The Small Tight Aspect Ratio Tokamak, or START was a nuclear fusion experiment that used magnetic confinement to hold plasma. The experiment began at the Culham Science Centre in the United Kingdom in 1991 and was retired in 1998. It was built as a low cost design, largely using parts already available to the START team. The START experiment developed the tokamak by changing the previous toroidal shape into a tighter, almost spherical, doughnut shape. The new shape increased efficiency by reducing the cost over the conventional design, whilst the field required to maintain a stable plasma was a factor of 10 less.
The START team holds the current highest record plasma pressure, which they achieved by using a neutral beam injector to heat the plasma. In March 1998, the START experiment finished and has since been disassembled and transferred to the ENEA research laboratory at Frascati, Italy. The START team began the Mega Ampere Spherical Tokamak Experiment or MAST in 1999 which still operates in the Culham Science Centre, UK.