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

Quasi-steady-state high β_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 β_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 β_p H mode plasma with H_89PL = 2.8, HH_y,2 = 1.4, β_p ≈ 2.0 and β_N ≈ 2.5 has been sustained for 1.3 s at small values of collisionality ν_e* and ion Larmor radius ρ_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 β_N^on and that at the mode disappearance β_N^off can be described as β_N^off/β_N^on = 0.05-0.4, which shows the existence of hysteresis. The value of β_N/ρ_i* at the onset of an m/n = 3/2 NTM has a collisionality dependence, which is empirically given by β_N/ρ_i* ∝ ν_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 be 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.