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.
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.
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.
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