The Small Tight Aspect Ratio Tokamak, or START was a nuclear fusion experiment that used magnetic confinement to hold plasma. START was the first full-sized machine to use the spherical tokamak design, which aimed to greatly reduce the aspect ratio of the traditional tokamak design.
Quick Facts Device type, Location ...
Close
The experiment began at the Culham Science Centre in the United Kingdom in 1990 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 revolutionized 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, while the field required to maintain a stable plasma was a factor of 10 less.
In order to successfully heat experiments in a spherical tokamak, physicists performed neutral beam injection. This involved interjecting hydrogen into hydrogen or deuterium plasmas, providing effective heating of both ions and electrons. Although the atoms were injected with no net electrostatic charge, as the beam passed through the plasma, the atoms were ionized as they bounced off the ions already in the plasma. Consequently, because the magnetic field inside the torus was circular, these fast ions were confined to the background plasma. The background plasma slowed the confined fast ions, in a similar way to how air resistance slows down a baseball. The energy transfer from the fast ions to the plasma increased the overall plasma temperature. The neutral beam injector used in START was on loan from Oak Ridge National Laboratory.[2]
The magneto-hydro-dynamic limit (MHD) was an operational limit of tokamaks, with START being no exception. The START team would test the MHD using forty-six sets of Mirnov coils at different heights on the center column of START. Plasmas being formed by compression within START limited the fluctuation of the MHD.[3]
Prior to October 1995, START had no rapid terminations. In October 1995, divertor coils were installed and images showed the plasma would interact with the coils before disruptions occurred. These suspicions were further strengthened when the divertor coils were moved closer to the plasma in December 1996, which resulted in a higher frequency of disruptions.[3]
The characteristics of plasma within START were also measured. Typical plasma within START had an aspect ratio A=1.3, elongation k=1.8, and a temperature of 400 eV.[2][4]
A number of experiments reached 32 percent beta with START, where the previous world record for beta in a tokamak was 12.6 percent. Factors that contributed to the significantly higher beta number include better vacuum conditions, a more powerful neutral beam injection, a lower toroidal field, a higher plasma pressure, and a lower magnetic pressure.[4] In 1998 a non-ohmic beta of 40% was achieved.[5]: 29