The UK Atomic Energy Authority (UKAEA) has announced a significant experimental result from its MAST Upgrade spherical tokamak, successfully operating a novel Super-X divertor. The experiment sustained a plasma for five seconds while using a new array of 16 magnetic coils to actively shape the plasma's exhaust path. This configuration expands the magnetic field lines near the divertor plates, increasing the surface area over which the exhaust heat is distributed. The primary goal is to mitigate the intense power loads that would otherwise damage the material surfaces of a compact fusion device, a key challenge for the commercial viability of spherical tokamaks. Source: UKAEA

Managing plasma-surface interactions is a critical engineering hurdle for sustained fusion reactions. In conventional divertor designs, particularly within the constrained geometry of a spherical tokamak, the exhaust power can reach several gigawatts per square meter, a load no current material can withstand long-term. The Super-X divertor addresses this by lengthening the path the exhaust particles travel from the core plasma to the divertor target plates. This extended path allows the superheated plasma to cool significantly through radiation before impact, reducing the peak heat flux on the material surfaces. The UKAEA team aims for at least a tenfold reduction in this heat load compared to conventional designs. Source: UKAEA

Managing plasma-surface interactions is a critical engineering hurdle for sustained fusion reactions.

The successful test on MAST Upgrade provides crucial validation for the design of the UK's planned Spherical Tokamak for Energy Production (STEP) prototype power plant. STEP's design relies on innovative solutions like the Super-X divertor to achieve a compact, economically attractive footprint while maintaining the material integrity required for continuous operation. By demonstrating control over the plasma exhaust in this new magnetic geometry, the UKAEA team has retired a significant risk for the STEP program. These experimental results are expected to be published in a scientific journal, providing the fusion community with detailed data on the divertor's performance characteristics. Source: UKAEA

This achievement positions the spherical tokamak as a more credible candidate for a commercial fusion reactor. While larger, conventional tokamaks like ITER also face divertor challenges, the problem is more acute in compact devices where the magnetic field lines are more compressed. The MAST Upgrade results will inform not only the UK's national program but also private fusion ventures pursuing similar high-beta, compact designs. The next steps will involve further experiments to quantify the precise reduction in heat flux and to test the divertor's performance under higher power and longer pulse durations, moving closer to the conditions required for a net-energy-gain power plant. Source: UKAEA