Gyrotron technology to accelerate Tokamak Energy fusion dream

Built by Japanese firm Kyoto Fusioneering the new gyroton, which will be installed in the ST40 later this year, will generate high-power electromagnetic waves for controlling and heating a hydrogen plasma many times hotter than the centre of the sun. It will also be used to start up and drive plasma current.

Using a gyrotron a beam of electrons travels through a strong magnetic field which accelerates them to the point where they emit microwave radiation. This is directed through a waveguide to the plasma of fusion fuels – isotopes of hydrogen.

The frequency of the microwaves is tuned to match the cyclotron resonance frequency of the electrons in the plasma (104GHz or 137GHz in the case of ST40). When the microwaves interact with the plasma, they transfer energy to the electrons, which heats and drives the plasma.

A gyrotron, which uses Electron Cyclotron Resonance Heating (ECRH), solves one of the key challenges for a spherical tokamak – limited space for a central solenoid, which would otherwise be required to induce the plasma current. A gyrotron means the central solenoid can be reduced in size. A big advantage over the current neutral beam heating approach is that gyrotrons can be positioned away from the device itself, whereas neutral beam heating needs to be very close.

Tokamak Energy plans to use both its current neutral beam heating and gyrotron heating simultaneously in order to build greater understanding of how a gyrotron works, the control systems needed and the best balance between the two forms of heating.

The gyrotron upgrade complements a recently announced collaboration between the U.S. Department of Energy (DOE), the UK’s Department of Energy Security and Net Zero (DESNZ) and Tokamak Energy. Once the gyrotron is in operation, the partners will be able to advance the fusion science and technology needed to deliver a future pilot plant by testing lithium on the inner wall of ST40.

Dr Ross Morgan, director of Strategic Partnerships at Tokamak Energy, said: “We’re excited to work with our partners Kyoto Fusioneering to add this important upgrade to our record-breaking fusion machine, and continue to operate ST40 to test and push new boundaries. The results from future experiments using the high-power gyrotron heating system will provide critical data to inform the design of future spherical tokamak pilot plants, on our mission to commercialise clean and limitless fusion energy in the 2030s.”

Tokamak Energy’s ST40 has reached a plasma ion temperature greater than 100 million degrees Celsius, the threshold required for commercial fusion energy and the highest ever achieved in a privately funded spherical tokamak. The company also recently presented the first details of its fusion energy pilot plant being designed as part of the DOE Milestone-Based Fusion Development Program, which was established for private firms to bring fusion towards technical and commercial viability.