Edited from an interview with Professor Steve Cowley in The IET Engineering and Technology Magazine June 2011
JET has been in operation there since 1984, until it was shutdown last year for a refurbishment in preparation for its role as an ITER proving ground. The upgrade, which is now almost complete, includes installing a new beryllium tungsten wall and increasing its power output.
We have to show that a beryllium tungsten wall can work on JET because it's never been used before. The other thing that we are doing with the upgrade is putting on more power, so when the JET comes back later this year it will be a different machine. So this next period of time with JET will be to start up with the new wall, ramp up the power so that we can get record breaking power into it, then put in a tritium fuel and break all the world's fusion records in 2015.
ITER is not very far off being the right size for a commercial reactor. We reason that a real commercial reactor will be less than 20 per cent bigger than ITER.
JET at full power can only do five seconds. The shots of JET are what we have used to extrapolate the performance of ITER - it was essentially designed around those 97 shots where we made 16MW.
It was not actually the 16MW shot that we designed ITER on, it was the 5MW for the full five seconds - you turn the machine on and it makes fusion for the full five seconds, then you turn the machine off and it stops.
We have no problem sustaining it, you just can't run JET for more than five seconds because the copper magnet gets really hot, all the systems get hot, and they won't last longer than five seconds at full power.
But ITER can because it's a superconducting machine that has its engineering systems designed to maintain a burst longer than that. It's not that the fusion can't go on longer than five seconds, it's just that the machine can't.
What we call the baseline of ITER is 500MW of fusion power for 400 seconds. But ITER could go on for thousands and thousands of seconds and some of the modes of operation that we have been working on at JET probably mean that, if we adopt them on ITER, we will do thousands of seconds.
Personally I think we should be more ambitious. I think we will probably get more out of ITER than that, but nobody wants to promise that. What we are promising I think is a fairly conservative estimate of how well it can perform. I am hoping that we will get more than 500MW and that the gain will go up to infinity. That
We would like to bring down the turbulence inside the plasma even further. If you ask the physics reason that JET's performance has improved over time, it is because we are suppressing more and more of the turbulence inside.
Inside the plasma it is bubbling; it is 170 million degrees in the middle and 10,000 degrees at the edge. That huge temperature gradient is like applying heat to a saucepan, it bubbles away. And those bubbles are little turbulent eddies inside the plasma that causes the heat to lean out.
The less turbulence you have, the better the fusion reactor. The big success of the last two-and-a-half decades of JET's operation is a reduction in the turbulence. But we can go further. On MAST, for instance, we have regimes where there is almost no turbulence inside the plasma. That is the perfect confinement device.
The second big challenge is that every now and then the plasma will erupt and throw itself against the wall. You are storing several hundred mega joules of energy inside the ITER plasma and you don't want it to throw itself against the walls. So we have to have ways of making sure that doesn't happen in a commercial reactor.
We are working on ways for ITER, but by the end of the ITER project we had better have a very good idea when it's going to go unstable. Because when it does, and it is rare, it can damage the walls.
Some scientist says 'how can you have a sun in a bottle?' - I would say we have done that! That is what JET does. The temperature in the middle of JET is a reactor working temperature; it is a sun in a bottle.
That is not the difficulty. The difficulty is a sun in a bottle that is cheap enough that it will make electricity at the sort of cost you want to pay for your electricity. I don't know if we can solve that problem.
I know we can make a sun in a bottle, but I don't know if we can produce electricity at five cents a KWh. That is the challenge and these days, it is as much an engineering challenge as a physics challenge.
Waste from a fusion power station is an interesting problem. We produce helium, it is just ordinary helium. But the neutron that comes out when it strikes steel not only moves the atoms around, it can make some of the nuclei in the steel radioactive, so the steel becomes radioactive.
That is the only waste that we really worry about, that radioactive steel - we have designed a special sort of steel called a low activation steel, so the radioactivity is short lived.
So the analysis of our proposed power plant shows that at the end of the lifetime of a fusion reactor, the waste that you have is a thousand times less active than a fission plant and that it decays in 200 years to have the same radioactivity as coal.
Tritium itself is quite a hazardous substance. It's a radioactive gas with a half life of 12 years. You don't store much tritium at a fusion plant because you make tritium from lithium and you take that tritium and put it in the plant.
The neutron comes out of the plasma and goes into the wall. In the wall is this blanket of lithium, where the energy of the neutron is deposited and goes into your coolant to power your turbine. The neutron hits lithium and makes a tritium, which you have to take and put back in your plasma to make more fusion, so actually you make tritium and consume tritium.
If you've got a clever plant you actually keep very little tritium around, maybe a kilogramme, so if you ever had an accident the amount of tritium that could be released would be relatively small.
In the European Fusion Power plant design that we participated in, you wouldn't need anybody to be evacuated beyond the site boundary in the event of a worst case scenario.
We won't know if we are right about ITER until after 2025. But by 2030 we will have to start building the first energy production reactor, something we call Demo.
Demo won't have to be perfect, it's a prototype, and it's not the last word. But it will have to be good enough to produce day in and day out electricity, so it's a demonstration that fusion works.