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, τ E = 0.97 s, n D (0) = 4.9 × 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 ∼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 χ eff had its minimum in the ITB. The values of H/q 95 and β t increased with the decrease of q 95 , and the highest performance was achieved at q 95 ∼3.1 (2.8 MA). The performance was limited by disruptive beta collapses with β N ∼2 at q min ∼2.