Pedestal radial flux measuring method to prevent impurity accumulation

Use of high-z wall materials attempts to shift the fusion challenge from heat handling to impurity removal. We demonstrate that not only the impurity density in-out asymmetry, but also the poloidal flow, have a major impact on the radial impurity flux direction. The realization provides the first method of measuring the flux from available diagnostics, without the need of a computationally demanding kinetic calculation of the full bulk ion response. Moreover, it affords insight into optimal tokamak operations to avoid impurity accumulation while allowing free fueling. Favorable operation occurs if the temperature is more than twice as steep as the density, and inboard impurity accumulation occurs with poloidal flow in the magnetic field direction.