Spherical tokamaks

Culham Centre for Fusion Energy has pioneered the innovative ‘spherical tokamak' fusion concept and continues to lead its development.

Plasma in the START tokamakSince tokamaks were developed in Russia in the 1960s, research has mainly concentrated on machines which hold the plasma in a doughnut-shaped vacuum vessel around a central column, with very successful results. JET and the next-generation ITER device are designed with this configuration, known as the ‘conventional tokamak'.

Another, potentially more efficient way of producing energy from fusion plasmas is also being explored. ‘Spherical tokamaks' (STs) hold plasmas in tighter magnetic fields, forming a more compact, cored apple shape. Topologically the same as conventional tokamaks, spherical tokamaks get their name from the natural shape of the plasma that forms as the width of the central column is reduced to a minimum.

Comparison of spherical and conventional tokamaks

Advantages and uses of spherical tokamaks

Spherical tokamaks could result in more economical and efficient fusion power. Designs demonstrating the feasibility of ST power plants have already been developed.

  • The ST's design results in a compact device where plasmas are confined at higher pressures for a given magnetic field. The greater the pressure, the higher the power output, and the more cost-effective the fusion device. In the 1990s, the START device at Culham set a world record for plasma pressure with a Beta value (the plasma pressure divided by the externally applied magnetic field pressure) of 40%. This was three times that of other contemporary machines.
  • The magnetic field needed to keep the plasma stable can be a factor of up to ten times less than in a conventional tokamak, also allowing more efficient plasmas.
  • STs will cost less to build as they do not need to be as large as conventional machines for the same performance. Neither do they require the use of superconducting magnets, which add greatly to the cost of conventional tokamak construction.

As STs are in a relatively early stage of development, they will probably not be used in the first generation of fusion power plants. In the nearer term, they could serve as component test facilities to ensure that systems and materials used in power plants can withstand neutron bombardment.

Additionally, plasma physics studies in STs are feeding into the development of ITER. As their geometry differs from conventional tokamaks, they are providing insights into the way changes in the characteristics of the magnetic field affect plasma behaviour, revealing trends that otherwise would be difficult to spot.

In January 1991, the world's first hot spherical tokamak began operating at Culham. START (Small Tight Aspect Ratio Tokamak) was a relatively low-cost device, mainly constructed from existing equipment. However, it achieved highly impressive results that surpassed expectations and confirmed the potential of spherical tokamaks.

Since 2000, the UK's ST programme has focused on the MAST (Mega Amp Spherical Tokamak) machine. Building on START's success, MAST is a larger, more sophisticated device with a suite of extremely advanced diagnostics for analysing the plasma.