The JET Experiment: Evolution, Present Status, and Prospects

Abstract

The background to the decision to build the Joint European Torus (JET) is described, and a brief introduction to the main aims, overall design philosophy, and the planned parameter range of the large tokamak device (major radius R = 2.96 m; horizontal and vertical minor radii a = 1.25 m and b = 2.10 m, respectively; plasma current Ip = 4.8 MA) is provided. JET is situated on the Culham Laboratory site, United Kingdom, and its main objective is to obtain and study plasmas in conditions and with dimensions approaching those needed in a fusion reactor. The main emphasis in the initial operation has been in the ohmic heating phase, in which results have covered a wide range of parameters: plasma currents Ip < 5 MA; toroidal magnetic fields BT = 1.3 to 3.4 T; elongation ratios b/a = 1.2 to 1.7; and safety factor values q = 2.2 to 12. Average electron densities ne = (1 to 4) × 1019 m-3, with high central electron temperatures (Te up to 5 keV) and ion temperatures (Ti up to 4 keV) have been achieved, although Zeff was in the range of 2.5 to 10. Energy confinement times (τE) of up to 0.8 s have been obtained. Some problems with metallic and low-Z impurities are noted, causing high radiation levels. Initial experiments, with ion cyclotron resonance frequency (ICRF) heating of hydrogen and 3He minorities in deuterium plasmas at megawatt levels, are reported. A discussion of certain limitations observed generally in tokamaks and how these might affect future developments of the JET program is presented. Planned future experiments on impurity control, additional heating (ICRF ≍ 15 MW, and neutral injection ≍ 10 MW), and preparations for tritium operation are also described.