High performance reversed shear plasmas with a large radius transport barrier in JT-60U

The operation of reversed. shear plasmas in JT-60U has been extended to the low-q, high-I-p region keeping a large radius transport barrier, and a high fusion performance has been achieved. Record values of deuterium-tritium (DT)-equivalent power gain in JT-60U have been obtained: Q(DT)(eq) = 1.05, tau(E) = 0.97 s, n(D)(0) = 4.9 X 10(19) m(-3) and T-i(0) = 16.5 keV. A large improvement in confinement resulted from the formation of an internal transport barrier (ITB) with a large radius, which was characterized by steep gradients in electron density, electron temperature and ion temperature just inside the position of q(min). Large negative shear regions, up to 80% of the plasma minor radius in the low-q(min) regime (q(min) similar to 2), were obtained by plasma current ramp-up after the formation of the ITB with the pressure and current profiles being controlled by adjustment of plasma volume and beam power. The ITB was established by on-axis beam heating into a low density target plasma with reversed shear that was formed by current ramp-up without beam heating. The confinement time increased with the radius of the ITB and the decrease of q(min) at a fixed toroidal field. High H factors, up to 3.3, were achieved with an L mode edge. The effective one fluid thermal diffusivity chi(eff) had its minimum in the ITB. The values of H/q(95) and beta(t) increased with the decrease of q(95), and the highest performance was achieved at q(95) similar to 3.1 (2.8 MA). The performance was limited by disruptive beta collapses with beta(N) similar to 2 at q(min) similar to 2.