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.

have started this year after a successful eight-year construction and test period of the fully superconducting facility. LHD investigates a variety of physics issues on large scale heliotron plasmas ͑Rϭ3.9 m, aϭ0.6 m͒, which stimulates efforts to explore currentless and disruption-free steady plasmas under an optimized configuration. A magnetic field mapping has demonstrated the nested and healthy structure of magnetic surfaces, which indicates the successful completion of the physical design and the effectiveness of engineering quality control during the fabrication. Heating by 3 MW of neutral beam injection ͑NBI͒ has produced plasmas with a fusion triple product of 8ϫ10 18 keV m Ϫ3 s at a magnetic field of 1.5 T. An electron temperature of 1.5 keV and an ion temperature of 1.4 keV have been achieved. The maximum stored energy has reached 0.22 MJ, which corresponds to ͗␤͘ϭ0.7%, with neither unexpected confinement deterioration nor visible magnetohydrodynamics ͑MHD͒ instabilities. Energy confinement times, reaching 0.17 s at the maximum, have shown a trend similar to the present scaling law derived from the existing medium sized helical devices, but enhanced by 50%. The knowledge on transport, MHD, divertor, and long pulse operation, etc., are now rapidly increasing, which implies the successful progress of physics experiments on helical currentless-toroidal plasmas.

The bootstrap current decreases in a more highly collisional regime, and in stellarator/heliotron it is predicted that a neoclassical current component proportional to the radial electric field exists when electrons and ions belong to different regimes of collisionality. To evaluate the bootstrap current in stellarator/heliotron in the whole range of collisionality from the collisionless 1/ν to the Pfirsch-Schluter regime, a new connection formula has been proposed. This connection formula has been applied to Large Helical Device (LHD) plasmas in which ions and electrons belong to different collisionality regimes, and finite beta MHD equilibria including the bootstrap current have been obtained. For LHD plasmas such as ECRH (T1 << Te) in electron root, the bootstrap current with an electric potential twice as high as the electron temperature reduces to about 1/5 to 2/3 of that with zero electric potential. Then, the MHD equilibrium configuration changes significantly, depending on collisionality and radial electric field, even for the same beta value of the LHD plasmas

Penetration of m/n = 1/1 resonant magnetic perturbation (RMP) in different magnetic configurations was investigated in the Large Helical Device (LHD). In the experiments with constant plasma parameters and heating condition, it was found that the mode penetration threshold increased linearly with increase in the magnetic shear. Also the threshold of penetration was increased by mitigating the magnetic hill. The amplitude of the perturbation field after the penetration was larger than that given by the RMP field. When the magnetic shear was further reduced by the plasma current and the plasma entered the ideal unstable regime, m/n = 1/1 minor collapse occurred after the mode rotation was decelerated and stopped. The occurrence of the collapse was independent of the existence of the error field.

In the Large Helical Device (LHD), non-rotating (NR) m/n = 1/1 mode has been observed in weak magnetic shear regime 1,2) , where the m and n is poloidal and toroidal mode number, respectively. When the magnetic shear is decreased by controlling external coils and/or driving plasma currents, the NR mode abruptly grow during a discharge and extends the flattening of pressure profiles.