Quasi-steady-state high βp H mode dischargesperformed by suppressing neoclassical tearing modes (NTMs) aredescribed. Two operational scenarios have been developed for longsustainment of the high βp H mode discharge: NTM suppressionby profile optimization, and NTM stabilization by local electroncyclotron current drive (ECCD)/electron cyclotron heating (ECH) atthe magnetic island. Through optimization of pressure and safety factorprofiles, a high βp H mode plasma with H89PL = 2.8,HHy,2 = 1.4, βp ≈ 2.0 and βN ≈ 2.5 hasbeen sustained for 1.3 s at small values of collisionality νe*and ion Larmor radius ρi* without destabilizing theNTMs. Characteristics of the NTMs destabilized in the region withcentral safety factor above unity are investigated. The relation betweenthe beta value at the mode onset βNon and that at themode disappearance βNoff can be described asβNoff/βNon = 0.05-0.4, which shows theexistence of hysteresis. The value of βN/ρi* at theonset of an m/n = 3/2 NTM has a collisionality dependence, which isempirically given by βN/ρi* ∝ νe*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 moderational surface. Stabilization of the NTM by local ECCD/ECH at themagnetic island has been attempted. A 3/2 NTM has been completelystabilized by EC wave injection of 1.6 MW.
Overview of JT-60U progress towards steady-state advanced tokamak