Fusion Power at JET

The Joint European Torus, or JET, is the biggest fusion experiment in the world right now - but how does it keep fusion reactions going? Meera went along to explore the...
19 October 2008

Interview with 

Andrew Kirk and Jef Ongena - JET


Split image showing interior view of the JET vacuum vessel with a superimposed image of an actual JET plasma, taken with an infra-red camera, 2005


Meera - This week I'm at the Culham Science Centre in Oxfordshire which is home to the Joint European Torus Project, also known as JET. The world's largest nuclear fusion research facility. Nuclear fusion is the process that occurs in our sun to keep it burning. If at all possible on Earth it could provide us with vast amounts of energy. Current work on fusion involves heating the hydrogen isotopes deuterium and tritium to high enough temperatures that they fuse together to form helium, releasing more energy as a result of this fusion. It's proving to be a real challenge because whilst there are techniques to heat and energise the atoms such as current and beams of high energy atoms the real trick to actually maintain these temperatures long enough for fusion to occur continuously. With me now is Andrew Kirk, a senior scientists here at the Culham Science Centre. So Andrew, fusion happens so naturally in our sun. Why is it so hard to recreate here on Earth?

JET vacuum vesselAndrew - To make fusion happen we have to get two atomic nuclei close enough together to make them fuse. That can only happen if the two nuclei collide at very high speeds, high temperatures. So tens to a hundred million degrees centigrade. Once you've got particles at these temperatures you've got to find a way of actually keeping them together and not making them melt any material surfaces. What we do is we use magnetic fields to actually constrain the charged particles in a plasma and keep them away from these surfaces while we try to heat them up to these extreme temperatures.

Meera - How do you actually go about doing this an creating fusion?

Andrew - We use a machine called the tokamac which is a Russian acronym which basically means a magnetic bottle. This allows us to actually constrain charged particles.

Meera - How does it go about doing that?

Andrew - A tokamac is a sealed vacuum vessel. The inside of a tokamac actually resembles a ring donut into which we inject a small amount of gas. Instead of using hydrogen we actually use the heavier forms of hydrogen called deuterium and tritium. We then take this gas and turn it into a plasma. A plasma is the fourth state of matter. You know you've got solids, liquids and gases. The next stage is a plasma in which you've stripped the electrons off from the atoms. You've got the positively-charged nuclei and the electrons together in effectively an electromagnetic gas.

Meera - What happens once you've created this plasma then?

Andrew - What we then do is we use the magnetic field to shape the plasma and to keep it away from touching the sides of the vessel. Then we actually start to heat it.

Meera - Why do you need to keep it away from the sides of the vessel?

Andrew - Because anywhere this plasma comes into contact with the vessel a) it would erode the material or damage the material of the vessel but more importantly it would actually cool down the plasma and it would stop the fusion happening. Or you'd have to put in a lot more energy to keep the plasma hot.

Meera - How does the tokamac actually do that?

Andrew - We generate the magnetic field in such a fashion that the charged particles would follow a magnetic field: spiral around and aournd the tokamac in a shap ethat resembles that of a slinky spring. They follow around in this helical pattern all around the tokamac. The slinky spring stops the charged particles escaping from the edge of the plasma and therefore keeps them away from the walls. We put a gap of about ten centimetres away from the plasma and the wall.

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Meera - So I've now come to the control room of the project. The tokamac isn't actually very far. It's about sixty metres away from us. With me now is Jeff Ongenar who's the task force leader on the JET project. How big is the actual tokamac here? We can only see a slight part of it but how big is the overall thing?

Jef - The overall thing is about 30 metres high and 30 metres in diameter. It has to be a certain size to produce a certain amount of energy. That is what physics teaches us. Small machines can only do a little bit, larger machines can do much better. Then a reactor will even be larger than JET.

Meera - So we're here now in the control room. What's monitored on the project here?

Jef - The control room is, in fact, the place where we control and plan the experiments. He set up all the physical parameters needed to run the machine for a particular experiment to start a particular idea. Every twenty minutes we can have a new experiment. A new experiment means that we change another parameter, see the effect. The final aim of all these experiments is to get to the best possible magnetic confining system that means we want to optimise the heat we need to get the reactions going. We want to optimise the time the heat stays in the machine because that will then allow to run as efficiently as possible.

Meera - So we heard a few minutes ago that a pulse had just taken place. We've got this screen here in front of us that has all the facts and the stats of the previous pulses that just happened in the past couple of hours. It's all to optimise the confinement of the plasma to keep it at as high a temperature as possible. What has the energy output in relation to input been so far with the project?

Jef - It's designed to show that fusion is scientifically possible. When JET was planned we had only small machines which fit more-or-less on the table. Europe decided to take a bold step and to build a much larger machine to show that the amount of heat produced by fusion reactors could be euqal to the amount of heat you need to get reactions going. We have proven that we get to about 70% of the heat back compared to the heat we put in. I think with current developments of the last years if we try again we will get much closer to one now. In fact the scientific possibility of fusion essentially is shown.

Meera - Are you now hoping to create more output - I.e. create more energy than you are putting in and therefore obviously having an energy source?

Jef - That will not be possible because JET is not built like this. To get more heat out you need a larger machine. This larger machine is designed and is now starting to be constructed in Cadarache in France. This machine is called ITER which stands for International Nuclear Experimental Reactor. ITER is there to show that science and technology now go together and can be used to realise a fusion reactor.


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