Inboard and outboard radial electric field wells in the H- and I-mode pedestal of Alcator C-Mod and poloidal variations of impurity temperature

摘要

We present inboard (HFS) and outboard (LFS) radial electric field (E_r) and impurity temperature (T_z) measurements in the I-mode and H-mode pedestal of Alcator C-Mod. These measurements reveal strong E_r wells at the HFS and the LFS midplane in both regimes and clear pedestals in T_z, which are of similar shape and height for the HFS and LFS. While the H-mode E_r well has a radially symmetric structure, the E_r well in I-mode is asymmetric, with a stronger ExB shear layer at the outer edge of the E_r well, near the separatrix. Comparison of HFS and LFS profiles indicates that impurity temperature and plasma potential are not simultaneously flux functions. Uncertainties in radial alignment after mapping HFS measurements along flux surfaces to the LFS do not, however, allow direct determination as to which quantity varies poloidally and to what extent. Radially aligning HFS and LFS measurements based on the T_z profiles would result in substantial inboard-outboard variations of plasma potential and electron density. Aligning HFS and LFS E_r wells instead also approximately aligns the impurity poloidal flow profiles, while resulting in a LFS impurity temperature exceeding the HFS values in the region of steepest gradients by up to 70%. Considerations based on a simplified form of total parallel momentum balance and estimates of parallel and perpendicular heat transport time scales seem to favor an approximate alignment of the E_r wells and a substantial poloidal asymmetry in impurity temperature.