So, the fusion tour was as great as I hoped! We planned our journey with lots of time to spare but after stopping off for dinner and getting a little lost we arrived just in time to pick up some leaflets and go into the introductory talk. This was pitched at the perfect level for all listeners and I think Dad was impressed. He wrote 'Tokomak!' on a post-it with quite a high level of excitement!
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| the MAST model |
I had forgotten that there were actually 2 experiments there called JET (the European one, the one we learnt about in our Alevel course) and MAST (the UK one) but MAST had been closed for refurbishment so that meant that we couldn't see inside.There was a model and an overview of the history of the site as substitute which was interesting but I was itching to go and see JET! The facilities are huge and apparently we walked about 4km and I can believe it! There was quite a variety of smells during the walk as well... That was a bit strange...
The fusion chamber is designed in a torus shape which is like the inside of a tyre. In here is a vacuum (so that there are no impurities) and a plasma of charged ions and electrons. Because the plasma can carry a current, magnets are used and currents are applied so it's all using Flemings left hand rule, which I describe a bit
here. Then the fuel, tritium (which is radioactive) and deuterium or deuterium and deuterium is injected but its such a tiny amount, like the weight of a postage stamp. But they are found in water so its a renewable fuel and radioactivity is relatively low. If there is a failure the walls of the torus just absorbs the fuel and there is no dangerous explosion like with fission.
The nuclei of the fuel fuse together and form a larger atom which is unstable so splits into a helium atom and a neutron. The weight of the products is actually less than the reactants and due to mass energy equivalence this mass is emitted as energy.You can collect this energy with lithium blankets which in turn creates tritium to go back into the reaction. In a fusion power plant this energy would then go onto heat water, turn it into steam which would turn a turbine converting kinetic energy into electrical. That's what they hope to achieve in the next experiment in Italy which they are building now using the things that they have learnt at JET.
One of the ways that they have improved fusion technology is by experimenting with the materials that the tiles within the torus are made of. They used to be Carbon but they used to absorb the fuel and become radioactive which is dangerous. Then the tiles were sprayed with beryllium because even though it is metallic it is relatively small and so won't add too many electrons to the plasma. It is ideal since it filled the gaps in the carbon so that it was no longer porous. But just recently all the tiles were replaced with purely beryllium tiles However beryllium is toxic and anyone who goes into the chamber must wear a full body suit! Due to this robotic arms are used for most repairs. To practise using the arms they play jenga! The arms actually measure the force so that the user feels the pressure of what they are doing.
We walked into a lab where they analyse the data and there were so many desks! There are 2 sessions a day which work on different projects that last about 6 months with about 25 runs a day. That's a lot of work. In JET the plasma can last 30 seconds before collapsing (which is long compared to MAST which lasts half a second). This is due to the limits of DC current. If the current changed direction so would the plasma which is not possible within the JET design. There are also limits because the copper has to cool. Super condcutors woudl be ideal but they are expensive at the moment.
We put on some bright yellow hard hats, walked though a giant room of wires and buttons and lights, then through giant concrete doors. This was our first peak of the JET and was actually the best view because of all the equipment surrounding!
The JET is made of 8 segments and when they built her they made an extra one so we could see that really clearly (well through a screen!) They built a pretend torus around it so it looked more realistic which you can see on the left hand side.
To protect the machine from splitting apart like a chocolate orange, it is built with movement so that it can lift up from the floor if necessary though it is better to monitor the reactions in such a way that it can be controlled. One day near the begining of experimentation it lifted up by 1.5cm and the vibrations could even be felt in the data analysis rooms.
I think I've covered just about everything... This was kind of my goodbye to my old physics class since we're all off to university now and it's quite emotional!
