Impurity expulsion in an RFP plasma and the role of temperature screening*

In the improved confinement discharges of the Madison Symmetric Torus reversed field pinch, the density profile of $C^{+6}$, measured using Charge-Exchange Recombination Spectroscopy, is hollow. The core impurity density decays in time after the transition to improved confinement, concurrent with an increase of impurity density outside mid-radius, indicating an outward convection of impurities from the core of the plasma. Our analysis using neoclassical impurity transport theory shows that the observed hollow profile could possibly be explained by a mechanism known as ``temperature screening,'' where a thermal force due to a strong ion temperature gradient and high collisionality of $C^{+6}$ ions expel impurities from the core of the plasma.