Poloidal variation of Ar$^{16+}$ impurity density in Alcator C-Mod plasmas*

Vertical poloidal asymmetries in impurity density have been observed in a number of tokamaks but have yet to be quantitatively explained by neoclassical theory. This up/down asymmetry, thought to be driven by a combination of ion-impurity friction and inertial forces, is being investigated in the wider context of verifying parallel impurity force balance theory necessary to utilize trace impurity flow measurements to calculate main-ion flows. The recent installation of an x-ray crystal imaging spectrometer allows simultaneous measurement of the up/down asymmetry in Ar$^{16+ }$density as well as radial profiles of the impurity temperature and both the poloidal and toroidal flows. The up/down density ratio was measured in Ohmic and ICRF-heated L-mode plasmas and shown to vary from 0.5 to 3.0 over a range of electron density, 0.2 $<$ n$_{e} <$ 2.0 10$^{20}$ [m$^{-3}$] and plasma current, 0.4 $<$ I$_{p}<$ 1.2 [MA]. The impurity density, n$_{z}$, is typically higher in the direction away from the $\nabla $B drift, regardless of x-point location and the asymmetry is localized to r/a $>$ 0.75. At low density, n$_{e} <$ 0.5x10$^{20}$ [m$^{-3}$] the ratio falls below unity indicating a reversal. For n$_{e}$ $>$ 0.5x10$^{20}$ [m$^{-3}$] the ratio is shown to scale linearly with n$_{e}$/I$_{p}$, qualitatively consistent with theory.