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

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Abstract
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- H-mode and reversed-shear mode. In the high- 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, . 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 . 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 in the low region . The performance was limited by disruptive beta collapses with and 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 ; , and 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.