Modeling of the transition mechanism from electrostatic drift-type modes to electromagnetic kinetic ballooning mode-dominant regime in high poloidal-beta discharges

Abstract

Recent analyses (for example, see the paper by Staebler et al (2018 Phys. Plasmas 056113)) indicate that in DIII-D high poloidal-beta discharges turbulent transport, particularly for ions, is governed by the electromagnetic kinetic ballooning mode (KBM) in most radial regions, including the outer core between the internal and external transport barriers (ITBs and ETBs). Considering that in the usual L- or H-mode plasmas turbulent transport is widely believed to be dominated by the electrostatic drift-type modes, such as the ion temperature gradient (ITG) or trapped electron mode (TEM), a modeling study is presented to show how this transition of dominant modes can occur. In the ITB region, where there already exist several theoretical models, the focus is put on clarifying the relative role between the Shafranov shift and linear electromagnetic effects. While these two can play an important role in the transition or ITB formation by stabilizing the electrostatic drift-type modes, a significant difference is found in that the former mainly works on the TEM, while the latter works on the ITG. In contrast to the ITB region, in the outer core between the ITB and ETB the transition appears to be more relevant to the destabilization of the KBM itself, rather than the stabilization of electrostatic modes. A jump of edge plasma beta through the pedestal formation is shown to play the critical role in this destabilization by making the KBM threshold temperature gradient smaller than that of the ITG/TEM, thus allowing an earlier excitation of KBM when background temperature gradients increase by external heating.