MAST Upgrade to tackle exhausting challenge in fusion power

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Nuclear fusion has long held out the promise of clean, safe energy but dealing with the extremely high temperatures generated in the reactors is no easy task.

Plasma test inside the original MAST machine
Plasma test inside the original MAST machine

Now the UK’s nuclear fusion experiment MAST Upgrade, at Culham in Oxfordshire, is to receive £21m to investigate new technology to cool the extremely hot plasma exhaust produced by the reaction.

Inside a fusion reactor, light atomic nuclei are fused together to form larger ones, releasing a huge amount of energy in the process.

This is achieved by confining the gas within a magnetic field and then heating it to temperatures of around 150 million Celsius, according to Dr James Harrison, Tokamak science programme delivery manager at Culham Centre for Fusion Energy.

The hot fusion plasma exhaust is then passed through an area of the reactor known as a “divertor” to allow it to dissipate some of this excess heat.

However, the extremely high temperatures involved mean the exhaust particles would damage the surfaces of a conventional divertor, meaning components would require regular replacement, which adds to the cost of the electricity produced.

“As the super-heated gas is gradually transported away from the hot fusion producing part of the machine into the area where it is exhausted, it comes into contact with solid surfaces of either tungsten or carbon, which are typically much closer to room temperature,” said Harrison. “The super-heated plasma can heat the surfaces, leading to gradual melting or erosion of the material over time.”

So the fusion researchers are investigating the use of a new “Super X” divertor, which is designed to cool particles down by sending them on a longer exhaust path out of the plasma.

“The Super X divertor aims to take the exhaust plasma, which has a very high density, and expand that into a much larger volume,” said Harrison.

This allows heat to be radiated away before it reaches solid material surfaces, he said.

The funding, from the European fusion research consortium EUROfusion and the EPSRC, will allow the researchers to improve their understanding of plasma exhaust physics, and to carry out better predictive modelling for the proposed prototype commercial fusion power plant, known as DEMO.