Recent results on investigations of Alfvén eigenmodes, fast ion confinement and fast ion diagnostics in JT-60U are presented. It was found that toroidicity induced Alfvén eigenmodes (TAEs) were stable in negative shear discharges with a large density gradient at the internal transport barrier (ITB). If the density gradient was small at the ITB, multiple TAEs appeared around the q = 2 surface (pitch minimum) and showed a large frequency chirping (∆f ≈ 80 kHz). In low-q positive shear discharges, the location of the TAEs changed from outside to inside the q = 1 surface, owing to a temporal change of the q profile. A significant depression of the megaelectronvolt ion population was observed only with high-n (n up to 14) multiple TAEs inside the q = 1 surface. Non-circular triangularity induced Alfvén eigenmodes were observed for the first time. Considerable depression of the triton burnup was observed in negative shear discharges. Orbit following Monte Carlo simulations indicated that ripple loss was responsible for the enhanced triton losses. The fast ion stored energies in ICRF heated negative shear discharges were comparable to those of positive shear plasmas. Tail ion temperatures in high (second to fourth) harmonic ICRF heating experiments were first analysed with an MeV neutral particle analyser. The behaviour of MeV ions produced by ICRF heating was studied with gamma ray diagnostics. A scintillating fibre detector system for detecting the 14 MeV neutron emission was developed for the triton burnup studies. Ion cyclotron emission measurements discriminating between parallel and perpendicular components of the electric field were carried out for the first time.

Instabilities with frequency chirping in the frequency range of Alfvén eigenmodes have been found in the domain 0.1% < β h < 1% and v b /vA ∼ 1 with high energy neutral beam injection in JT-60U. One instability with a frequency inside the Alfvén continuum spectrum appears and its frequency increases slowly to the toroidicity induced Alfvén eigenmode (TAE) gap on the timescale of an equilibrium change (≈200 ms). Other instabilities appear with a frequency inside the TAE gap and their frequencies change very quickly by 10-20 kHz in 1-5 ms. During the period when these fast frequency sweeping (fast FS) modes occur, abrupt large amplitude events (ALEs) often appear with a drop of neutron emission rate and an increase in fast neutral particle fluxes. The loss of energetic ions increases with a peak fluctuation amplitude of Bθ /B θ . An energy dependence of the loss ions is observed and suggests a resonant interaction between energetic ions and the mode.

In this paper, experimental observations of spontaneously excited waves in the ion cyclotron range of frequency (ICRF) on JT-60U are described. The fluctuations in ICRF are driven by the presence of non-thermal ion distribution in magnetically confined plasmas. Two types of magnetic fluctuations are detected: one is due to high-energy D ions from neutral beam (NB) injections and the other is due to fusion products (FPs) of 3He and T ions. These fluctuations have been reported as ion cyclotron emissions (ICEs) in the burning plasma experiments on large tokamaks. This paper describes the first measurement of the spatial structures of the excited modes in the poloidal and toroidal directions. It is confirmed by using ICRF antennas as pickup loops that all modes excited spontaneously have magnetic components. The modes due to D ions have zero or a small toroidal wave number k z . On the other hand, the measurement of finite k z in the modes due to FP ions supports the excitation of the Alfvén waves, which is the possible origin of FP-ICEs. It is also observed that the excited modes due to FP ions (3He and T ions) have different characteristics: driven by different NBs and having different parameter dependences. ICE due to T ions has no harmonics and the value of ω/Ωci is smaller than that due to 3He. Both beam-driven ICEs and FP-ICEs are clearly observed and their spatial structures are obtained on JT-60U.

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.