ST40, our compact spherical tokamak, is moving us closer to fulfilling our exciting mission to achieve commercial fusion energy.

In early 2022 we demonstrated a world-first by reaching a plasma temperature of 100 million degrees Celsius in our ST40 spherical tokamak, the threshold required for commercial fusion energy.

This achievement further substantiates spherical tokamaks as the optimal route to the delivery of clean, secure, low cost, scalable and globally deployable commercial fusion energy.

The ST40 device is now undergoing an upgrade and will be used to develop technologies for future devices.

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    Explore the ST40:
  • ST40 outside view
  • ST40 inside view
  • Toroidal field coils
  • Centre column
  • Poloidal field coils
  • Plasma start-up
  • Inner vacuum chamber
  • Outer vacuum chamber
  • ST40 outside viewOur latest fusion device

    This robust structure is manufactured from 30mm thick stainless steel and houses the inner vacuum chamber, magnets and other key components.

  • ST40 inside viewOur latest fusion device

    All of the key components that are required for fusion are neatly packaged inside the tokamak.

  • TOROIDAL FIELD COILS Confining the plasma

    The toroidal field (TF) magnets work with the poloidal field magnets to create a closed field pattern that confines the hot plasma and holds it away from the walls of the Inner Vacuum Chamber. The charged plasma particles follow the closed magnetic field lines, continuously spiralling around the tokamak.

    The 250 000 amp current in the TF coil interacts with the magnetic fields to both expand the coils outwards and push them sideways. These large forces are transferred to the Outer Vacuum Vessel.

    The strength of the toroidal field is about the same as an



    The magnets are also known as coils because they are electromagnets made by winding insulating conducting wire round and round into a coil. The number of windings in the coil determined the strength of the magnetic field.

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    Return limb – the outer part of the toroidal field magnets that are needed to create a closed field pattern and hold the hot plasma away from the Inner Vacuum Chamber walls.
    Flexible joints – passing 250,000 amps through the toroidal field coils causes them to heat up and expand. These ‘flexi’ joints allow the magnet to expand and contract without breaking.

  • CENTRE COLUMN Confining the plasma

    The centre column has two parts: the central wedges of the toroidal field magnets, and a large solenoid. The solenoid maintains a current flowing through the plasma, which is important for plasma stability, but the toroidal field magnets generate the magnetic field that keeps the plasma confined.

    The solenoid stack is made up of 24 central wedges. Each wedge has a


    twist so they can all be joined up in a circuit.


    Centre Stack – this contains 24 twisted copper wedges that make up part of the TF magnet. Each wedge is twisted by 15 degrees to allow the TF magnets to be connected in one continuous circuit.

  • poloidal field coilsControlling the plasma

    The poloidal field coils control the shape and position of the hot plasma. The properties of plasma are heavily influenced by its shape so these coils help create optimum fusion conditions.

    The divertor coils – centre, top and bottom – stretch the plasma vertically and guide the plasma exhaust to a dedicated region where it can be effectively removed.

    Temperature gradients inside the tokamak are the largest in the solar system. The coils will be cooled with liquid nitrogen to


    while 40cm away will be a plasma that is 10 times hotter than the centre of the sun


    Liquid nitrogen-cooled coils – cooling the coils with liquid nitrogen extends the ST40 pulse length.
    Divertor coils – these powerful coils stretch the plasma into what’s known as a divertor configuration. Having a divertor at the top and bottom reduces the heat that must be removed by each divertor.

  • Plasma Start-UPGenerating the plasma

    ST40 uses a novel technique to generate and heat the plasma – Merging Compression.

    Normally start-up is the role of the solenoid. While ST40 still has a solenoid, it is used to maintain the plasma current rather than generate it.

    Not relying on the solenoid is important in spherical tokamaks as there is only limited space in the centre of the machine.

    The plasma inside the tokamak will reach more than 100 million degrees Celsius during the fusion process, which is


    hotter than the centre of the sun


    Merging Compressions Coils – used to breakdown the neutral fuel and form a plasma. When fired, a plasma ring forms around each of the two coils. These plasma rings snap together, converting magnetic energy into heat. Merging Compression will heat ST40 first to 15 million degrees and then on to 100 million.

  • inner vacuum chamber (ivc)Where the particles collide

    To get the right conditions for fusion, you must create plasma – an electrically-charged gas. If the charged plasma particles that make up the plasma are moving fast enough (if the plasma is hot enough), the particles may overcome the repulsive force.

    At the top and bottom of the IVC is the divertor region where the plasma exhaust is removed.

    The divertor needs to handle heat loads that are larger than those experienced by the

    Space shuttle

    during re-entry


    Divertor region – the energetic particles that make up the plasma exhaust are diverted to this dedicated region where they can be effectively removed. The divertor stops unwanted impurities entering the plasma and will allow ST40 to reach 100 million degrees.

  • Outer VACUUM CHAMBER (ovc)Supporting against strong magnetic fields

    The OVC has an internal vacuum that provides thermal insulation for the liquid nitrogen-cooled copper field coils. It also has a vital role to play in supporting the toroidal and poloidal field coils against strong magnetic forces.

    The tokamak needs to withstand huge forces and torque loads


    times greater than those in an F1 car


    Ports used for plasma heating, diagnostics and vacuum pumping.

Further information


Research Library

Our research builds on decades of work on tokamaks and applies new technologies to make improvements. Explore our library of technical publications to find out more about the key scientific evidence behind our approach to fusion.


News and Press

Our mission is producing lots of interesting stories that we want to tell the world about and the media are keen to communicate to their global audiences. Visit our news and press section to see what’s making the headlines.