The Engineer 9 November 2015
Controlled fusion has the potential to be a long-term energy source. David Kingham explains the next steps
The world needs abundant, clean energy. Fusion – with no CO2 emissions, no risk of meltdown and no long-lived radioactive waste – is the obvious solution and has been for decades, but it is so hard to achieve. Controlled fusion is the ideal long-term energy source, complimentary to renewables. But why has it proven so difficult?
The challenge is that fusion only happens in stars, where the huge gravitational force creates pressures and temperatures so intense that usually repulsive particles will collide and fuse. On Earth we need to create similar conditions and hold a hot electrically charged plasma at high enough pressure for long enough for fusion reactions to occur. This is understandably tricky and the problem has occupied some of the world’s brightest minds for over half a century. Different approaches to fusion energy are being pursued, from cold fusion, which lacks evidence and may never work, to inertial fusion, which could work, to magnetic fusion, which does work.
Two recent papers from MIT and from a group in Durham and Culham in the UK add weight to magnetic fusion methods by throwing the spotlight on the use of high-temperature superconductors (HTS). Both focus on the same novel material – REBCO second- generation HTS tape – and, taken together, add confidence that the engineering of high field HTS magnets is feasible, resulting in more compact and commercially viable tokamak fusion power devices.
These papers reinforce the approach taken by Tokamak Energy, which has recently demonstrated a small tokamak with all its magnets made from REBCO HTS and been recognised as a Technology Pioneer of the World Economic Forum as a result of its progress and bold plans.
Magnetic fusion uses strong magnetic fields to pressurise and trap the hot plasma fuel. There are many configurations of magnets to achieve this, but the best performance has been achieved in ring-shaped tokamak devices, the simplest shape that has no open-ended magnetic field lines. The JET tokamak at Culham Laboratory achieved 16MW of fusion power in 1997 with 24MW of input power.
However, progress since then has slowed because the successor device, ITER, reached such gargantuan proportions that it has succumbed to numerous delays. In recent years efforts have focused on a smaller way to fusion. Can this most studied and top-performing device be reduced in scale?
Within the class of tokamaks there are two choices – the conventional doughnut shape such as JET or the apple-shaped spherical tokamak, described recently by Dan Clery in Science magazine as “the new kid on the block”. The spherical tokamak has two big advantages: being a squashed-up version of the tokamak it is inherently compact. Additionally, it uses the magnetic field more efficiently.
Its limitation has always been the tricky engineering due to lack of space in the centre for magnets and associated temperature controlling and protective elements. But the rapid development of HTS materials is overcoming this. Exceptionally high-field magnets can now be made that allow simpler solutions to the problems of cooling and protection, thanks to their ability to conduct high currents with zero resistance at temperatures well above absolute zero and in a strong magnetic field.
Earlier this year, Tokamak Energy scientists published two ground-breaking papers in Nuclear Fusion. One showed for the first time that it is feasible to build a low power tokamak with a high power gain. The second tackled one of the toughest engineering challenges of a compact spherical tokamak – the shielding of the centre – with HTS materials.
So instead of building ever larger tokamak devices, with huge costs and long timescales, we are now moving forward by increasing the magnetic field in more compact devices. This turns the pursuit of fusion energy from a big moonshot to a series of engineering challenges.
Currently, HTS technology has allowed Tokamak Energy to build and demonstrate a tokamak with all its magnets made from HTS, achieving that first challenge and moving us on to the next: designing a compact tokamak to get to fusion temperatures. When we succeed with one challenge, we can raise the investment to tackle the next. Tokamak Energy is deliberately trying to tackle difficult engineering challenges as rapidly as possible, something HTS materials is helping us do.
Fusion energy projects and start-ups around the world are pursuing fusion in different ways. This concerted effort towards fusion is the best way to reduce greenhouse gas emissions and ensure the supply of safe, clean energy long into the future.
Dr David Kingham is chief executive of Tokamak Energy