Study of carbon transport in the scrape-off layer of HL-2A with impurity sources located at limiter, dome, and divertor

摘要

Impurity transport in the scrape-off layer (SOL) has been studied in ohmically heated discharges of the HL-2A tokamak based on space-resolved vacuum ultraviolet spectroscopy. The vertical profile (from the plasma center to the lower X-point) of carbon emissions of CⅢ (977 A: 2s~2 ~1So-2s2p ~1P_1) and CIV (1548 A: 2s ~2S-2p ~2P) as well as the ratio of CⅣ to CⅢ were measured to investigate the edge impurity transport with relation to impurity source locations and sputtering characteristics. The experimental result shows that the impurity profile in the SOL has been clearly changed against different source locations. The emission of CⅢ and CⅣ from the mid-plane is stronger than that from the Ⅹ-point when the impurity source is located at the divertor plate. The profile becomes flat as a result. When the impurity source changes to the dome source, the profile clearly changes to a slightly peaked one, indicating the edge carbon emission at the Ⅹ-point is stronger than the mid-plane. The change to the limiter source makes the profile further peaked by increasing the carbon emission at the Ⅹ-point. In the case of the dome impurity source, the intensity of CⅢ_e and CⅣ_e, normalized to line-averaged electron density, n_e, decreases with n_e at low n_e (n_e ≤ 2.6 × 10~(19) m~(-3)) and becomes saturated at high n_e (n_e ＞ 2.6 × 10~(19) m~(-3)). In contrast, the ratio of CⅣ to CⅢ increases with n_e at low n_e and starts to decrease at high n_e. A numerical simulation with 3D edge plasma transport code, EMC3-EIRENE, suggests that a poloidal asymmetry in the impurity flow profile and an enhanced physical sputtering play an important role in the edge impurity distribution, particularly in the screening efficiency of C~(2+) and C~(3+) ions in the SOL region of the HL-2A tokamak.