Edge and core impurity behavior and transport in EAST H-mode plasma with internal transport barrier

After installation of tungsten divertor in EAST, impurity accumulation of tungsten ions has been frequently observed in H-mode discharge with internal transport barrier (ITB) due to an enhancement of the impurity confinement inside the ITB. A strong plasma cooling induced by the tungsten impurity ions caused a collapse of the ITB formation. To study the impurity transport in high βN discharges with ITB, temporal behaviors and radial profiles of spectral lines emitted from low- and high-Z impurity ions were analyzed. Line emissions from moderately ionized ions, e.g. O6+, Fe17+ and Mo25+ locating outside the ITB (ρ ⩾ 0.4), maintain low intensities and remain unchanged during the ITB formation. However, line emissions from highly ionized high-Z impurity ions such as Fe22+, Cu21+, Cu25+, Mo30+ and W26+-W37+ locating inside the ITB (ρ < 0.4) are strongly influenced by peaking effects of electron density and ion temperature profiles. The impurity screening effect due to the ion temperature peaking is dominant during Ti-ITB phase because the line intensities of high-Z impurity ions are reduced and the radial high-Z impurity profiles are flattened. In contrast, during ne-ITB phase accompanied by electron and ion temperature ITB, an increase in the electron density gradient from R/Lne = 3.4–4.9 results in a significant increase in the high-Z impurity density, and leads to the impurity accumulation. Statistical analysis on the tungsten impurity density (IW-UTA/ne) with toroidal rotation velocity (Vt0) and ion temperature gradient (R/LTi) suggests that the tungsten impurity accumulation can be effectively mitigated and the IW-UTA/ne can be reduced to below 18 (phs·m·s−1·Sr−1), when R/Lne < 3.4, Vt0 < 150 km·s −1 and R/LTi > 2.5. During two ITB phases, however, low-Z impurity ions like O7+ locating at edge of the ITB appear to be sensitive to only the electron density gradient.