The stellarator W7X – the modern endeavour to compare stellarators to the long-time favoured tokamaks. Picture: IPP/ Tino Schulz

The stellarator W7X – the modern endeavour to compare stellarators to the long-time favoured tokamaks. Picture: IPP/ Tino Schulz

 

We all know that ITER will be a tokamak. Read about the hard times scientists had to overcome in order to make the tokamak the winning concept in the 60s.

Secrecy until Geneva

In the 1920s scientists realised that the nucleus stores an immense amount of energy. One way to harvest its energy is thermonuclear fusion. Just after WWII, the United Kingdom created the first devices designed to achieve controlled fusion in a few research centres. After the Argentinean president Juan Peron bravely claimed in 1951 that his scientists had succeeded in producing fusion energy, the USSR and the USA became more interested in taming the fusion fire. France, Sweden and Japan also joined the race later in the 50s. Besides building fusion devices, studies of fusion plasma physics were also boosted in the 50s, for example, investigations into plasma stability, Bohm diffusion and the Lawson criterion.

Some phenomena were even “discovered” multiple times, since fusion research was classified back then – thanks to the Cold War and the potential military uses of fusion.

Bohm diffusion
The Bohm diffusion theory describes the particle transport across the magnetic field lines that are confining the fusion plasma. This was necessary to estimate huge losses in early fusion devices. However, experiments showed that the losses are over 100 times larger than the Bohm diffusion had predicted which mad the Bohm diffusion the most optimistic transport theory for magnetically confined plasma.

“Cold” but not too cold

However, things were not entirely “cold” all the time as two Soviet scientists were able to visit Western laboratories, where they spoke of their research in a notably open manner. The UK was the first to declassify their documents as early as January 1957. Other Western countries followed them, while the USSR waited for the next conference to open up about their fusion research. During the Geneva Conference in 1958, scientists from both sides of the Iron Curtain openly discussed their research for the first time. Most of the presented results were on so-called “pinches and magnetic mirrors”. In addition, the USA introduced the first stellarator concept.

The Pinch concept

The Pinch concept uses a magnetic field to squeeze plasma. Pinches could be open with a cylindrical shape or closed with a ring-like shape, also called toroidal. Depending on the direction of the current through the plasma, the pinch could be a Z-pinch (current along cylinder or torus) or a Theta-pinch (poloidal, around the ring). The open Z-pinch was investigated in Moscow and Berkeley, while other centres in the USA investigated open Theta-pinches. The most prominent example of Theta-pinches, Scylla, located in Los Alamos, demonstrated the first thermonuclear neutrons. At the time, the UK was leading research into the toroidal pinch system with their massive ZETA device. In addition to the UK, the USSR, the USA, France and Sweden also presented results from their toroidal devices.

Mirror, Mirror

The other approach is that of a magnetic mirror which stores plasma as particle trap – whereby a stronger magnetic field at the edge repels the particles towards the centre, where the magnetic field is weaker. Soviet as well as American scientists attempted to create continuous plasma with the help of low magnetic field mirrors. A beam of energetic ions was often used to reach the thermonuclear conditions, but the estimated radial losses were too large for practical purpose. Some centres in the USA developed pulsed magnetic mirrors with stronger magnetic fields, which slowly evolved into the Theta-pinch.

The third approach, the stellarator was an attempt to produce plasma without current flowing through it, thus providing full control over plasma by the magnetic coils. This is executed either by bending the torus and/or bending the coils. Almost all reports contained neutrons that would sooner or later be proven to be of non-thermonuclear origin. Researchers later identified that these neutrons came from the beam-plasma interaction or fast-particle vs. wall interaction.
The Conference summary in Geneva finally concluded that fusion research has to ‚fill the missing gaps‘ and it was obvious that obtaining fusion power will be far more difficult than had been expected ten years previously. Soon after the Conference, West Germany, The Netherlands and Switzerland joined the fusion community.

