Mega Ampere Spherical Tokamak

The Mega Ampere Spherical Tokamak (MAST) experiment was a nuclear fusion experiment. It tested a spherical tokamak nuclear fusion reactor. MAST was commissioned by EURATOM/UKAEA. The experiment took place at Culham Centre for Fusion Energy, Oxfordshire, England. The experiment ran from December 1999 to September 2013. A successor facility called MAST Upgrade began operation in 2020.
Design.
A spherical tokamak is shaped more like a cored apple than the conventional, doughnut-shaped toroidal design used by experiments such as ITER. Spherical tokamaks are more efficient in their use of the magnetic field. 
MAST included a neutral beam injector for plasma heating. It used a merging compression technique for plasma formation instead of the conventional direct induction. Merging compression saves central solenoid flux, which can then be used to increase the plasma current and/or maintain the required current flat-top.
MAST's plasma volume was about 8 m3. It confined plasmas with densities on the order of 1020/m3.
MAST's plasma had an 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.
Experiments.
MAST confirmed the increased operating efficiency of spherical tokamaks – demonstrating a high beta (ratio of plasma pressure to the pressure from the confining magnetic field). MAST performed experiments on controlling and mitigating instabilities at the edge of the plasma – so-called Edge Localised Modes or ELMs.
History.
The Small Tight Aspect Ratio Tokamak (START) experiment (1991-1998) was an earlier spherical tokamak. START exceeded the most optimistic predictions and MAST confirmed its on a larger, more purpose-built experiment.
The MAST design occupied 1995-1997. Construction consumed 1997-1999. First plasma came in 1999.
Over its lifetime MAST produced 30471 plasmas (in pulses up to 0.5 sec). In October 2013 the reactor was shut down for upgrade.
MAST Upgrade.
MAST Upgrade is the successor experiment, also at Culham Centre. The upgrade cost £45M. It started in 2013 and was expected to significantly exceed MAST’s heating power, plasma current, magnetic field and pulse length.
MAST Upgrade began operation on 29 October 2020.
One of MAST Upgrade's most notable features is the Super-X divertor. The divertor removes excess heat and impurities from the plasma. Conventional divertor designs, at powerplant scale, will experience high heat loads and will need to be regularly replaced. The Super-X divertor was expected to produce heat loads that are lower by around a factor of 10.
Spherical Tokamak for Energy Production.
The design of the next generation Spherical Tokamak for Energy Production (STEP) began in 2019 with £220 million in government funding. The plan is to begin operations in the 2040s. Current plan is to not include a tritium generation facility.