The unique roles played by ICRH in the preparation, formation and sustainment of internal
transport barriers (ITBs) in high fusion performance JET optimized shear experiments using the Mark II
poloidal divertor are discussed. Together with LHCD, low power ICRH is applied during the early ramp-up phase
of the plasma current, `freezing in' a hollow or flat current density profile with q(0)>1. In
combination with up to ∼20 MW of NBI, the ICRH power is stepped up to ∼6 MW during the main
low confinement (L mode) heating phase. An ITB forms promptly
after the power step, revealed by a region of reduced central energy transport and peaked profiles,
with the ion thermal diffusivity falling to values close to the standard neoclassical level near the
centre of both DD and DT plasmas. At the critical time of ITB formation, the plasma contains an
energetic ICRF supported hydrogen minority ion population, contributing ∼50% to the total
plasma pressure and heating mainly electrons. As both the NBI population and the thermal ion
pressure develop, a substantial part of the ICRF power is damped resonantly on core ions (ω = 2 ωcD = 3ωcT), contributing to the ion heating. In NBI step-down experiments, high
performance has been sustained by maintaining central ICRH; analysis shows the efficiency of central
ICRH ion heating to be comparable to that of NBI. The highest DD fusion neutron rates
(RNT = 5.6 × 1016 s-1) yet achieved in JET plasmas have been produced by combining a low
magnetic shear core with a high confinement (H mode) edge. In DT, a fusion triple product
niTiτE = (1.2 ± 0.2) × 1021 m-3 keV s was achieved with 7.2 MW of fusion power obtained in the L mode and with up to 8.2 MW of fusion power in the H mode phase.