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 central electron temperature has successfully reached up to 7.5 keV in Large Helical Device (LHD) plasmas with a central high-ion temperature of 5 keV and central electron density of 1.3 × 10 19 m −3 . The result was obtained by heating with a newly-installed 154 GHz gyrotron and also optimization of injection geometry in electron cyclotron heating (ECH). The optimization has been carried out by using the ray-tracing code "LHDGauss," which has been upgraded to include the rapid post-processing three-dimensional (3D) equilibrium mapping obtained from experiments. For ray-tracing calculations, LHDGauss can automatically read the relevant data registered in the LHD database after a discharge, such as ECH injection settings (e.g., Gaussian beam parameters, target positions, polarization, and ECH power) and Thomson scattering diagnostic data along with the 3D equilibrium mapping data. The equilibrium map of the electron density and temperature profiles is then extrapolated into the region outside of the last closed flux surface. Mode purity, or the ratio between the ordinary mode and the extraordinary mode, is obtained by calculating the 1D full-wave equation along the direction of the rays from the antenna to the absorption target point. Using the virtual magnetic flux surfaces, the effects of the modeled density profiles and the magnetic shear at the peripheral region with a given polarization are taken into account. Power deposition profiles calculated for each Thomson scattering measurement timing are registered in the LHD database. Adjustment of the injection settings for the desired deposition profile from feedback provided on a shot-by-shot basis has resulted in an effective experimental procedure.

A simultaneous high ion temperature (T i ) and high electron temperature (T e ) regime was successfully extended due to an optimized heating scenario in the LHD. Such hightemperature plasmas were realized by the simultaneous formation of an electron internal transport barrier (ITB) and an ion ITB by the combination of high power NBI and ECRH. Although the ion thermal confinement was degraded in the plasma core with an increase of T e /T i by the on-axis ECRH, it was found that the ion thermal confinement was improved at the plasma edge. The normalized ion thermal diffusivity χ T i i 1.5 /at the plasma edge was reduced by 70%. The improvement of the ion thermal confinement at the edge led to an increase in T i in the entire plasma region, even though the core transport was degraded.

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