to 150 million degrees temperatures that we need.  So, we have a few extra methods.  One of them is called radio frequency heating and that's basically analogous to a microwave oven, the same way that your microwave oven excites water molecules to kind of heat up your food.  Radio frequency heating fires radio waves into the plasma to then heat up the plasma, excite the plasma particles.  And the final heating method is called neutral beam heating and that's analogous to a cappuccino maker when you have a jet of steam that you shoot into a cup of milk to warm up the milk.  We basically send a jet of neutral particles into the Tokamak to heat up the plasma because the energy is transferred from those energetic fast particles to the particles in the plasma.

Dave -   So, when JET is actually fired up, what actually happens?

Sarah -   Every time we run an experiment on JET, that's called a plasma pulse and that's basically because we have sort of 60 seconds where weve got a plasma inside the machine and the machine is operating properly and we kind of get all our data from the experiments that we do.  And then we finish off the pulse and we do another one 20 minutes later.  So, we do that throughout the day and then all the data from those experiments is what we use to kind of progress fusion research further.

Dave -   So, they're about to fire off JET and create a plasma several hundred million degrees centigrade in the centre of the Tokamak next door.  They're counting down.  So there's a series of radio screens in front of us.  So, you can just see light being created.  I'm guessing that's from the plasma itself that its getting very, very hot at the bottom and I'm not sure if there's a nice plasma going on in there?

Sarah -   That look like a nice plasma to me.

Dave -   So, this whole process is incredibly difficult and incredibly expensive.  Why are we bothering?

Sarah -   It would produce so much energy, but also in a clean and reliable way that it really would be the energy source of the future.



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