Extended JT-60U plasma regimes toward high integrated performance

With the main aim of providing physics basis for ITER and the steady-state tokamak reactor, JT-60U has been optimizing operational concepts and extending discharge regimes toward simultaneous sustainment of high confinement, high βN, high bootstrap fraction, full noninductive current drive and efficient heat and particle exhaust utilizing variety of heating, current drive, torque input and particle control capabilities. In the two advanced operation regimes, the reversed magnetic shear (RS) and the weak magnetic shear (high-βp) ELMy H modes characterized by both internal (ITB) and edge transport barriers and high bootstrap current fractions fBS, discharges have been sustained near the steady-state current profile solutions under full noninductive current drive with proper driven current profiles (High-βp; HHy2∼1.4 and βN∼2.5 with N-NB, RS; HHy2∼2.2 and βN∼2 with fBS∼80%). Multiple pellet injection has extended the density region with high confinement. These operational modes have been extended to the reactor relevant regime with small values of collisionality and normalized gyroradius and Te∼Ti. In the RS regime, QDTeq=0.5 has been sustained for 0.8s. Stability has been improved in these regimes by suppression of the neoclassical tearing mode with local ECCD and enhanced βN-values with wall stabilization. The ITB structure has been controlled by toroidal rotation profile modification and transport studies have revealed a semi-global nature of the ITB structure. The both-leg divertor pumping has enhanced He exhaust by ∼40%. Ar-puff experiments have improved confinement at high density with detached divertor due to high pedestal temperature Ti-ped. In H-modes, the core confinement degraded with decreasing Ti-ped suggesting stiff core profiles. The operational region of grassy ELMs with small divertor heat load has been established at high triangularity, high q95 and high βp. The record value of the neutral beam current drive efficiency of 1.55x1019 A/m2/W has been demonstrated by N-NB. Abrupt large amplitude events causing neutron drop have been discovered with frequency inside the TAE gap. Disruption studies have clarified that runaway current is terminated by MHD fluctuations when the surface q becomes 2∼3. (author)