Effect of edge magnetic island on carbon screening in the J-TEXT tokamak

The effect of externally applied resonant magnetic perturbation (RMP) on carbon impurity behavior is investigated in the J-TEXT tokamak. It is found that the m/n = 3/1 islands have an impurity screening effect, which becomes obvious while the edge magnetic island is generated via RMP field penetration. The impurity screening effect shows a dependence on the RMP phase with the field penetration, which is strongest if the O point of the magnetic island is near the low-field-side (LFS) limiter plate. By combining a methane injection experimental study and STRAHL impurity transport analysis, we found that the variation of the impurity transport dominates the impurity screening effect. The impurity diffusion at the inner plasma region (r/a < 0.8) is enhanced with a significant increase in outward convection velocity at the edge region in the case of the magnetic island's O point near the LFS limiter plate. The impurity transport coefficient varies by a much lower level for the case with the magnetic island's X point near the LFS limiter plate. The interaction of the magnetic island and the LFS limiter plate is thought to contribute to the impurity transport variation with the dependence on the RMP phase. A possible reason is the interaction between the magnetic island and the LFS limiter.