Two-dimensional study of edge impurity transport in the Large Helical Device

Edge impurity transport has been studied in the stochastic magnetic field layer called the ergodic layer of the Large Helical Device. Chord-integrated full vertical profiles of C IV (312.4 Å: 1s²3p–1s²2s) near edge X-points are observed at horizontally elongated plasma cross sections. Measured C IV profiles are almost flat at low density (⩽2 × 10¹³ cm⁻³), while two peaks called the ‘X-point peak’ newly begin to appear near edge separatrix X-points with increase in density in addition to ordinary edge peaks. The X-point peaks become very clear at high-density range (⩾6 × 10¹³ cm⁻³). The C³⁺ ions analyzed with three-dimensional edge plasma transport code, EMC3–EIRENE, move upstream at low density due to large thermal force. The widely expanded C³⁺ distribution makes the C IV profile flat. With increasing the density, the friction force becomes dominant and the impurity ions start to move downstream. The C³⁺ ions stay in the vicinity of the edge X-point where magnetic field lines are very short. The X-point peaks are then clearly formed with increase in the C³⁺ density. Two-dimensional C IV distribution is also observed for different plasma axis positions. It is found that the C IV emission becomes strong along the poloidal trajectory of X-points and the C IV poloidal trace moves from inboard to outboard X-point trajectories when the plasma axis is changed from 3.60 to 3.75 m. The three-dimensional simulation can well reproduce the enhancement of C IV trajectory along the X-point. However, a discrepancy with the experiment is still seen in the C IV intensity between inboard and outboard X-points. (paper)