As the finalization of the hydrogen experiment towards the deuterium phase, the exploration of the best performance of the hydrogen plasma was intensively performed in the Large Helical Device (LHD). High ion and electron temperatures, Ti, Te, of more than 6 keV were simultaneously achieved by superimposing the high power electron cyclotron resonance heating (ECH) on the neutral beam injection (NBI) heated plasma. Although flattening of the ion temperature profile in the core region was observed during the discharges, one could avoid the degradation by increasing the electron density. Another key parameter to present plasma performance is an averaged beta value . The high regime around 4 % was extended to an order of magnitude lower than the earlier collisional regime. Impurity behaviour in hydrogen discharges with NBI heating was also classified with the wide range of edge plasma parameters. Existence of no impurity accumulation regime where the high performance plasma is maintained with high power heating > 10 MW was identified. Wide parameter scan experiments suggest that the toroidal rotation and the turbulence are the candidates for expelling impurities from the core region.

The deuterium operation was initiated in the LHD in 2017. In the first campaign of the deuterium experiments, we successfully extended the high temperature regime in the LHD. The new record of the ion temperature of 10 keV associated with the ion internal transport barrier (ITB) was achieved due to several operational optimization. The confinement characteristics of ITB plasmas were compared between hydrogen and deuterium discharges. The ion thermal diffusivity was reduced in the ion-ITB plasmas with deuterium compared with the plasmas without deuterium. It was also found that the electron thermal confinement of the electron-ITB plasmas was clearly improved in the deuterium case.

We report the experimental finding of n=1 helical cores (HCs) accompanied by saturated m/n=2/1 tearing modes (TMs) with low mode frequencies in JT-60U. The HCs accompanied by TMs were observed after an increase in the mode amplitude and a decrease in the mode frequency of m/n=2/1 precursors with tearing parity. The decreased mode frequency is typically lower than 20 Hz. With various diagnostics, the coupling of n=1 HCs and m/n=2/ 1 TMs has been clearly observed. Because the coherent oscillations in the ion temperature are observed in both the core region and the edge region, the flux surfaces including the m/n=2/1 magnetic island appear to have m=1 helical deformation. It has also been suggested that the m/n=2/1 TM and the HC rotate in the electron diamagnetic direction keeping f m/n=1/1(HC) =2f m/n=2/1(TM) in several plasmas. Here, f m/n=1/1(HC) is the mode frequency of HCs and f m/n=2/1(TM) is the mode frequency of TMs. In addition, the core seems to be shifted to the high-field side when the O-points of the m/n=2/1 magnetic island line up in the midplane, which is confirmed by reconstructions of magnetohydrodynamic equilibria with motional Stark effect measurement and the MEUDAS code. Our observation of m/n=2/1 TMs having HCs contributes to the understanding of the excitation mechanism of HCs in tokamak plasmas.

The resistive interchange mode destabilized by the resonant interaction with the trapped energetic ions is fully suppressed when the injected power of electron cyclotron heating exceeds a certain threshold. It is shown for the first time that the complete stabilization of the energetic-particle-driven mode without relaxing the energetic particle (EP) pressure gradient is possible by reducing the radial width of the eigenmodes δ_{w}, especially when δ_{w} narrows to a small enough value relative to the finite orbit width of EP.

Energetic ion driven resistive InterChange modes (EICs) accompanying repeated bursts of the magnetic fluctuations were found in hydrogen campaign of Large Helical Device. The pressure gradient of helically trapped energetic particles, which are mainly produced by perpendicularly injected beams, drives this mode. Recently, perpendicular neutral beam injection (PERP-NBI) systems are upgraded for deuterium plasma campaign. The beam energies of the two PERP-NBIs are increased from 40/40 keV to 60/80 keV, respectively. And the injection powers increase from 6/6 MW to 9/9 MW, as well. As results of these upgrades of NBIs, the β value of helically trapped energetic ions, β h⊥ , has increased up to ∼ 0.35 % and induced EICs with larger bursts and smaller repetition frequencies. It is found that the threshold of β h⊥ to excite EICs increases with deuterium PERP-NBIs. The amplitude of each burst and effect on energetic beam ions become larger when β h⊥ is larger. In addition, a large electrostatic potential about -25 kV is observed when EICs are excited, which is about two times larger than the potential observed in hydrogen campaign. The transient increases of the electron density and temperature in edge regions are clearly observed when the electrostatic potential is formed.