Transport Characteristics of Deuterium and Hydrogen Plasmas with Ion Internal Transport Barrier in LHD

Full text: A remarkable achievement of T_i0 = 10 keV with Z_eff = 2 was obtained in Large Helical Device (LHD). In order to clarify transport characteristics in ion internal transport barrier (ion ITB) formation with isotope effect, a dataset of pure deuterium (D) (n_D/n_e > 0.85) and pure hydrogen (H) (n_H/n_e > 0.85) plasmas for high-ion-temperature (high-T_i) regime were analyzed, and two mechanisms of transport improvement were characterized. A significant reduction of ion heat transport in D plasmas was observed in comparison between D and H plasmas, indicating non-gyroBohm mass dependence. The dependence of the heat transport on temperature ratio (T_e/T_i) and normalized T_i-gradient (R/LT_i = -(R/T_i)(d T_i/dr)) was investigated in the core region, in which gyrokinetic simulations with GKV code predicts the destabilization of ITG modes. The T_e/T_i dependence shows ITG-like property, while a significant deviation from the ITG-like property is found in the R/LT_i dependence. Moreover, the density fluctuation is well correlated with the heat transport dependence on T_e/T_i and R/LT_i , indicating suppression of ITG mode in large R/LT_i regime and resultant ion ITB formation. The similarity of instabilities found by GKV indicates that both ITG suppression and isotope effect contribute to production of high-T_i plasmas (T_i0 ∼ 10 keV) with multiple-ion conditions. (author)