Speaker
Description
Since the electron density profile is a key factor determining the overall plasma performance, we investigated its impact on the behavior of core impurities in NBI-heated LHD plasmas. The experiments have been carried out in the LHD standard configuration, R$_{\rm{ax}}$ = 3.6 m and B$_{\rm{t}}$ = 2.75 T. A flat (slightly peaked) density profile was obtained when a tangential NBI was injected. In contrast, a peaked density profile was formed when a perpendicular NBI was injected. In addition to the baseline heating, a modulated perpendicular NBI was used as a probe beam for charge-exchange spectroscopy. The line-averaged electron density was set at around 2.4 x 10$^{19}$ m$^{-3}$ for both cases. It should be noted that the central electron temperatures are ~2.0 keV and ~1.6 keV for the flat and peaked density profiles, respectively, due to the different NBIs employed. A Fe impurity, the amount of which was the same in both cases, was injected into the plasma using different TESPEL sizes; the temporal behavior of the Fe impurity was tracked with EUV/VUV spectrometers, SOXMOS, EUV_Short, and EUV_Short2. In the case of a peaked-density plasma, for the smaller TESPEL, the electron density and temperature profiles did not respond significantly to the TESPEL injection; the Fe line emissions gradually decreased over time, then finally returned to the pre-injection level. For the larger TESPEL, the electron and ion temperatures in the core decreased significantly, and then they gradually recovered. The emission lines from higher charge-state Fe ions decreased below their pre-injection levels immediately after TESPEL injection and became nearly undetectable. At the same time, the emission lines from lower charge-state Fe ions increased significantly. These results indicate that, for the smaller TESPEL, the impurity accumulation predicted from the peaked density profile did not occur, and the injected Fe was almost completely expelled from the plasma. In contrast, for the larger TESPEL, Fe remained in the plasma for a relatively long period, while undergoing progressive changes in its ionization balance in response to the evolving electron temperature. In the case of a flat density plasma, for the smaller TESPEL, the impact on the plasma was similarly limited to that observed in the peaked density case, and no significant changes were observed. For the larger TESPEL, the electron temperature decreased in the region penetrated by the TESPEL. However, unlike in the peaked density case, the reduction in electron temperature did not propagate toward the plasma core. In this density-profile case, the line emissions from Fe ions exhibited similar behavior in both the smaller and larger TESPELs. The Fe line emissions were dominated by the higher charge states, Fe XXIII-XXIV, and remained at levels higher than those observed before the TESPEL injection, indicating that impurity accumulation occurred in both cases. These experimental results demonstrate that the impurity deposition location and the background ne profile both play essential roles in determining impurity confinement in LHD plasmas.