Our 2010 School Lecture presenter Dr Melanie Windridge on her lecture tour and fusion reaction.
Summer seems to fly by! It has been raining most of August and with all the wind this week it’s been feeling quite autumnal, which can only mean one thing – the summer holidays are coming to an end and I’ll soon be off on tour again. I did say I would write a bit about JET over the holidays, so here we go….
JET (the Joint European Torus) is the biggest tokamak in the world and the only operational experiment capable of producing fusion energy. As the name suggests, it is a European joint venture used by over 40 European labs and more than 350 scientists and engineers. JET was built at the end of the 1970s on the site of a former airfield at Culham in Oxfordshire, England. Its construction took five years and it was happily completed on time and on budget. Its very first plasma was created in 1983 and it is still running today over 25 years later.
In November 1991 JET achieved the world’s first controlled release of fusion power, operating with the deuterium-tritium fuel mix of commercial reactors. The world record for fusion power was set in 1997 when JET produced 16MW. At this time the energy input was 24MW, so the power produced was only 65% of the energy put in to create the fusion reactions, but it was still a massive achievement. JET has made real fusion reactions and it hopes to get closer to break-even in the coming years after a big upgrade that will be completed by 2011.
JET is a purely scientific experiment and has evolved over the years as the fusion field has progressed. It has been modified and upgraded frequently to answer new questions and test technologies for the next step fusion device – ITER. (ITER is currently under construction and will be a reactor-scale experiment that hopes to prove the technological feasibility of fusion power. I’ll be telling you more about ITER in my next blog.)
Pictures 2 and 3 show the impressive size of JET. The machine is 12m high and 15m in diameter and the whole ensemble weighs thousands of tonnes. In the centre, the doughnut-shaped vacuum vessel is about 8m across from the outside edges and holds a volume of 80 cubic metres of gas. There are powerful auxiliary heating systems (the Neutral Beam Injection and resonance heating described in the last blog), a pellet injector for refuelling (a bit like a huge pea-shooter that fires tiny frozen pellets of deuterium into the plasma) and a gas injector for disruption studies (if the plasma is unstable and about to hit the wall, pumping lots of impurity gas into the vessel will smother the plasma and it will lose all its heat and current very quickly).
JET also has about 100 diagnostic instruments for looking at the plasma and taking measurements. Scientists measure properties of the plasma like the temperature, density, its position and how fast it is rotating, and machine parameters like the strength of the magnetic fields and currents flowing. There are video cameras taking pictures in various parts of the spectrum, including X-ray, visible and infrared.
Picture 4 is a photograph of the inside of the JET vacuum vessel. The left-hand half shows it empty and the right-hand half shows plasma in JET taken with a visible-light camera. Now you’ll notice that it looks like there’s not much there in the very centre of the machine where we expect the fusion reactions to be taking place. But there is. Most of the plasma is in the middle of the vessel, but it’s invisible.
That’s because it is so hot that the only light that the plasma emits (if it’s emitting any at all, because light is only given off when electrons recombine with the nucleus of the atom and this is unlikely to happen much in the hot centre) is X-rays or ultraviolet light. Only cooler things emit visible light, so we can only see the cooler parts of the plasma, which is at the edge, near the wall. The centre is at about 100 million degrees and the edge is at a few thousand degrees.
Here’s a great video of what a whole experimental plasma shot looks like in JET. In a couple of weeks I’ll be travelling to Scotland and even doing an extra talk on the Isle of Mull, and in the next blog we’ll be talking about the new fusion experiment, ITER, that is currently being built in France.