Operation at high performance in optimized shear plasmas in JET

Heating during the early part of the current rise phase gives a low or negative magnetic shear (= r/q(dq/dr)) in the centre of JET plasmas. Under these conditions the confinement improves with high additional heating power heating during the current ramp-up phase of the discharge. The reduction in the transport manifests itself as a peaking of the profiles with a large gradient region near r/a = 0.55. The best discharges have no transport barrier at the edge of the plasma(l-mode). This allows central power deposition by the neutral beams in JET. A control of the plasma pressure, using feedback of the additional heating power in real-time, minimizes the impact of magnetohydrodynamic instabilities. As a result, these discharges achieve the highest D-D neutron rates in JET; S-n = 5.6 x 10(16) neutrons s(-1), with n(e0) approximate to 6 x 10(19) m(-3), T-e0 approximate to 12 keV and T-i0 approximate to 26 keV.