Losing the optimism

It is true that Scylla lifted the open Theta-pinch research, but due to the edge losses and impractical dimensions of the experimental power plant measured in hundreds of meters, the concept was abandoned. In Moscow, scientists worked on Field-Reversed Configuration (FRC) for better plasma stabilisation. The idea was to create a self-organised opposite toroidal field of the external toroidal field in the device, like a ring in the cylinder.

The ZETA device at Harwell. The size of it was unmatched for almost two decades. Picture: © protected by United Kingdom Atomic Energy Authority

The ZETA device at Harwell. The size of it was unmatched for almost two decades.
Picture: © protected by United Kingdom Atomic Energy Authority

From ZETA to stellarator

The British pinch ZETA withstood the disappearance of toroidal pinches and detected the spontaneous reversed toroidal field at the plasma edge. Later this discovery would lead to the Reversed-Field Pinch (RFP) which is currently being investigated at EUROfusion’s Italian Research Unit in Padua. “Baseball-like” shaped coils reduced the losses in the fusion devices using the mirror approach, but these improvements were unimportant for practical purposes. “Cusp” mirrors were tested to stabilise the plasma. This lead to toroidal devices with the conductor implemented inside the vacuum chamber. These units are named Levitron because the conductor would levitate inside the chamber. The Levitron was never considered to be a potential power plant, but it helped with our understanding of the plasma stability and transport.

In the 60s, stellarators appeared to be the best candidate for fusion power plants. This way primarily due to them having an inherent continuous closed plasma system. After the Conference, many labs built their stellarators in line with the American approach. Unfortunately, designing the stellarator was cumbersome and included complicated calculations in an era without super-computers. As a result, the main design principle was “trial- and-error”. During the 1960s there was no additional big advances, either the particle losses were too large, the heating was insufficient or the plasma would be unstable and too short.

The Siberian Sun

In 1968, there was 3rd Fusion Energy Conference organised in Novosibirsk. As the future of fusion seemed a bit discouraging at the time, one could say that this was the best moment for the USSR to present their “ace in the sleeve”: the tokamak. The strong toroidal field was what made the tokamak different from the other devices.

Lev Artsimovich was honoured to present the tokamak T-3 results. T-3 immediately achieved the temperatures and an energy confinement much larger than any other device. The community was sceptic, especially in the USA and the United Kingdom. Moreover, the methods used to estimate the electron temperature were very indirect and this only increased the criticism. Yet, Artsimovich’s numbers were confirmed by direct measurements by UK scientists a year later. The astonishing results of the tokamak made this concept superior to all other fusion concepts. This drove all research centres worldwide to build their own tokamak. Note that, Australia was the only country, besides the USSR, to have a tokamak prior to the Novosibirsk Conference and around 15 tokamaks were constructed in total up to that time.

During the 1970s, fusion research was conducted on almost 80 tokamaks and the 3 largest tokamaks ever built (JET, TFTR and JT-60) had already been designed. Even the USA let their favourite stellarator go for few years.

The star bottle

At the end of the 1960s, fusion was still far from the power plant level. But it took scientists only two decades to get from simple pinches and mirrors to the tokamak concept, which already had improved the quality of fusion plasma immensely. Finally, after choosing the best “star bottle”, the tokamak, scientists worldwide could start to think about a “star-like” power plant on Earth. With the kickoff of Wendelstein 7-X in Greifswald last year, stellarators quickly returned to the fusion stage. The following article will compare the stellarator to the tokamak.

Milos Vlainic. Picture: private
I am a PhD student of Fusion Science and Engineering at the International Doctoral College. In my opinion there is nothing more practical than fusion – an abundant energy source for mankind. A year ago I found myself interested in popularising fusion science and I thank EUROfusion for giving me the opportunity to further disseminate my two passions: fusion and history.

Miloš Vlainić (27) from Serbia is currently based at: Ghent University. Blog: www.fom-fen.net (Picture: private)