Extended JT-60U plasma regimes for high integrated performance

With the main aim of providing the physics basis for ITER and steady state tokamak reactors, JT-60U has been optimizing operational concepts and extending discharge regimes towards simultaneous sustainment of high confinement, high βN, high bootstrap fraction, full non-inductive current drive, and efficient heat and particle exhaust utilizing a variety of heating, current drive, torque input and particle control capabilities. In the two advanced operation regimes (reversed magnetic shear (RS mode) and weak magnetic shear (high βp mode)), ELMy H mode discharges characterized by both internal and edge transport barriers and by high bootstrap current fraction fBS have been sustained near the steady state current profile solutions under full non-inductive current drive with appropriate driven current profiles (high βp: HHy2 ≈ 1.4 and βN ≈ 2.5 with negative ion based neutral beams (N-NBs); 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 0.8 s. Stability has been improved in the high βp mode regime by suppression of the neoclassical tearing mode with local ECCD and in the RS mode regime by wall stabilization. The internal transport barrier structure has been controlled by toroidal rotation profile modification, and transport studies have revealed a semiglobal dynamics of the internal transport barrier structure. Simultaneous pumping at the inner and the outer divertor leg has enhanced He exhaust by ~40%. Argon puff experiments have improved confinement at high density with detached divertor owing to high pedestal temperature Tiped. In H modes, the core confinement degraded with decreasing Tiped, suggesting stiff core profiles. An operational region of grassy ELMs with small divertor heat load has been established at high triangularity, high q95 and high βp. A record value of the neutral beam current drive efficiency of 1.55 × 1019A m-2 W-1 has been demonstrated using N-NB. Abrupt large amplitude events causing a drop in neutron emission 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 decreases to 2-3.

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