Improvements of Ion Energy Confinement in Helium Rich Plasma of LHD

Full text: The improvement of the ion energy transport was observed in the He rich plasma of LHD. The ion thermal diffusivity around the edge pedestal region is lower than that of the gyro- Bohm prediction taking into account the effective ion mass and charge. The transport of different ion species is an important issue to predict the performance of ITER and the future reactor operation. In this paper, we report the results of a systematic study of effective ion mass and effective ion charge on ion energy transport of ion ITB plasma in LHD. With constant NB heating power (20-23 MW) at the same line averaged electron density (1.35 x 10¹⁹/m³), clearly higher T_i was observed at higher He concentration plasma. The fuelling ratio R, which is defined as R = n_H/(n_H + n_He) were controlled by the discharge cleaning and external gas fuelling. R changed from 0.34 to 0.79. At lower R (with higher He concentration), total ion density n_i, which was consist of n_H+, n_He²⁺, n_C⁶⁺, became lower. However, ion deposition powers per ion (P_i/n_i) were almost constant for different R. Thus, the achieved higher T_i at lower R is not due to the difference of P_i/n_i but due to the improvements of ion energy transport at lower R (He rich plasma). The T_i profiles shows edge pedestal formation both in He rich and H rich plasma. But T_i at pedestal top at p∼0.9 was higher in the He rich plasma. While in the inner region (p < 0.9), the normalized T_i gradients were almost constant for different R. At p = 0.4-0.9, ion scale turbulence (k ⊥ p_i∼0.4) were observed. The linear gyrokinetic analysis showed ITG was the dominant instability. Thus, within the T_i pedestal (p < 0.9), it is likely that the normalized T_i gradient is limited by ITG threshold. The effective ion thermal diffusivities (X_{i, eff}), which is the representative ion thermal diffusivity of H⁺, H^e2+ and C⁶⁺, were estimated from the power balance analysis. When X_{i, eff} is normalized by the gyro-Bohm factor, which is m_ieff^0.5T_i^1.5/(q²_ieff), where m_ieff is effective ion mass, q_ieff is the effective ion charge, the normalized X_{i, eff} is almost identical at p < 0.8 for different R, while the normalized X_{i, eff} is clearly lower at lower R (larger He contamination). This suggests transport improvements in edge region and corresponds to the higher edge T_i pedestal in He rich plasma. (author)