Our compact spherical tokamak, called the ST40, is the latest development in our exciting mission to achieve commercial fusion power.
And it’s a fully-commissioned device that has already achieved notable success. During a first round of testing in June 2018, ST40 achieved a plasma temperature of 15 million degrees Celsius – hotter than the core of the Sun.
As we develop our high temperature superconductor (HTS) technology further, we aim to demonstrate ST40 as the first privately-funded fusion machine to achieve the temperatures required for fusion – 100 million degrees. This will allow us to take the next step on our strategic fusion development plan.
100 million degrees is the temperature required to force together charged deuterium and tritium particles that naturally repel each other and get them close enough together to fuse.
Along the way, ST40 will be investigating a new domain in tokamak operation: the combination of high magnetic field and low aspect ratio (a squashed shape). We want to find out whether the plasma behaves in the way we think it will under these new conditions.
Achieving these objectives will prove that commercially-viable fusion power can be produced in compact spherical tokamaks.
- 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
magnetThe 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.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 universe. 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 sunLiquid 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 sunMerging 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
during re-entryDivertor 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 carPorts used for plasma heating, diagnostics and vacuum pumping.
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.
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