Merging compression is a technique whereby the tokamak is started-up without use of a central solenoid. Most tokamaks have a large spiral magnet, a solenoid, at their centre. Running current through the many turns of the solenoid generates a high current in the torus ring, ionising the gas to make a plasma and then providing stability and heat to that plasma. This plasma current is crucial. However the solenoid takes up a lot of space in the centre of the machine. If we want to reduce machine size, we need to start up without the solenoid. But then we need a new way of producing the hot plasma and high current. Tokamak Energy are investigating merging compression.

Internal workings of a conventional tokamak with a central solenoid. Credit: Max-Planck Institut für Plasmaphysik

Internal workings of a conventional tokamak with a central solenoid. Credit: Max-Planck Institut für Plasmaphysik

For merging compression, two symmetric magnet rings are constructed inside the torus chamber. Running high currents through these magnetic coils creates two rings of plasma around them, and as the coil current is reduced to zero the plasma rings attract and combine. When the rings combine, their magnetic fields reconfigure in a process called “reconnection”. Stretched field lines break and release huge amounts of energy, in the same way that a stretched catapult releases enough energy to launch a missile. This energy heats the plasma.

merging compression

The progression of merging compression seen in one half of the spherical tokamak.

The challenge for Tokamak Energy is to heat the plasma to 100 million degrees using this method. That’s ten times more than the record so far. But if we succeed, we can make smaller tokamaks with higher magnetic fields.

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