Turbulent transport of impurities and their effect on energy confinement

By presenting linear and nonlinear gyrokinetic studies, based on a balanced neutral beam injection deuterium discharge from the DIII-D tokamak, we demonstrate that impurities alter the scaling of the transport on the charge and mass of the main species, and even more importantly, they can dramatically change the energy transport even in relatively small quantities. A poloidally varying equilibrium electrostatic potential can lead to a strong reduction or sign change of the impurity peaking factor due to the combined effect of the in-out impurity density asymmetry and the E×B drift of impurities. We present an approximate expression for the impurity peaking factor and demonstrate that impurity peaking is not significantly affected by impurity self-collisions.