Although largely a coffee drinker, occasionally I will order tea in a café. When I do so, one of my pet hates is being served a cup of hot water with an individually wrapped tea bag sitting on the saucer beside it. Quite apart from the unnecessary environmental cost of individually wrapping tea bags, there is the problem with the resultant cup of tea. Hot water poured onto tea (preferably in a pot) allows the tea to infuse by a mixture of turbulence, convection and diffusion as the hot water swirls around carrying the tea with it. A tea bag placed into hot water on the other hand relies on infusion by convection and diffusion only and so takes a lot longer to brew. Oddly enough, there is at this moment, a major scientific project being built in the south-west of France that has the opposite problem. The aim of the project is to generate electricity by nuclear fusion in extremely hot clouds of gas that are confined into the shape of a doughnut. To achieve this, they must reduce the turbulence within their doughnuts. Unlike the tea, nuclear fusion seems to require diffusion and convection to prevail over turbulence.
Supplying the growing energy demands of the planet is a major problem for us all. How can we simultaneously generate the electricity that we want while limiting our carbon dioxide emissions to levels that will cause minimal damage to our planet? Renewable energy is part of the solution, some have argued that nuclear fission could be another part of the solution (all of our current “nuclear” power plants run by nuclear fission). The “ITER” project in the Provence-Alpes-Côtes d’Azur region of France aims to demonstrate the feasibility of nuclear fusion to supply our energy needs instead.
Unlike nuclear fission which works by exploiting the decay of radioactive elements, nuclear fusion ‘fuses’ elements together to produce energy. Gazing up at the sky you can see thousands of nuclear fusion generators: Each star (including our Sun) produces light and heat, by nuclear fusion. First the stars fuse hydrogen into helium (as our Sun does now), then, as the star ages, the heavier elements combine until finally iron is formed in the core of the dying star. All the elements found on our planet and elsewhere in space have, ultimately been formed in the core of a star (or in reactions as the star dies in a final explosion). Every atom in us has been formed by such reactions in stars and so it is very true to say “from dust you came and to dust you will return”, the dust in question being star dust. If we can exploit it on Earth, nuclear fusion offers a method of providing energy with no long term radioactive by-products and limited carbon dioxide emissions. It is a possible, but very long term, route out of our quandary about energy generation.
So why can’t we start using it immediately? A clue comes from the fact that the nuclear fusion reactors that we know of (stars) are very hot and relatively dense. It is not easy to smash two hydrogen atoms together such that they fuse, it requires them to have a lot of energy (ie. be very hot) and be quite close together. To build a nuclear fusion reactor requires us to heat a gas until it becomes a ‘plasma’ which means heating the gas to temperatures of around 150 million ºC. At this temperature we need to confine the plasma with very high magnetic fields so that it does not hit the walls of its container and it turns out that the best way to do this is to manipulate the plasma into a ring doughnut shape. This doughnut confinement, known as a ‘Tokamak’ has become the standard way of confining the plasma. At the moment, we cannot keep the plasmas hot enough for long enough (the current record is 6min30 sec confinement) for fusion to generate more energy than is required to form the plasma in the first place. One of the things limiting the lifetime of the plasma is the fact that the plasma cools down and one of the things that cools the plasma down is the turbulence in the plasma carrying the heat energy from the centre to the edge of the doughnut. Increasing the time it takes for the heat to escape from the centre of the doughnut to the outer edge is one of the challenges facing the ITER team. Just as with the pot of tea, were the cooling by diffusion and convection only, the plasma would take a lot longer to cool down. Understanding the turbulence inside the plasma is one of the challenges facing the team at ITER.
Our method of making tea can tell us a lot, not just about the problems for nuclear fusion generators, but also about diffusion and turbulence generally. It is worth pondering that brew a little more deeply next time you make your pot.