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 (R/LTi) is the strongest organizing parameter for both the B diffusion and convection and large R/LTi 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/χi) in qualitative agreement with theoretical predictions.

Peaking and hollowness of low-Z impurity profiles: an interplay between ITG and TEM induced turbulent transport

“…In this section, we test whether the quasi-linear results bear any meaning in the non-linear regime. For that, full numerical simulations of the simplified model ( 13) are performed with plasma parameters in line with values found in ASDEX Upgrade conditions [5,6,23]…”

“…While the transport of impurities has been explored in a variety of tokamak devices in the past decades, in the present work we choose as an experimental benchmark a database [23] obtained at ASDEX Upgrade [5,32] using CXRS diagnostics for the transport of Boron impurities. The authors validated their theoretical setup (gyrokinetic GKW and neoclassical NEO codes) on the measured data with a good overall level of agreement, but hollow profiles were, in general, poorly reproduced.…”

“…the ion-temperature-gradient (ITG) and trappedelectron-mode (TEM) instabilities [3], that affects mostly the ion dynamics. Unfortunately, the exact behavior of impurities cannot be always precisely predicted, especially in the case of hollow profiles, not even using state-of-the-art gyrokinetic simulations coupled with neoclassical codes [5,17,22,23].…”

“…The second goal of this work is to use the theoretical framework to compare our findings with realistic fusion experiments. For that, a recent database [23] developed at ASDEX Upgrade [32] from the measurement of transport coefficients of Boron impurities is considered. The results were obtained using charge exchange recombination spectroscopy (CXRS) diagnostic [5,6].…”

Radiation pressure by electron cyclotron waves on density fluctuations

“…The formalism leads to the conclusion that the radiation force of electron cyclotron waves on core fluctuations will lead to a peaking of the density profile. In experiments on ASDEX-U, peaking of the impurity density profile by EC waves has been observed [19].…”