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-βp H-mode and reversed-shear mode. In the high-βp 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, δ. 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 qmin. 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 qmin in the low qmin region (qmin ∼ 2). The performance was limited by disruptive beta collapses with βN ∼ 2 and qmin ∼ 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 (q95 3.1); QDTeq = 1.05, τE = 0.97 s, nD(0) = 4.9x1019m-3 and Ti(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. (author)