Non-Neoclassical Up/Down Asymmetry of Impurity Emission on Alcator C-Mod

We demonstrate that existing theories are insufficient to explain up/down asymmetries of argon X-ray emission in Alcator C-Mod Ohmic plasmas. Instead of the poloidal variation, ñz/〈nz〉, being of order the inverse aspect ratio, ǫ, and scaling linearly with Btn̄e/I p , it is observed over 0.8 < r/a < 1.0 to be of order unity and exhibits a threshold behavior between 3.5 < Btn̄e/Ip < 4.0 [T10 20m−3/MA]. The transition from symmetric to asymmetric impurity distribution is shown to occur at densities just below those that trigger a reversal of the core toroidal rotation direction, thought to be linked to the transition between the linear and saturated Ohmic confinement regimes. A possible drive is discussed by which anomalous radial transport might sustain the impurity density asymmetry as the ratio of the perpendicular to parallel equilibration times, τ⊥,z/τ‖,z, approaches unity. This explanation requires a strong up/down asymmetry in radial flux which, while not observable on C-Mod, has been measured in TEXT and Tore Supra Ohmic plasmas.