A study of charge dependence of particle transport using impurity pellet injection and high-spatial resolution bremsstrahlung measurement on the Large Helical Device

The spatial structure and Z dependence of transport coefficient were studied by injecting impurity pellets (C, Al, and Ti) and measuring high-spatial resolution bremsstrahlung in steady-state neutral beam heated plasmas on the Large Helical Device [A. Iiyoshi et al., Fusion Techol. 17, 169 (1990)]. The D and V were evaluated from a diffusive/convective model with minimizing the residual error between the experimental and computational bremsstrahlung intensities taking the recycling rate into account. As a result, the existence of an inward convection was observed at ρ>0.6, and no convection was required at ρ<0.6. Furthermore, it is found that the dependence of D on the electron density, the species and charge state of impurity is weak [typically D=0.15–0.25 (m2/s) in the range of ne=1.4–5.2×1019 (m−3)], but the inward convection has a strong dependence not only on the electron density but also the charge state. The spatial structure and Z dependence of the convective velocity for carbon and titanium ions were evaluated.