A comprehensive study on impurity behavior in LHD long pulse discharges

Impurity behavior is studied in a variety of LHD (Large Helical Device) long pulse discharges, i.e. standard hydrogen plasmas, super dense core plasmas, helium plasmas with ICH (Ion Cyclotron Frequency Heating), multi-species plasmas mixed with H and He. Density scan experiments show a specific density range of impurity accumulation for only hydrogen discharges. Strong suppression of impurity accumulative behavior is observed in high temperature plasmas with high power heating. The main contributions to impurity transport are extracted by a comprehensive study on impurity behavior, i.e. investigating the critical conditions for impurity accumulation and the parameter dependences. It is found that the impurity behavior is determined by three dominant contributions, i.e. neoclassical transport mainly depending on radial electric field, turbulent transport increasing with heating power and impurity screening at high edge collisionality in the ergodic layer. The mapping of impurity behavior on n-T (electron density and temperature) space at the plasma edge shows a clear indication of the domain without impurity accumulation and provides operation scenarios to build up fusion-relevant plasmas. Keywords: Long pulse discharge, Impurity accumulation window, Radial electric filed, Impurity screening, Turbulent impurity transport