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, , and scaling linearly with , it is observed over 0.8 < r/a < 1.0 to be of order unity and exhibits a threshold behaviour between (T1020 m−3 MA−1). The transition from a poloidally 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.
Poloidal asymmetry of parallel rotation measured in ASDEX Upgrade