Mega Ampere Spherical Tokamak

The Mega Ampere Spherical Tokamak (MAST) 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.[1]

MAST
Mega Ampere Spherical Tokamak
Plasma in the MAST reactor
Device TypeSpherical tokamak
LocationCulham, Oxfordshire, UK
AffiliationCulham Centre for Fusion Energy
Technical specifications
Major Radius~ 0.9 m (2 ft 11 in)
Minor Radius~ 0.6 m (2 ft 0 in)
Plasma volume8 m3
Magnetic field0.55 T (5,500 G)
Heating power5 MW
Plasma current1.3 MA
History
Date(s) of construction1997
Year(s) of operation1999 – 2013
Preceded bySmall Tight Aspect Ratio Tokamak (START)
Succeeded byMAST-U

MAST followed the highly successful Small Tight Aspect Ratio Tokamak (START) experiment (1991-1998) 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 (JET) 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
  • ~1995 design started
  • ~1997 construction started
  • 1999 First plasma
  • 2013 Oct. Final plasma (#30471) before shutdown for upgrade.[2]

Operation

From 1999 to 2013, MAST ran 30471 plasmas (in pulses up to 0.5 sec). It confirmed the increased operating efficiency of spherical tokamaks as shown on START – especially demonstrating a high beta (ratio of plasma pressure to the pressure from the confining magnetic field). 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 (by around a factor of 10), potentially solving one of the major challenges of commercially viable fusion power in the future.

See also

References
  1. "MAST Upgrade Research Plan, November 2019" (PDF). Culham Centre for Fusion Energy. Retrieved 2019-12-13.
  2. "News: It's goodbye to MAST - and hello to MAST Upgrade". Ccfe.ac.uk. Retrieved 2015-12-11.
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