Mega Ampere Spherical Tokamak


The Mega Ampere Spherical Tokamak experiment was a nuclear fusion experiment in operation at Culham Centre for Fusion Energy, Oxfordshire, England, from December 1999 to September 2013. A successor facility called MAST Upgrade is being commissioned in 2020, with physics campaigns planned to start in late 2020.
MAST followed the highly successful Small Tight Aspect Ratio Tokamak experiment and used the same innovative spherical tokamak design as START. Spherical tokamaks have been shown to be more efficient in use of the magnetic field than the more conventional toroidal design, adopted by Joint European Torus and ITER. START proved to exceed even the most optimistic predictions and MAST confirmed the results of its forerunner on a larger, more purpose-built experiment.
It was fully commissioned by EURATOM/UKAEA and took two years to design and a further two years to construct. MAST included a neutral beam injector similar to that used on START for plasma heating, and used the same merging compression technique for plasma formation instead of the conventional direct induction. Merging compression provides a valuable saving of central solenoid flux, which can then be used to further ramp up the plasma current and/or maintain the required current flat-top.
Its plasma volume was about 8 m3 and it confined plasmas with densities on the order of 1020/m3.
Image to right shows plasma in the MAST device, displaying its almost circular outer profile. The extensions off the top and bottom are plasma flowing to the ring divertors, a key feature of modern tokamak designs.

Timeline

From 1999 to 2013, MAST ran 30471 plasmas. It confirmed the increased operating efficiency of spherical tokamaks as shown on START – especially demonstrating a high beta. MAST also performed valuable experiments on controlling and mitigating instabilities at the edge of the plasma – so-called Edge Localised Modes or ELMs.

MAST Upgrade

MAST Upgrade is a new tokamak fusion experiment at UKAEA’s Culham Centre for Fusion Energy, building on the success of it predecessor – the MAST tokamak. The upgrade, which is costing £45M, started in 2013 when MAST stopped operating and is designed to significantly enhance MAST’s capabilities in terms of heating power, plasma current, magnetic field and pulse length.
MAST Upgrade is currently being commissioned, ready to start operation in late 2020.
One of the most notable features on MAST Upgrade is the Super-X divertor. A divertor is part of the tokamak that is designed to exhaust the excess heat and impurities from the plasma. Conventional divertor designs, when scaled up to future powerplants, will experience very high heat loads and will need to be replaced every few years. The Super-X divertor should demonstrate much lower heat loads, potentially solving one of the major challenges of commercially viable fusion power in the future.