Long sustainment of quasi-steady-state high βp H mode discharges in JT-60U

Quasi-steady-state high beta (p) H mode discharges performed by suppressing neoclassical tearing modes (NTMs) are described. Two operational scenarios have been developed for long sustainment of the high beta (p) H mode discharge: NTM suppression by profile optimization. and NTM stabilization by local electron cyclotron current drive (ECCD)/electron cyclotron heating (ECH) at the magnetic island. Through optimization of pressure and safety factor profiles, a high beta (p) H mode plasma with H-89PL = 2.8. HHy.2 = 1.4. beta (p) approximate to 2.0 and B-N approximate to 2.5 has been sustained for 1.3 s at small values of collisionality v(e*) and ion Larmor radius rho (i*) without destabilizing the NTMs. Characteristics of the NTMs destabilized in the region with central safety factor above unity are investigated. The relation between the beta value at the mode onset beta (on)(N) and that at the mode disappearance beta (off)(N) can be described as beta (off)(N) / beta (on)(N) = 0.05-0.4, which shows the existence of hysteresis. The value of beta (N) / rho (i*), at the onset of an m/n = 3/2 NTM has a collisionality dependence, which is empirically given by beta (N) / rho (i*) proportional to v(e*)(0.36). However, the profile effects such as the relative shapes of pressure and safety factor profiles are equally important. The onset condition seems to he affected by the strength of the pressure gradient at the mode rational surface. Stabilization of the NTM by local ECCD/ECH at the magnetic island has been attempted. A 3/2 NTM has been completely stabilized by EC wave injection of 1.6 MW.