Stability and Transport of Gyrokinetic Critical Pedestals

Abstract

A gyrokinetic threshold model for pedestal width-height scaling prediction is applied to multiple devices and to a shaping and aspect-ratio scan giving $Δ_{\mathrm{ped}} = 0.92 A^{1.04} κ^{-1.24} 0.38^δ β_{θ,\mathrm{ped}}^{1.05}$ for pedestal width $\hat{I}{”}_{\mathrm{ped}}$, aspect-ratio $A$, elongation $\hat{I}º$, triangularity $\hat{I}´$, and normalized pedestal height ${\hat{I}}_{\hat{I}¸,\mathrm{ped}}^2$. We also find a width-transport scaling $\hat{I}{”}_{\mathrm{ped}}=0.028{\left(q_e/\hat{I}{“}_e-1.7\right)}^{1.5}\sim \hat{I}{\cdot }_e^{1.5}$ where $q_e$ and $\hat{I}{“}_e$ are turbulent electron heat and particle fluxes and $\hat{I}{\cdot }_e=\nabla \ln T_e/\nabla \ln n_e$ for electron temperature $T_e$ and density $n_e$. Pedestals close to those limited by kinetic-ballooning-modes (KBMs) have modified turbulent transport properties compared to strongly driven KBMs. The role of flow shear is studied as a width-height scaling constraint and pedestal saturation mechanism for a standard and wide pedestal discharge.

34 pages, 16 figures