This paper reports results on the progress in steady-state high-βp ELMy H-mode discharges in JT-60U. A fusion triple product, nD(0)τETi(0), of 3.1 × 1020 m−3 s keV under full non-inductive current drive has been achieved at Ip = 1.8 MA, which extends the record value of the fusion triple product under full non-inductive current drive by 50%. A high-beta plasma with βN ∼ 2.7 has been sustained for 7.4 s (∼60τE), with the duration determined only by the facility limits, such as the capability of the poloidal field coils and the upper limit on the duration of injection of neutral beams. Destabilization of neoclassical tearing modes (NTMs) has been avoided with good reproducibility by tailoring the current and pressure profiles. On the other hand, a real-time NTM stabilization system has been developed where detection of the centre of the magnetic island and optimization of the injection angle of the electron cyclotron wave are done in real time. By applying this system, a 3/2 NTM has been completely stabilized in a high-beta region (βp ∼ 1.2, βN ∼ 1.5), and the beta value and confinement enhancement factor have been improved by the stabilization.

The paper discusses edge stability, beta limits and power handling issues for negative triangularity tokamaks. The edge MHD stability is the most crucial item for the power handling. For the case of negative triangularity the edge stability picture is quite different from that for conventional positive triangularity tokamaks: the 2nd stability access is closed for localized Mercier/ballooning modes due to the absence of magnetic well, and nearly internal kink modes set the pedestal height limit weakly sensitive to diamagnetic stabilization just above the margin of localized mode Mercier criterion violation. While negative triangularity tokamak is thought to have low beta limit with its magnetic hill property, it is found that plasmas with reactor relevant values of normalized beta β N > 3 can be stable to global kink modes without wall stabilization with appropriate core pressure profile optimization against localized mode stability and also with increased magnetic shear in the outer half radius. The beta limit is set by n=1 mode for the resulting flat pressure profile. The wall stabilization is very inefficient due to strong coupling between external and internal modes. The n>1 modes are increasingly internal when approaching the localized mode limit and set a lower beta in case of peaked pressure profile leading to Mercier unstable core. With the theoretical predictions supported by experiments, a negative triangularity tokamak would become a perspective fusion energy system with other advantages including larger separatrix wetted area, more flexible divertor configuration design, wider trapped particle free SOL, lower background magnetic field for internal poloidal field coils and larger pumping conductance from the divertor room.

The results of stabilizing neoclassical tearing modes (NTMs) with electron cyclotron current drive (ECCD) in JT-60U are described with the emphasis on the effectiveness of the stabilization. The range of the minimum EC wave power needed for complete stabilization of an m/n = 2/1 NTM was experimentally identified for two regimes using unmodulated ECCD to clarify the NTM behaviors with different plasma parameters: 0.2 < j EC /j BS < 0.4 for W sat /d EC ∼ 3 and W sat /W marg ∼ 2, and 0.35 < j EC /j BS < 0.46 for W sat /d EC ∼ 1.5 and W sat /W marg ∼ 2. Here, m and n are the poloidal and toroidal mode numbers; j EC and j BS the EC-driven current density and bootstrap current density at the mode rational surface; W sat , W marg and d EC the full island width at saturation, marginal island width and full width at the half maximum of the ECCD deposition profile, respectively. Stabilization of a 2/1 NTM using modulated ECCD synchronized with a mode rotation of about 5 kHz was performed, in which it was found that the stabilization effect degrades when the phase of the modulation deviates from that of the ECCD at the island O-point. The decay time of magnetic perturbation amplitude due to the ECCD increases by 50% with a phase shift of ±50 • from the O-point ECCD, thus revealing the importance of the phasing of modulated ECCD. For near X-point ECCD, the NTM amplitude increases, revealing a destabilization effect. It was also found that modulated ECCD at the island O-point has a stronger stabilization effect than unmodulated ECCD by a factor of more than 2.

Optimization studies have been done for the helical axis heliotron configuration. One purpose is to find a configuration suitable for experimental studies of the basic properties of a helical axis heliotron. In the present study, the role of the bumpy field component (toroidal mirror ratio) in MHD stability and neoclassical confinement for this type of configuration is examined. The physical mechanism of the improvement of the neoclassical transport through control of the bumpy field component is clarified. The physics design and current status of the new helical axis heliotron device, Heliotron J, are also described.