Edge impurity transport study in the stochastic layer of LHD and the scrape-off layer of HL-2A

Edge impurity transport has been investigated in the stochastic layer of the Large Helical Device (LHD) and the scrape-off layer (SOL) of the Huan Liuqi-2A (HL-2A) tokamak, as a comparative analysis based on the three-dimensional (3D) edge transport code EMC3-EIRENE and on the carbon emission profile measurement. The 3D simulation predicts not only an impurity screening effect in both devices, but also different impurity behaviour against collisionality and impurity source location between the two devices. The difference is caused by geometrical structures of the magnetic field lines in the stochastic layer and X-point poloidal divertor SOL, i.e. the number of poloidal turns of flux tubes affecting the poloidal distribution of plasma parameters and the impact of perpendicular transport on parallel pressure conservation and energy transport. These processes have an influence on the impurity screening efficiency at upstream and downstream positions of field lines. The carbon emission measured in the stochastic layer of LHD clearly indicates the screening effect in the high-density region. The result can be qualitatively interpreted by the present modelling, although the modelling shows a slight difference in the quantitative behaviour of carbon ions in the stochastic layer of LHD. On the other hand, a comparison of the carbon emission profile from HL-2A with the modelling is not straightforward. It is found that the impurity distribution in the HL-2A SOL is very sensitive to the impurity source location. In order to interpret the experimental observation a further study is necessary, in particular, on the impurity source distribution in the divertor plate and the first wall.