Fusion nails its colours to the MAST

A £30 million upgrade to the MAST experiment at Culham Centre for Fusion Energy will enable the UK to carry out three tasks central to the success of the international fusion programme – and offer exciting opportunities for research collaborations

Bringing nuclear fusion – the process that keeps the Sun burning – down to Earth offers a tantalising prize: abundant supplies of carbon-free energy for a world that needs it more than ever. The Sun generates huge amounts of energy by fusing hydrogen into heavier elements. In the same way, man-made fusion devices join hydrogen isotopes (deuterium and tritium) to produce helium and release the highly energetic neutrons that will one day be used to provide the heat for our powerplants.

Engineering a miniature version of the Sun is no easy task, as over half a century of research has proved. The route being followed by many fusion scientists around the world is ‘magnetic confinement’ – in which a magnetic ring-shaped bottle, known as a tokamak, is used to create and control plasmas at temperatures of over 100 million degrees Celsius, hot enough for nuclei to overcome their electrostatic forces and fuse. This approach has already been shown to be feasible; the Joint European Torus (JET) tokamak has briefly released 16 megawatts of fusion power using the deuterium-tritium fuel mix; enough to supply a town if JET were connected to the electricity grid.

The next stage is to build a larger machine to prove the technology on an industrial scale. The ITER device, now under construction at Cadarache, France, will make that step by producing 500 megawatts of fusion power – equivalent to the capacity of a small power station. A huge project involving the European Union, the United States, Russia, China, India, Japan and South Korea, it will be one of the largest international scientific collaborations in history and, if successful, will pave the way for the first fusion power station in around 30 years from now.

Ensuring ITER’s success when it comes online in 2019 is now the main priority of fusion laboratories around the world, including Culham Centre for Fusion Energy (CCFE), home to the UK’s research programme. Culham’s role in this international mission has just been boosted with the approval of a £30 million upgrade to its Mega Amp Spherical Tokamak (MAST) experiment by the Engineering and Physical Sciences Research Council, the main funding body for UK fusion. £20 million of the project cost is new funding, with the rest being met by savings in Culham’s existing programme.

‘Spherical tokamaks’ like MAST confine the plasma in a tighter magnetic configuration than the conventional JET or ITER-style design, resulting in a cored apple-shaped plasma rather than the usual ring doughnut shape. This offers the potential for more compact and economical fusion operation. Since being commissioned in 2000, MAST has made significant contributions to fusion physics, particularly in understanding the instabilities that form at the edges of the plasma and disrupt its performance (which can be compared with solar flares on the edge of the Sun).

The upgrade will allow MAST to add to this progress and perform three specific roles in global tokamak development. Firstly, it will provide important physics data to assist the operation of ITER. Today’s tokamaks – there are over 30 currently running – are carrying out plasma experiments which will be scaled up to help predict performance on the much larger ITER device. MAST Upgrade will simulate ITER scenarios in the spherical tokamak configuration, assessing how the alternative shape affects plasma parameters and providing a different spectrum of results to be fed into the ITER knowledge base.

Secondly, MAST Upgrade will test systems for fusion power reactors. In

particular, it will be the first device to incorporate a novel plasma exhaust system, the ‘Super-X’ divertor. The area at the bottom of tokamaks, a trench known as the divertor, is used to exhaust heat and the helium by-product from the plasma core, and is subject to very high power loads. In the Super-X design, particles leaving the plasma will be steered along magnetic fields in such a way that they travel a much longer distance before interacting with the divertor targets designed to withstand the impact. This radiatively cools the particles and spreads them over a larger area, reducing the power loads on the tiles in the machine – an important objective for next-generation fusion devices. If Super-X performs well in MAST Upgrade, this divertor type could be included in the DEMO prototype power station that will follow ITER.

While ITER confirms the physics of commercial fusion, another facility will be very valuable to help validate the engineering of full-size reactor parts, such as blanket modules, ahead of DEMO’s operation. Many in the fusion community advocate the construction of a Component Test Facility (CTF) for this purpose. The spherical tokamak’s compact design makes it a good candidate for a cost-effective CTF. MAST’s third key task when upgraded will therefore be to explore the viability of a spherical tokamak CTF by examining a range of areas such as plasma start-up, sustainment and exhaust.

The MAST Upgrade project will provide ample opportunities for involvement by physicists and engineers at other laboratories and at universities. Over 20 institutions are already involved in MAST, and remote participation facilities have been installed allowing university-based researchers to run plasma experiments and analyse data direct from the campus. This collaborative network is expected to widen as Culham seeks partnerships to design and build the enhancements. For example, MAST is particularly well known for its outstanding suite of diagnostic tools that measure the plasma. Collaborators will be able to develop diagnostics for the upgrade – the recent project to replace MAST’s Thomson scattering laser diagnostic, a joint initiative with the University of York, points the way forward. Once the new machine is online in 2015, Culham plans to operate it as a user facility, allowing researchers from the UK and overseas to advance their studies by exploiting MAST Upgrade’s improved capabilities, which will include much longer plasma pulse lengths and increased heating power.

The green light for MAST Upgrade is part of a new 20-year strategy for fusion in the UK, published by the Engineering and Physical Sciences Research Council and Science and Technologies Facilities Council in February 2010. The strategy follows an independent review of the fusion programme, commissioned by the two Research Councils to provide a strategic UK vision for fusion in the international context. Chaired by Professor Keith Burnett (Vice-Chancellor of the University of Sheffield), the review supported continued long-term funding for Culham’s research activities, recognising fusion’s potential to contribute as a major component of the future global energy system.

MAST Upgrade is expected to operate well into the 2020s and will give the UK a crucial advantage in fusion technology as this much-needed new energy source approaches commercial fruition.