Asymptotic mass and charge scaling for trace impurity transport in tokamaks

Using an approximate quasilinear formulation of gyrokinetic theory, we derive a simple formula for the impurity particle transport in the limit of large charge za and mass ma. When za and ma are large and comparable, the flux decomposes into four separate contributions from curvature drift, parallel motion, density, and temperature gradients. In the extreme limit that ma≫za2, the flux obeys a simple 2-term scaling law proportional to za/ma. The results are limited to electrostatic gyrokinetic theory and are confirmed by direct nonlinear gyrokinetic simulations. The new findings contrast with historical emphases on charge-number scalings alone and have direct implications for tungsten transport in fusion plasmas. In particular, they indicate that most tungsten charge states, which share the same mass but differ in ionization, experience a mass-dependent suppression of impurity particle transport arising from finite-Larmor-radius effects in ion-scale turbulence.