Evolution of internal transport barriers in jet optimised smear plasmas

High performance discharges with optimised magnetic shear having a broad or hollow current density profile have been produced in JET. These plasmas ar e characterised by improved core confinement within a region called the Int ernal Transport Barrier (ITB). Near the plasma centre, the ion thermal diff usivity is reduced to levels close to the standard neo-classical level in b oth D-D and D-T plasmas. With D-D fuel, a JET record neutron yield (R-NT = 5.6 x 10(16) s(-1)) was produced and, with D-T fuel, L-mode discharges with 7.2 MW of fusion power, and H-mode discharges with up to 8.2 MW of fusion power are obtained. Central ion temperatures approaching 40 keV, ion temper ature gradients of 150 keVm(-1) and plasma pressure gradients of 10(6) Pa m (-1) are observed in D-T leading to a fusion triple product n(i)T(i)tau(E) = (1.1 +/- 0.2) x 10(21) m(-3) keVs. Central toroidal rotation frequencies up to 37 kHz are observed and the rotational flow near the radius of the IT B is strongly sheared. These plasmas are prepared by applying Lower Hybrid Current Drive (LHCD) and Ion Cyclotron Resonance Heating (ICRH) during the early current ramp-up phase, "freezing-in" a flat or possibly slightly holl ow current density profile; into this target the main heating, consisting o f combined Neutral Beam Injection (NBI) and ICRH, is applied. By superposin g an ITB with an edge barrier (ELMy H-mode), near steady-state conditions ( H-89 = 2 for four energy confinement times) with high fusion yield have bee n achieved, offering a possible route towards quasi-steady-state. High fusi on yield, quasi-steady-state plasmas with confinement significantly above t he standard sawtoothing ELMy H-mode can be achieved in this way. The techni ques previously developed with D-D plasmas were modified to allow an L-mode ITB to form in D-T with a lower L to H-mode power threshold. The heating p ower and the q-profile were similar in both cases. ICRH plays a key role in the direct heating of core ions which allows real-time control of the cent ral pressure profile.