Reproducing what goes on in the Sun and other stars in a machine on Earth has been the quest of fusion scientists for more than 40 years. The Sun successfully fuses hydrogen ions by gravitationally confining them and their electrons in a hot spherical plasma. Here on Earth, however, it is not possible to confine ions using their own gravitational attraction. Instead, we often use magnetic fields to coax the ions into doing what we want them to do.
Due to the lack of magnetic monopoles, we cannot make a spherically symmetric magnetic field. However, there have been many attempts to achieve fusion in straight cylinders, and in various toroidal or doughnut-shaped configurations. Currently, the best plasma confinement has been achieved in tokamaks. These torus-shaped machines have a strong magnetic field in the toroidal direction (the long way around the doughnut) together with a large toroidal plasma current that produces another magnetic field in the poloidal direction (the short way around the doughnut). Although the resulting helical field is not perfect, it corrects for the natural drifts of the ions and electrons in the plasma, thus confining them reasonably well within the torus.
As the "hole" in the torus becomes smaller, the machine begins to resemble a sphere and the physical properties of the plasma change. There have been attempts to make a toroidal plasma that is almost spherical using a machine called a spheromak, but the resulting plasmas are not as stable and the confinement is relatively poor.
Machines called spherical tokamaks approach a spherical geometry but have current-carrying coils that pass through the core of the torus. Several of these machines have been built around the world. In 1995 the START experiment at the Culham Laboratory in the UK became the first spherical tokamak to achieve high temperatures and high confinement. The START plasma is relatively small, approximately 1 m wide and 0.5 m high, and has a toroidal magnetic field less than 0.3 T. Nevertheless, the geometry and the efficient use of the magnetic field means that START can carry plasma currents in excess of 300 000 Amps. Indeed, the stored plasma energy has recently exceeded all expectations based on predictions from conventional tokamaks (A Sykes et al. 2000 Phys. Rev. Lett. 84 495).
In the April issue of Physics World magazine, Joseph Snipes from the Massachusetts Institute of Technology, US explains the growing interest in spherical tokamaks.