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, (n) over tilde (z)/< n(z)>, being of order the inverse aspect ratio, is an element of, and scaling linearly with B-t(n) over bar (e)/I-p(2), it is observed over 0.8 < r/a < 1.0 to be of order unity and exhibits a threshold behaviour between 3.5 B-t<(n) over bar (e)/I-p < 4.0 (T10(20) 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, tau(perpendicular to),(z)/tau(parallel to,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.