Comparison of resonant magnetic perturbation-induced particle transport changes in H-mode (DIII-D) and L-mode (MAST)

Abstract

Recent experiments show the impact of resonant magnetic perturbations (RMPs) on the density (Schmitz et al 2008 Plasma Phys. Control. Fusion 50 124029, Evans et al 2008 Nucl. Fusion 48 024002, Kirk et al 2008 Nucl. Fusion 50 024002, Liang et al 2007 Phys. Rev. Lett. 98 265004), leading to a so-called density pump-out. Previous comparisons between DIII-D and TEXTOR have focused on the similarities of the deformation of the separatrix and the creation of striations at the intersection of the main chamber wall (Schmitz et al 2008 Plasma Phys. Control. Fusion 50 124029; Schmitz et al 2009 Phys. Rev. Lett. 103 165005). In this paper, we compare the difference in magnitude of the experimentally observed density pump-out in L-mode with H-mode in two diverted tokamaks: MAST and DIII-D. In order to address the differences in magnetic field from the coils, plasma shape and q95 between the two devices, we compute a weighted magnetic diffusion coefficient with a vacuum field line tracing code. This allows us to compare the changes in density pump-out with the weighted magnetic diffusion coefficient, using a simple particle diffusion model. We find that the density pump-out is vastly different in the two confinement regimes, suggesting different particle transport mechanisms. Since one main difference in transport characteristics between L- and H-mode is turbulence, we compare turbulent particle characteristics. We find that in L-mode (MAST) the fluctuations and E × B shear increase at the plasma edge, whereas in H-mode (DIII-D) the fluctuations decrease at the plasma edge. Deeper inside the core, the E × B shear remains similar in L-mode (MAST), whereas a large decrease that quickly saturates with RMP strength is observed in H-mode (DIII-D). These results suggest that the RMP-induced particle transport at the plasma edge in L-mode (MAST) is the result from increases in turbulent particle transport, whereas the results in H-mode (DIII-D) suggest a decrease in turbulent particle transport.