The Discerning Physicist January 17, 2016
Nuclear fusion, the power source of the stars, seems like the perfect solution to all of our energy problems. It has an abundant fuel, produces no greenhouse gases and has an efficiency four million times greater than that of burning fossil fuels. Unfortunately, problems arise when we are faced with the practicalities of creating the extreme conditions required to make fusion happen here on Earth.
A number of projects around the globe are trying to make fusion a viable energy source. One is a massive nuclear reactor currently under construction in France. The project is called ITER and is the result of the collaboration of 35 countries including the US, Russia and India. It will use magnetic confinement in a tokamak to try to overcome the difficulties posed in forcing atoms to undergo nuclear fusion. A tokamak is a doughnut-shaped vessel into which hydrogen fuel is injected. The fuel is heated to huge temperatures (up to 150 million degrees) so that the atoms break down into a plasma of positive ions and electrons and fusion can occur between the ions. This plasma can be controlled by magnetic fields as the charged particles are confined to moving along the field lines. This means that the plasma can be held away from the walls, which would be vaporised if it came into contact with them. The ITER project is building on research carried out at Culham in England where the nuclear fusion reactors JET and MAST are housed. JET is a European project that has been running for over 30 years while MAST is a newer UK-funded reactor. MAST is currently being upgraded to allow further research into the effects of running reactors for longer periods of time, as will be required in a working nuclear fusion power plant.
At the National Ignition Facility in the US a different method for inducing nuclear fusion is being trialled. 192 lasers are fired at a tiny pellet of hydrogen, causing it to be compressed and heated to the point where its atoms undergo nuclear fusion. This is known as inertial confinement fusion and is showing some promise. In 2013 researchers managed to achieve “ignition”, the amount of energy released through nuclear fusion was greater than the energy absorbed by the fuel in order to start the process. Unfortunately the energy released was still less than the total energy supplied as not all of it is actually absorbed by the fuel, but this was still an important milestone.
Recently it has been suggested that a hybrid of the above two methods, called magnetic target fusion, should be invested in. This would involve using magnetic confinement as well as mechanical compression to cause fusion in the fuel. This approach would require smaller and significantly less expensive power plants than magnetic confinement and much less input energy than inertial confinement using lasers meaning that it would be easier to achieve a net gain from the fusion process.
There is a running joke in the industry that nuclear fusion as an energy source will always be 30 years away. This makes decisions about funding research difficult, but the potential of fusion power is so great that ultimately it seems worth the investment.
If you are interested in finding out more about the projects mentioned they all have great websites with more information about the general science of nuclear fusion as well as the specifics of their own efforts to achieve it:
JET and MAST: https://www.ccfe.ac.uk/index.aspx