TRANSPORT IN JET DEUTERIUM PLASMAS WITH OPTIMIZED SHEAR

Plasmas with the highest deuterium fusion neutron rates (R-DD = 5.6 x 10(16) s(-1)) yet achieved in JET have been produced by combining a ho llow or flat central current profile with a high-confinement (H-mode) edge. In these discharges, lower hybrid current drive (LHCD) and ion c yclotron resonance heating (ICRH) preheating, applied early in the cur rent ramp-up phase, 'freezes in' a hollow or hat current density profi le. When the combined neutral beam injection (NBI) and ICRH heating po wer is increased, a region of reduced transport and highly peaked prof iles 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 inte rnal transport barrier or ITB) which expands with radial velocity simi lar to 0.5 m s(-1). The dearest signatures of this region ate large gr adients in the ion temperature and toroidal rotation profiles. Ion the rmal diffusivities in the central region are of the order of the neocl assical value. The position and rate of expansion of the ITB radius co rrelates well with the calculated rational q = 2 surface. The confinem ent improvement can also be seen in electron density, and, to a lesser extent, in electron temperature. The ITB can persist in combination w ith the edge transport barrier characteristics of the H-mode.