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

Quasi-steady state high βp H-mode discharges performed by suppressing neoclassical tearing modes (NTM) is described. Two operational scenarios have been developed for long sustainment of the high βp 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. By optimizing pressure and safety factor profiles, a high βp H-mode plasma with H89PL=2.8, HHy2=1.4, βp=2.0, βN=2.5 has been sustained for 1.3 s at small values of νe* and ρi* without destabilizing the NTMs. Characteristics of the NTMs destabilized in the region of the central safety factor above unity are investigated. Relation between the beta value at the mode onset βNon and that at the mode disappearance βNoff can be described as βNoff=0.05-0.4 βNon, which shows the existence of hysteresis. The value of βN/ρi* at the onset of an m/n=3/2 NTM has collisionality dependence, which is empirically given by βN/ρi*∝νe*0.36. However, the profile effects are equally important, such as relative shapes of pressure and safety factor. Onset condition seems to be affected by 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. The 3/2 NTM has been completely stabilized by EC wave injection of 1.6 MW. (author)