Plasmas with the highest deuterium fusion neutron rates yet achieved in JET have been produced by combining a hollow or flat central current profile with a high-confinement (H-mode) edge. In these discharges, lower hybrid current drive (LHCD) and ion cyclotron resonance heating (ICRH) preheating, applied early in the current ramp-up phase, `freezes in' a hollow or flat current density profile. When the combined neutral beam injection (NBI) and ICRH heating power is increased, a region of reduced transport and highly peaked profiles forms during the L-mode phase and persists into the later H-mode phase when the fusion reactivity reaches a maximum. Transport analysis shows the formation of a central region of good confinement (the internal transport barrier or ITB) which expands with radial velocity . The clearest signatures of this region are large gradients in the ion temperature and toroidal rotation profiles. Ion thermal diffusivities in the central region are of the order of the neoclassical value. The position and rate of expansion of the ITB radius correlates well with the calculated rational q = 2 surface. The confinement improvement can also be seen in electron density, and, to a lesser extent, in electron temperature. The ITB can persist in combination with the edge transport barrier characteristics of the H-mode.
Deuterium-tritium operation in magnetic confinement experiments: results and underlying physics