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.

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A tungsten block is supposed to be used as a divertor armor material on the helical reactor FFHR-d1. On the other hand, material selection of the heat sink and bonding technique between armor and heat sink are currently under investigation. On the material selection, copper alloy has a large advantage for the thermal conductivity, but its material properties such as toughness and thermal conductivity, are dramatically decreased due to the neutron irradiation. However, from the assessment of the neutronics environment on the divertor region of the FFHR-d1, copper alloys could be used for a heat sink especially at the outer divertor. In the ITER case, copper alloy (CuCrZr) pipes are joined by a brazing technique with Nicuman37 filler material. This combination has not been optimized for the FFHR-d1, because the toughness of the CuCrZr at high temperature over 450 • C is dramatically decreased with increasing the temperature. As such, another candidate is an oxide dispersion-strengthened copper alloy (ODS-Cu) such as GlidCop R . For the bonding technique, a reliable brazing combination between "two kinds of copper alloys" and "three kinds of filler materials (MBF-20, BNi-6, Nicuman37)" were investigated from a viewpoint of mechanical strength. The most superior fracture strength among the three filler materials was BNi-6 with GlidCop R .

Static discontinuous recrystallization was studied during room temperature annealing of a newly designed 80% room temperature rolled oxide dispersion strengthened copper. At early stages of annealing, fine new oriented nanosized/submicron grains were recrystallized in the unique matrix of single brassoriented deformed grain. Upon longer annealing time up to 14 months, the size, area fraction and number density of the recrystallized grains increased significantly along with changing the crystallographic textures. The analysis of misorientation angle distribution of boundaries indicated transformation of low angle boundaries to high angle boundaries results in nucleation of recrystallized grains by significant contribution of static recovery. Furthermore, the constant level of mechanical hardness after recrystallization was interpreted by the balance between grain size hardening, oxide particle hardening and strain hardening.