The world of fusion

Materials World magazine
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1 Sep 2018

In the wake of recent advances in fusion energy, Ellis Davies investigates the ever-on-the-horizon technology.

Fusion power is a promising option for generating large amounts of carbon-free energy. Although still in development, several countries are working on projects to make fusion power stations a reality.

A new record in fusion power was set in June 2018, at the Wendelstein 7-X reactor in Germany. The device was able to produce pulse lengths of 26 seconds – a world record for stellarator technology. In the UK, the ST40 tokamak device at Tokamak Energy reached a plasma temperature of 15 million degrees Celsius in the same month – hotter than the centre of the sun. This achievement is just one on the way to the company’s goal of 100 million degrees Celsius – the temperature at which a tokamak device becomes viable for commercial energy production.

However, due to its developmental nature, fusion is less frequently discussed, and its potential is both unrealised and unrecognised.

Fusion – the basics

The process of fusion heats the sun and other stars. This involves the collision of atomic nuclei – deuterium and tritium, types of hydrogen – that causes the release of energy in the form of neutrons – helium. Obtaining energy from this process requires the formation of plasma, which is done by heating the nuclei to temperatures of over 100 million degrees Celsius. The resulting plasma is fragile with a density less than one million times that of air.

The fragility of the material means that it needs to be kept in a magnetic confinement system – a vessel that controls the plasma using magnetic fields in order to keep it from being contaminated and cooled by contact with material surfaces. In the case of the Wendelstein 7-X, the magnetic cage is a ring of 50 superconducting magnet coils around 3.5 metres-high.

The history

Scientists first discovered fusion in the 1930s, and in the ‘40s set about looking for ways to initiate and control reactions to produce energy. The technology was kept a secret at first as researchers thought it could have military applications. One such application came in the form of Ivy Mike, an experimental bomb dropped on the Pacific island of Elugelab in 1952, partly consisting of plutonium and a hydrogen tank. The explosion of the plutonium bomb heated the hydrogen to 150 million degrees Celsius, amounting to an explosion comparative with 10 megatons of TNT – 700 times as powerful as the Hiroshima bomb. The design was deemed non-credible and has not been pursued since 1992.

The stellarator device was invented at Princeton University, USA, in 1951. Its basic concept was to create magnetic fields to allow particles to circulate around the long axis of the device following twisted paths. This would cancel out the instabilities seen in toroidal devices. The hope was that this would keep plasma confined long enough to heat it to the point of fusion.

Once fusion became public knowledge, a partnership developed between the USA and the former USSR in the early 1960s. With international collaboration made possible, research into a major approach to fusion energy, inertial confinement fusion (ICF). Proposed in 1961, a year after the invention of the laser, researchers intended to use large pulses of energy to reach the high temperatures needed to create energy from fusion. This approach was known as inertial confinement.

It was soon discovered that the key problem was the tendency of plasma to develop instabilities and escape the magnetic confinement. In 1964, the USA and former USSR developed a plasma focus device to combat this issue, which used the instabilities to compress the energy rather than suppress it.

The tokamak was invented by the USSR in the 1960s, and the USA began to focus all its energies on this device in the mid-1970s. For the following 20 years, several billions of dollars were spent on the development of the tokamak, with the devices getting larger. Plasmas, however, remained unstable.

A tokamak device works with a toroidal pattern, rather than the twisting paths of a stellarator. David Kingham, co-founder of Tokamak Energy, explained its function to Materials World. ‘To create the conditions for fusion, you need to hold a hot plasma at high temperature and pressure for a long time – you don’t want the energy to leak away too quickly, and you certainly don’t want the plasma touching the sides of the vessel. One of the best ways to confine it is using magnetic fields – the stronger the field/bottle, the more compact your device can be and the better chance you have of reaching energy break-even conditions and beyond,’ he says.

Kingham believes the tokamak is the most promising of the fusion devices being used today because of the efficiency of its magnetic bottle configuration. ‘It is partly the geometrical shape – the toroidal shape has no end, so the ions can spiral round in the plasma following magnetic field lines and never escape. The combination of toroidal and vertical magnetic fields can make very stable plasma with high temperature and sharp temperature gradients in the plasma,’ he says.

Tokamak Energy has achieved 15 million degrees Celsius using a tokamak device – specifically, a fairly small one standing at 2.5m-tall with a major radius of plasma of 40cm.

Hotter than the sun

The sun is the natural example of fusion energy. It is able to produce energy at a temperature below 15 million degree Celsius due to its size and age – the sun can sit for many years and not burn its fuel too quickly, and its size means it doesn’t need as many reactions per cubic metre to keep the plasma hot. On a smaller scale, however, higher temperatures are needed.

Kingham explains some of the materials challenges involved in raising plasma to such high temperatures. ‘There are various challenges around the vacuum conditioning and getting a clean enough plasma for it to stay hot for a few milliseconds – if you get a lot of contamination then radiation cools the plasma much too quickly.’ Kingham also stressed the importance of getting the correct magnets for the device, as the magnetic field required is powered by very high current density – the materials used need to be able to handle such currents. ‘This is mainly a matter of careful mechanical engineering, and choice of a particular cold rolled hardened copper that has the right conductivity and strength for purpose. Eventually on this device, in another year’s time, we’ll be getting up to six-million amps in the centre column of the device – we’ve only achieved two-million so far, but you need extremely high currents to get to the magnetic fields that we want to get to.’

Looking to the next level, the company is implementing high-temperature superconductor magnets, which are a yttrium barium copper oxide ceramic. These materials have high conductivity properties even in a high field and at temperatures of 20 kelvin, rather than very close to absolute zero like most superconductors.

New walls for new records

In Germany, the Wendelstein 7-X made advancements in the time the plasma lasted. This was down to the implementation of new wall elements in the vessel. These walls were covered in graphite tiles, which, researchers say, allow for higher temperatures and longer plasma discharges. The tiles are also able to control the purity and density of the plasma through the twisted contour, which protects the walls from particle escape and avoids contamination.

In the recent experiment, the device reached an ion temperature of around 40 million degrees Celsius and a density of 0.8x1020 particles per cubic metre, attaining a fusion product affording a good 6x1026 degrees/second per cubic metre. Professor Dr. Thomas Sunn Pedersen, Head of Stellarator Edge and Divertor Physics at Max-Planck-Institut für Plasmaphysik, Germany, said, ‘This is an excellent value for a device of this size, achieved, moreover, under realistic conditions, i.e. at a high temperature of the plasma ions.’

The device is to swiftly move forward. The graphite tiles will be replaced by carbon-reinforced carbon components that are water-cooled. They will produce plasma discharges of up to 30 minutes possible, researchers say.

Always 20 years away

It is commonly said that fusion energy is forever 20 years away, but Kingham believes it is now on the horizon. ‘It’s still a long-term challenge, but we’re targeting getting to industrial scale by 2025 and having electricity from fusion in the grid by 2030,’ he says. What has held fusion back is that, according to Kingham, most of the government funding has gone into increasingly large devices that have demonstrated impressive performance, but have been relatively slow to develop. ‘We are suggesting that a faster way forward is to get devices smaller with high performance, and essentially learn quickly on a series of prototypes rather than try and produce one big perfect device,’ he says.

Fusion energy still faces many challenges. However, these are being tackled by devices such as the ST40 and Wendelstein 7-X, using advanced materials including high temperature superconductors, graphite, and carbon-reinforced carbon components to solve the issues that have plagued fusion energy for over half a century.