Experiments of suppressing fast-ion-driven MHD instabilities such as energetic particle modes (EPMs) and global Alfvén eigenmodes (GAEs) have been made by using a second harmonic X-mode electron cyclotron heating (ECH) and current drive (ECCD) in the helical-axis heliotron device, Heliotron J. ECCD experiments show that the GAE destabilized by fast ions of neutral beam injection (NBI) with the observed frequency around 140 kHz are fully stabilized, and the EPMs with the observed frequency around 90 kHz are suppressed when the EC-driven plasma current flowing in the counter direction reaches approximately 0.7 kA. The low magnetic shear under the vacuum condition is modified into positive magnetic shear when counter-ECCD is applied, and the amplitude of GAEs and EPMs decreases with an increase of the EC-driven plasma current. These results indicate that magnetic shear is a key role in controlling GAEs as well as EPMs. The comparison of the calculation of shear Alfvén spectra with experimental results shows that the increasing continuum damping rate with an increase in local magnetic shear by EC-driven current is important for both EPMs and GAEs. Moreover, the increase in plasma current lead to the inward movement of GAEs. This effect would also contribute to suppression of GAEs because the continuum damping rate increases more and more toward core. Steady ECH is also found experimentally to be effective to control the amplitude of both GAEs and EPMs. The amplitude of EPMs, and especially for GAEs decreases with an increase in the ECH power under fixed density conditions.

The positive isotope effects have been found in ECRH plasma of LHD. The global energy confinement time ( E ) in deuterium (D) plasma is 16% better than in hydrogen (H) plasma for the same line averaged density and absorption power. The power balance analyses showed that clear reduction of ion energy transport, while electron energy transport does not change dramatically. The global particle confinement time ( p ) is degraded in D plasma.  p in D plasma is 20% worse than in H plasma for same line averaged density and absorption power. The difference of the density profile was not due to the neutral or impurity sources, but rather was due to the difference of the transport. Ion scale turbulence levels show isotope effects. The core turbulence ( = 0.5 -0.8) level is higher in D plasma than in H plasma in low collisionality regime and is lower in D plasma than in H plasma. Density gradient and collisionality play a role in core turbulence level.

Isotope effects of ECRH plasma in LHD were investigated in detail. A clear difference of transport and turbulence characteristics in H and D plasmas was found in the core region, with normalized radius <0.8 in high collisionality regime. On the other hand, differences of transport and turbulence were relatively small in low collisionality regime. Power balance analysis and neoclassical calculation showed a reduction of the anomalous contribution to electron and ion transport in D plasma compared with H plasma in the high collisionality regime. In core region, density modulation experiments also showed more reduced particle diffusion in D plasma than in H plasma, in the high collisionality regime. Ion scale turbulence was clearly reduced at  < 0.8 in high collisionality regime in D plasma compared with H plasma. The gyrokinetic linear analyses showed that the dominant instability  =0. 5 and 0.8 were ion temperature gradient mode (ITG). The linear growth rate of ITG was reduced in high D plasma than in H plasma. This is due to the lower normalized ion temperature gradient and density gradient. More hollowed density profile in D plasma is likely to be the key control parameter. Present analyses suggest that anomalous process play a role to make hollower density profiles in D plasma rather than neoclassical process. Electron scale turbulence were also investigated from the measurements and linear gyrokinetic simulations.

The particle diffusion coefficient and convection velocity of electron cyclotron resonant heating plasmas have been evaluated using a density modulation experiment in the Large Helical Device. The diffusion coefficients and convection velocities were estimated by a newly developed analysis method. This method combines conventional quasi-Newton optimization and a genetic algorithm (GA). The GA finds the appropriate initial values. By giving the appropriate initial values for the quasi-Newton method, falling into an inappropriate solution which is not applicable for transport physics such as negative diffusion coefficients or extremely high values can be avoided. The diffusion coefficient obtained with the modulation experiment (D mod ) in the core region increases monotonically with the increase of the effective collisionality (n h *). The experimental results of D mod and the convection velocity (V mod ) can reconstruct the density profile at n < 1.5 h * , but they do not reconstruct it at n > 1.5 h * . The values of D mod were from several times to three orders higher than those obtained by neoclassical prediction. On the other hand, V mod was comparable with the neoclassical estimation. Ion-scale microturbulence was measured and compared with D mod and V mod . The comparison suggested possible linkage between the two.

Isotope effects are one of the most important issues for predicting future reactor operations. Large helical device (LHD) is the presently working largest stellarator/helical device using super conducting helical coils. In LHD, deuterium experiments started in 2017. Extensive studies regarding isotope effects on transport have been carried out. In this paper, the results of isotope effect studies in LHD are reported. The systematic studies were performed adjusting operational parameters and nondimensional parameters. In L mode like normal confinement plasma, where internal and edge transport barriers are not formed, the scaling of global energy confinement time (τ E) with operational parameters shows positive mass dependence (M 0.27; where M is effective ion mass) in electron cyclotron heating plasma and no mass dependence (M 0.0) in neutral beam injection heating plasma. The non-negative ion mass dependence is anti-gyro-Bohm scaling. The role of the turbulence in isotope effects was also found by turbulence measurements and gyrokinetic simulation. Better accessibility to electron and ion internal transport barrier (ITB) plasma is found in deuterium (D) plasma than in hydrogen (H). Gyro kinetic non-linear simulation shows reduced ion heat flux due to the larger generation of zonal flow in deuterium plasma. Peaked carbon density profile plays a prominent role in reducing ion energy transport in ITB plasma. This is evident only in plasma with deuterium ions. New findings on the mixing and non-mixing states of D and H particle transports are reported. In the mixing state, ion particle diffusivities are higher than electron particle diffusivities and D and H ion density profiles are almost identical. In the non-mixing state, ion particle diffusivity is much lower than electron diffusivity. Deuterium and hydrogen ion profiles are clearly different. Different turbulence structures were found in the mixing and non-mixing states suggesting different turbulence modes play a role.