Isotope effect on energy confinement time and thermal transport in NBI-heated plasmas on LHD

Energy confinement time and thermal transport have been investigated in NBI heated hydrogen (H), deuterium (D), and mixture (H&D, H&D&He) plasmas in a large-scale stellarator-heliotron LHD. Comparison of H and D plasmas has not shown significant mass dependence in energy confinement time, which is not consistent with unfavourable mass dependence predicted by a gyro-Bohm model. Nonetheless, different aspect emerges when the scaling expression of energy confinement is rephrased into dimensionless parameters. The expression in dimensionless parameters ? _E^scl_,thOi f M ^0.99 ?_* -^2.98 suggests co-existence of a clear mass dependence and gyro-Bohm nature. Comparison of thermal diffusivity for dimensionally similar H and D plasmas in terms of ?*, ?*, and ß has clearly shown robust confinement improvement in deuterium to compensate unfavorable mass dependence predicted by the gyro-Bohm model. Whichever mixture plasmas of H and D, or H,D, and He lie on the scaling derived from H and D plasmas. Even regression analysis of energy confinement time including M and Z_eff of extended dataset including mixture plasmas has shown significant dependence on neither M nor Zeff.