JET, or Joint European Torus, is a magnetic confinement plasma physics experiment located in Oxfordshire, United Kingdom. Its main purpose is to open the way to future nuclear fusion experimental tokamak reactors, such as ITER (International Thermonuclear Experimental Reactor) and DEMO (Demonstration Power Plant). Scientists at Oxfordshire are preparing for fusion testing, set to begin in 2015.
A tokamak is a device using a magnetic field to confine a plasma in the shape of a torus (“doughnut” shape). Achieving a stable plasma equilibrium requires magnetic field lines that move around the torus in a helical shape. A helical field can be generated by adding a toroidal field (traveling around the torus in circles) and a poloidal field (traveling in circles orthogonal, or non-overlapping, to the toroidal field). In a tokamak, the toroidal field is produced by electromagnets that surround the torus, and the poloidal field is the result of a toroidal electric current that flows inside the plasma.
The word tokamak is a transliteration of the Russian word токамак, an acronym of either “тороидальная камера с магнитнымикатушками” (toroidal'naya kamera s magnitnymi katushkami)—toroidal chamber with magnetic coils, or “тороидальная камера саксиальным магнитным полем” (toroidal'naya kamera s aksial'nym magnitnym polem)—toroidal chamber with axial magnetic field.