Microtearing Thresholds and Second-Stable Ballooning in the DIII-D Pedestal: Reduced Modeling and Core-Edge Implications

Abstract

Global and local linear gyrokinetic simulations of 42 pedestal equilibria from three DIII-D discharges are used to investigate pedestal stability and its impact on pedestal structure and confinement. Microtearing modes (MTMs) and kinetic ballooning modes (KBMs) represent the main ion scale instabilities. For all three discharges, MTMs lie near a stability boundary in the mid-pedestal and exhibit threshold behavior, with growth rates increasing at and beyond pre-ELM pressure gradients. Pedestal MTMs retain conventional signatures but also show enhanced particle transport and partial density-gradient drive, indicating they can constrain pedestal {\it pressure} rather than electron temperature alone. KBMs are typically second-stable in this region due to low magnetic shear and large pressure gradients, though they can become active near the pedestal foot where magnetic shear is higher. These findings suggest MTMs play the role of inter-ELM pressure limit in the mid-pedestal when KBM is second stable. A preliminary quasilinear mixing-length transport model, with properly tuned free parameters, reproduces experimental temperature and density profiles when coupled to ASTRA. When applied to a case with doubled separatrix density, the model predicts reduced pedestal pressure consistent with ITPA H-mode confinement trends, attributable to increased MTM and ETG transport. These results clarify pedestal-limiting mechanisms and establish a physics-based link between separatrix conditions, pedestal structure, and global confinement. This work lays the foundation for new predictive modeling capabilities for core-edge integration in burning plasma regimes.

Figures (27)

• Figure 1: Pre-ELM electron temperature (top) and density (bottom) profiles for the three DIII-D discharges.
• Figure 2: The ensemble of density and temperature profiles for DIII-D shot 162940. Each panel shows a single density profile along with all corresponding temperature profiles. These are parameterized by the variations shown in Table \ref{['tab:variations']}.
• Figure 3: Plots related to classification and k-means clustering. Red is MTM and blue is all other instabilities (notably, KBM). Top: two-dimensional projection of the six-dimensional feature space onto the first two principal components (PCA), with points colored by k-means cluster assignment (k = 2). Middle: plot of the normalized frequency and parity of $A_{||}$ with symbols and colors denoting the two clusters. Bottom: plot of the normalized frequency and electromagnetic heat flux ratios with symbols and colors denoting the two clusters.
• Figure 4: Global growth rates (left) and frequencies (right) for low toroidal mode numbers (top) and high toroidal mode numbers (bottom) for GENE simulations of DIII-D discharge 162940. Colors denote the pressure gradient variations with lighter colors (yellow) corresponding to steeper pressure gradients. Note that parameter points with negative growth rates (i.e., no instabilities) are not plotted. Compare with mixing length estimates shown in Fig. \ref{['global_Dmix_162940']}, which will be discussed later.
• Figure 5: Global growth rates (left) and frequencies (right) for low toroidal mode numbers (top) and high toroidal mode numbers (bottom) for GENE simulations of DIII-D discharge 174082. Colors denote the pressure gradient variations with lighter colors (yellow) corresponding to steeper pressure gradients. Note that parameter points with negative growth rates (i.e., no instabilities) are not plotted. Compare with mixing length estimates shown in Fig. \ref{['global_Dmix_174082']}, which will be discussed later.