High-performance experiments towards steady-state operation in JT-60U

High-performance experiments with the aim of establishing a physics basis for advanced steady-state tokamak reactors have been carried out in JT-60U using two approaches; high-beta(p) H-mode and reversed-shear mode. In the high-beta(p) H-mode, where an internal transport barrier (ITB) formed in the positive-shear region is combined with an edge-transport barrier (H-mode), a quasi-steady state with the ELMy H-mode edge has been obtained through pressure profile control and its beta limit has been improved by increasing the plasma triangularity, delta. In the reversed-shear mode, a radially localized ITB including a clear electron-temperature pedestal is formed in the negative-shear region and very high confinement is obtained; H factors up to 3.3 have been achieved with an L-mode edge. The location of the ITB was well correlated to the location of q(min). Clear electron-and ion-temperature pedestals were sustained with a small density gradient in the combined heating experiments with ICRF + NBI. Large confinement improvement resulted from the large radius of the ITB and that of q(min) in the low q(min) region (q(min) similar to 2). The performance was limited by disruptive beta collapses with B(N) similar to 2 and q(min) similar to 2 and no steady-state was attained. The fusion performance was enhanced with the plasma current and the highest performance was achieved at 2.8 MA (q(95) = 3.1); 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. Optimization of both regimes will be continued, especially on the non-inductive current drive fraction and particle and heat control in the radiative divertor, using the negative-ion-based NBI and a newly installed W-shaped divertor.