Experimental and theoretical research has shown ‘spherical’ tokamaks to be a “fast route to fusion” compared with more “conventional” tokamak devices such as Joint European Torus (JET), according to David Kingham, chief executive of Tokamak Energy.
“By pursuing this route, fusion researchers around the world, including at Tokamak Energy, are developing new materials and technologies to help us get fusion power into the grid by 2030,” Kingham told a meeting held last week by the International Energy Agency (IEA) on developing fusion power. Tokamak Energy was invited as “one of the three most promising fusion concepts”, along with General Fusion and Tri-Alpha Energy.
The UK’s Tokamak Energy grew out of Culham Laboratory, home to JET – the world’s most powerful tokamak – and the world’s leading centre for magnetic fusion energy research. Tokamak Energy’s technology revolves around high temperature superconducting (HTS) magnets, which allow for relatively low-power and small-size devices, but high performance and potentially widespread commercial deployment.
The world’s first tokamak with exclusively HTS magnets – the ST25 HTS, Tokamak Energy’s second reactor – demonstrated 29 hours continuous plasma during the Royal Society Summer Science Exhibition in London in 2015 – a world record.
“The plasma is where the fusion reaction takes place, and its stability is crucial,” Kingham said.
The next reactor in construction – the ST40 – would produce plasma temperatures of 15 million degrees Celsius – hotter than the centre of the Sun – this year. The ST40 is currently being built at Tokamak Energy’s facility at Milton Park in Oxfordshire.
Center column of the Spherical Tokomak ST40
“The ST40 is designed to achieve 100 million degrees C and get within a factor of ten of energy break-even conditions. To get even closer to break-even point, the plasma density, temperature and confinement time then need to be fine-tuned,” Kingham said.
“The next step is to build a reactor that takes this knowledge and uses it to demonstrate first electricity from fusion by 2025. This will then form the basis of a power plant module that will deliver electricity into the grid by 2030,” he added.
This huge challenge requires, he said, “massive investment, many important collaborations, an excellent supply chain, many dedicated and creative engineers and scientists – and, no doubt, some good luck and good management” in order to succeed.
Tokamak Energy has raised private investment of £20 million ($25 million) from Oxford Instruments, L&G Capital, the Institution of Mechanical Engineers and others. It has a “valuable dialogue”, Kingham said, with Princeton Plasma Physics Laboratory on spherical tokamaks, and with the Plasma Science and Fusion Centre at MIT on HTS magnets. Both institutions are “leading laboratories that share our vision”, he said.
Elsewhere private ventures can be seen “tackling challenges previously assumed to be the realm of governments” – Virgin Galactic and Space X being two examples, he said.
In an interview with World Nuclear News (WNN) on 26 January, a day after his meeting with the Paris-based IEA, Kingham said the ST40 is due to be completed and start commissioning this Spring.
“This signals a defining moment for Tokamak Energy, as the ST40 will be the most powerful compact spherical tokamak in the world that will aim to produce plasma temperatures hotter than the centre of the sun well before the end of the year,” he said.
Tokamak Energy is “unique among nimble, privately funded fusion energy ventures”, he said, in the way that the majority of them are looking for alternative and quicker routes to fusion energy, in comparison to large publicly funded companies, which often make slow progress but do sometimes produce new scientific breakthroughs. Tokamak Energy is unique amongst privately funded fusion energy ventures, he added, as it is aiming to accelerate the development of fusion energy based on the tokamak.
Other “routes to fusion” are being taken by, for example, General Fusion and Tri-Alpha Energy, he noted. General Fusion is taking the approach of Magnetised Target Fusion, with the aid of modern electronics, materials, and advances in plasma physics. Tri-Alpha Energy is utilising proprietary advanced beam-driven field reversed configuration technology to create a superheated plasma environment. Tri Alpha Energy has operated a national lab-scale machine, which in many aspects resembles a future power plant, in which hydrogen and boron would fuse generating helium and energy.
The tokamak as a class of device has had “unprecedented global support backed up by scientific consensus”, Kingham said. More than 200 tokamaks have been built in laboratories worldwide, he noted, and there has been a €20 billion ($21 billion) international agreement to build Iter, a huge tokamak, in France.
Tokamak Energy’s schedule is, he said: build a small prototype tokamak to demonstrate the concept; build a tokamak with all magnetics of high temperature superconductor (achieved in 2015); reach fusion temperatures in a compact tokamak (aiming for 100 million degrees in 2018); achieve close to energy breakeven conditions by 2019; produce electricity for the first time by 2025; put fusion electricity into the grid by 2030.
The energy confinement time of tokamak plasmas scales positively with plasma size and so it is generally expected that the fusion triple product, nTτ E, will also increase with size, and this has been part of the motivation for building devices of increasing size including ITER. Here n, T, and τ E are the ion density, ion temperature and energy confinement time respectively. However, tokamak plasmas are subject to operational limits and two important limits are a density limit and a beta limit. We show that when these limits are taken into account, nTτ E becomes almost independent of size; rather it depends mainly on the fusion power, P fus. In consequence, the fusion power gain, Q fus, a parameter closely linked to nTτ E is also independent of size. Hence, P fus and Q fus, two parameters of critical importance in reactor design, are actually tightly coupled. Further, we find that nTτ E is inversely dependent on the normalised beta, β N; an unexpected result that tends to favour lower power reactors. Our findings imply that the minimum power to achieve fusion reactor conditions is driven mainly by physics considerations, especially energy confinement, while the minimum device size is driven by technology and engineering considerations. Through dedicated R&D and parallel developments in other fields, the technology and engineering aspects are evolving in a direction to make smaller devices feasible.