The ring master

Prof Chris Llewellyn Smith, director of nuclear fusion research at the UK Atomic Energy Authority, Culham, heads ground-breaking work into new tokamak reactors. Stuart Nathan reports

In a primary school classroom in the early 1950s, fate threw together two eight-year-olds who would become hugely significant to science and technology in the UK. Sitting next to each other in maths classes were the young Adam Hart-Davies, eventually to become one of the country’s best-known popularisers of science, and Prof Chris Llewellyn Smith, less famous but certainly more significant to the development of science.

Enthused by those early maths lessons, Llewellyn Smith became fascinated by particle physics, studying the subject at Oxford. Since then, a glittering career has taken him to some of the most highly-regarded centres for high-energy physics and he is now director of the UK’s nuclear fusion research programme at UKAEA’s Culham site in Oxfordshire.

After completing his DPhil at Oxford in 1967, Llewellyn Smith’s studies took him to the Lebedev Institute in Moscow, then to CERN in Geneva and the Stanford Linear Accelerator in the US. Returning to the UK, he spent 20 years back in the physics department at Oxford, then revisited CERN, where he served as director-general during an eventful five years. During his tenure, the Large Hadron Collider project — the world’s largest physics project — was approved.

His work at CERN earned Llewellyn Smith a knighthood for ‘services to particle physics’ but his efforts in ‘big science projects’ have not abated.

Culham is best known as the home of the Joint European Torus (JET), the world’s largest nuclear fusion reactor and the only one capable of using tritium to fuel fusion reactions. JET is the forerunner of the first reactor to generate net energy from fusion, ITER, soon to be built in France (The Engineer, 12 February).

Although UKAEA Culham operates JET, the project is funded by the EU’s nuclear research directorate, EURATOM. The site is also home to the UK’s independent fusion research, as ground-breaking and potentially valuable as JET.

The pride of Culham’s research is a tokamak fusion reactor, which is very different from JET. Unlike the usual doughnut shape of a tokamak, the reactor is in a cylindrical vessel, 4m high and 4m in diameter, which contains the coils and magnets to confine and control the plasma. The plasma is still in a doughnut shape with transformer coils running down the centre and electromagnets surrounding it, but this configuration squeezes the plasma to form a shape like a cored apple. Because of this, it is known as a spherical tokamak.

The device operating at Culham, MAST (Mega-Ampere Spherical Tokamak), is the second of its type and began operating in 1999. Its predecessor, built in 1991, was START (Small Tight Aspect Ratio Tokamak), one of the first spherical tokamaks to be built. Hardly a model of robust engineering, START was assembled from spare parts at the Culham site. ‘But it was sort of a sensation,’ said Llewellyn Smith.

Research in the 1980s indicated the spherical tokamak may have advantages over the conventional toroidal shape. The cored-apple shape means the plasma is closer to the magnets and coils, so it was predicted it would use the magnetic field more efficiently. ‘One of the key parameters in fusion is a ratio known as beta, which is the plasma pressure divided by the magnetic pressure,’ said Llewellyn Smith. ‘You want the plasma pressure as high as possible, because if you double it, you double the density of the plasma, which means you have four times as many collisions of the nuclei in the plasma and therefore four times as much fusion. But the magnetic pressure costs money [for power to generate the fields], so you want that as small as possible.’

There were attempts, mostly in the US, to make spherical tokamaks, but ‘nobody built anything bigger than a teapot; they couldn’t get hot, they couldn’t prove anything’. A proposed larger device in the US was never funded, so the Culham team stepped in and built its spare-parts reactor. The results surprised everyone.

‘Prior to START, the world record for beta was about 13 per cent; it’s still not much higher for conventional tokamaks. But START raised it to 40 per cent. That’s like someone going to the Olympics and running the 100m in three seconds.’

MAST, twice the size and about eight times the volume of START, is further investigating the properties of spherical tokamaks. The main advantage, says Llewellyn Smith, is that the greater beta ratio means the tokamak could run at a steady state without superconducting magnets to maintain the magnetic field, which is impossible with conventional tokamaks. There is a disadvantage — the heat loads generated by a spherical plasma with fusion would be greater than than in a toroidal tokamak.

The immediate goal for the spherical tokamak research is not power generation. ‘We want to devise a component testing facility,’ said Llewellyn Smith. If commercial fusion reactors are to be developed, there will be a need to test relatively large pieces of equipment, from different types of welds and joints to the components that will remove heat from the reactor and generate the tritium fuel for fusion, in the exact environment of the fusion power station. But a toroidal reactor producing these conditions would be at least as big as JET, hugely expensive, and would burn up the entire world supply of tritium in about three days.

This is where the spherical tokamak could have huge potential. Because it is compact, a spherical tokamak running power station conditions need not, in theory, be much larger than MAST. ‘It wouldn’t be a power station; in fact, it would consume power. But it would produce the power station conditions on a much, much smaller scale.’

However, this is still some distance off, hence the continuing UK work on MAST. Llewellyn Smith is preparing a grant proposal for an upgrade to the reactor that will provide all the information needed to build a spherical tokamak capable of sustaining steady-state fusion. ‘Nobody has run a spherical tokamak into a steady state regime yet; you need to run it at higher power, for longer. MAST runs for about half a second, and that’s the time needed just to get to a steady state.’

The Culham team is anxious to make sure that the upgrade will be a single step from a component test-type reactor. ‘We don’t want to be in a position where we upgrade MAST, get positive results, then have to upgrade again to answer more questions,’ said Llewellyn Smith. ‘So we’re trying to work out what all the questions are, and make sure we’re equipped to answer them.’

The upgrade should cost £35m, he estimated, with a quarter coming from the EU. There would be a year or two of final design and reviews, then the upgrade would take four years. MAST could not run with tritium and so would not be capable of fusion, he said, but research in JET indicates it would be possible to extrapolate the results and predict its behaviour accurately.

Llewellyn Smith is keen to develop other lines of research for the UK, including developing heat injection systems and monitoring equipment for ITER. ‘These haven’t been designed yet, because they have to be bolted on to the reactor, so you want to design them as late as possible so they’ll be state of the art. We think we have a special advantage here, because we’re the JET operator and we have all the experience of running these systems on the closest thing to ITER in the world.’

This would also help UK fusion scientists, he said: ‘On the whole, the people who design the instruments then get to do the science with them, because they know how to use them best.’

Another goal is a programme to build prototype systems, research axed by 1980s budget cuts. ‘We want to rebuild that technology capability, but we want to do it by working with universities, through the Keeping Nuclear Options Open programme. That’s aimed at the fission sector, but a lot of the systems, such as heat handling and cooling, are in common with what would be needed for fusion.’