Validation of low-Z impurity transport theory using boron perturbation experiments at ASDEX upgrade

Abstract

An experimental technique has been developed at ASDEX upgrade (AUG) to separately identify the diffusive and convective components of the boron particle flux. Using this technique a database of B transport coefficients has been assembled that shows that the normalized ion temperature gradient () is the strongest organizing parameter for both the B diffusion and convection and large is a necessary ingredient to obtain hollow B density profiles in AUG. This database also shows that large changes in the applied neutral beam injection (NBI) have a relatively small impact on impurity transport compared to similar changes in electron cyclotron resonance heating (ECRH). Even low levels of ECRH power dramatically increase both the diffusive and convective fluxes and lead to peaking of the impurity density profile. Comparisons to a combination of neoclassical and quasi-linear gyrokinetic simulations show good agreement in the measured and predicted diffusion coefficients. The outward convection measured in NBI dominated plasmas, however, is not well captured by the simulations, despite the inclusion of fast ions. In contrast, the convection is reasonably well reproduced for plasmas with flat or peaked boron density profiles. This dataset provides an excellent experimental validation of the non-monotonic, predicted, convective-particle-flux created by the combination of pure-pinch, thermo-diffusion, and roto-diffusion. In addition, this dataset demonstrates a non-monotonic dependence of the experimental particle diffusivity to ion heat conductivity (D/χ i) in qualitative agreement with theoretical predictions.