Ion cyclotron heating of JET DD and DT optimized shear plasmas

The unique roles played by ICRH in the preparation, formation and sustainme nt of internal transport barriers (ITBs) in high fusion performance JET opt imized shear experiments using the Mark II poloidal divertor are discussed. Together with LHCD, low power ICRH is applied during the early ramp-up pha se of the plasma current, 'freezing in' a hollow or flat current density pr ofile with q(0) > 1. In combination with up to similar to 20 MW of NBI, the ICRH power is stepped up to similar to 6 MW during the main low confinemen t (L mode) heating phase. An ITB forms promptly after the power step, revea led by a region of reduced central energy transport and peaked profiles, wi th the ion thermal diffusivity falling to values close to the standard neoc lassical level near the centre of both DD and DT plasmas. At the critical t ime of ITB formation, the plasma contains an energetic ICRF supported hydro gen minority ion population, contributing similar to 50% to the total plasm a pressure and heating mainly electrons. As both the NBI population and the thermal ion pressure develop, a substantial part of the ICRF power is damp ed resonantly on core ions (omega = 2 omega(cD) = 3 omega(cT)), contributin g to the ion heating. In NBI step-down experiments, high performance has be en sustained by maintaining central ICRH; analysis shows the efficiency of central ICRH ion heating to be comparable to that of NBI. The highest DD fu sion neutron rates (R-NT = 5.6 x 10(16) s(-1)) yet achieved in JET plasmas have been produced by combining a low magnetic shear core with a high confi nement (H mode) edge. In DT, a fusion triple product n(i)T(i)tau(E) = (1.2 +/- 0.2) x 10(21) m(-3) keV s was achieved with 7.2 MW of fusion power obta ined in the L mode and with up to 8.2 MW of fusion power in the H mode phas e.