Alan Costley, a physicist working for the company Tokomak Solutions in Culham, Oxfordshire, has written a paper in the journal Nuclear Fusion which suggests that there may only be a weak link between the size of a magnetic-confinement fusion reactor and the point at which it produces more power than it takes to operate.
Until now, most fusion research has focused on the principle that ‘bigger is better’. The best-researched and most advanced form of nuclear fusion device is the tokamak, a toroidal (doughnut-shaped) vacuum vessel surrounded by powerful electromagnets that both confine a hydrogen plasma (a mixture of different isotopes of hydrogen, in a state where their charged nuclei are separated from their associated electrons) and force the plasma particles to circulate around the torus. The combination of the magnetic squeezing and the speed of the particles (to which other devices also contribute) forces the nuclei to collide at high energies, fusing together to form helium nuclei and releasing a burst of energy.
The problem is that powering the electromagnets and heating the plasma requires a large energy input; so much that, to get to the point where fusiuon occurs, the energy input far outweighs the energy output. Current projects are using superconducting materials to form the electromagnets, to reduce the power requirement for the magnetic field. But it has been thought that even with this technology, a large volume of plasma was still needed to produce enough fusion to increase the power output. This has led to projects such as ITER, the international attempt to build the world’s largest tokamak to date, twice as large as its predecessor. However, ITER is a vastly complex and expensive undertaking which is already behind its original schedule.
Costley’s analysis contradicts this thinking. Taking data from the operation of the largest currently-operating tokamak, JET at Culham, and experimental ‘spherical’ tokamaks (so called because the ‘hole’ in the doughnut is very much smaller than the overall diameter) at the same site, Costley looked at how power output depends on size, plasma current and magnetic field strength, and combined this with the ‘operational limits’ which dictate the plasma conditions needed for ther best results. This, he claims, shows that tokamak performance doesn’t depend on size.
“The implications for the design of pilot plants and reactors are significant and potentially positive,” he said. “Providing sufficient fusion power and in-vessel power handling capabilities can be achieved, relatively small devices can have a high fusion gain. Such devices would open the possibility of a much faster development path and also, perhaps, lead to fusion reactors based on multiple modules rather than one large power unit.” This would mean that the path to fusion is potentially cheaper and less costly than had previously been thought: Costley estimates that, with development accelerating on high-temperature superconductors and neutron-absorbing materials, commercial fusion could be ten years off, rather than the oft-quoted three decades.
Dr David Kingham, chief executive of Tokamak Energy, said: “This work by Dr Alan Costley is further validation for our approach of accelerating the development of fusion power; our earlier paper looking at small scale fusion reactors published in 2015 is the most downloaded paper in the history of the journal and has set in motion this fundamental shift in tokamak fusion science. We will make fast progress by keeping devices small, using the efficient spherical tokamak shape and by using the latest generation of high temperature superconductors to generate the strong magnetic fields necessary while reducing the energy input. This work adds further weight to our ideas for compact 100MW fusion power modules. They are shown to be feasible from a physics perspective and are now primarily a materials and engineering challenge.”