15-tonne double beam box to support higher-power fusion experiments

Engineers at the UK Atomic Energy Authority have installed a 12-tonne double beam box vessel on the MAST Upgrade fusion machine at Culham, a major step towards doubling its neutral beam heating power and enabling experiments at plasma temperatures of around 60 million degrees Celsius.

The new vessel is designed to house two additional neutral beam injectors, increasing the machine's neutral beam heating capacity from 4.4 MW to 8.8 MW. Neutral beam injection works by firing high-energy neutral atoms into the plasma, where they transfer energy and raise its temperature to more than 30 million degrees Celsius.

With the additional injectors, researchers expect plasma conditions to roughly double, from 30 million to 60 million degrees Celsius, opening access to higher-pressure and higher-performance plasma conditions that are more representative of those required in future fusion power plants.

The double beam box has a double-decker internal layout to accommodate the new injection equipment. The vessel has a volume of around 20 m3 and is manufactured from 316L stainless steel, a grade widely used in offshore infrastructure and medical implants and selected here for its strength, durability, corrosion resistance and compatibility with the ultra-high vacuum conditions required inside.

Once fully assembled with its lid, the vessel will weigh approximately 15 tonnes. Engineers are now progressing the internal systems needed to make the new heating arrangement operational, including the beam line module used to form and inject the neutral beam, a cryopump to control pressure within the vessel and the positive ion neutral injectors themselves.

The installation forms part of a wider upgrade programme at MAST. A new electron Bernstein wave microwave heating system is also being installed, while power supply equipment from the former Joint European Torus machine is being transferred and repurposed.

The higher heating power is intended not simply to increase plasma temperature, but to allow researchers to examine how the machine behaves under more demanding operating conditions. Andrew Thornton, Head of MAST Upgrade Operations at UKAEA, said the lift was a huge step forward, allowing the team to explore plasma conditions previously out of reach and to study the stability of higher-pressure plasmas while testing how the exhaust system copes with hotter, higher-performance plasmas.

Plasma exhaust is one of the central research areas, where heat and particles leaving the plasma must be directed towards components known as the divertor. Managing these heat loads is one of the major challenges in developing commercial fusion power, as future machines will need divertor systems capable of handling intense energy fluxes without requiring excessive maintenance or frequent replacement.

The additional heating will therefore allow MAST Upgrade to test both plasma stability and the performance of its exhaust system under higher-pressure conditions. These experiments are expected to provide data relevant to the UK's STEP programme, which is developing a prototype spherical tokamak fusion power plant.

All major hardware for the double beam box is expected to be manufactured and fitted inside the vessel by the end of autumn 2027. The vacuum-tight vessel will then be commissioned, with scientific experiments scheduled to begin in 2028.

The installation represents an important engineering step in moving MAST Upgrade towards the more extreme operating conditions needed to study the physics, materials and heat-management systems required for future fusion energy.