HIGH-PERFORMANCE EXPERIMENTS TOWARDS STEADY-STATE OPERATION IN JT-60U

High-performance experiments with the aim of establishing a physics ba sis for advanced steady-state tokamak reactors have been carried out i n 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-tra nsport barrier (H-mode), a quasi-steady state with the ELMy H-mode edg e has been obtained through pressure profile control and its beta limi t has been improved by increasing the plasma triangularity, delta. In the reversed-shear mode, a radially localized ITB including a clear el ectron-temperature pedestal is formed in the negative-shear region and very high confinement is obtained; H factors up to 3.3 have been achi eved with an L-mode edge. The location of the ITB was well correlated to the location of q(min). Clear electron-and ion-temperature pedestal s were sustained with a small density gradient in the combined heating experiments with ICRF + NBI. Large confinement improvement resulted f rom the large radius of the ITB and that of q(min) in the low q(min) r egion (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 s teady-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 conti nued, especially on the non-inductive current drive fraction and parti cle and heat control in the radiative divertor, using the negative-ion -based NBI and a newly installed W-shaped divertor.