Stability and transport of gyrokinetic critical pedestals

Abstract

A gyrokinetic threshold model for pedestal width-height scaling prediction is applied to multiple devices. A shaping and aspect-ratio scan is performed on NSTX equilibria, finding $\Delta_{\mathrm{ped}} = 0.92 A^{1.04} \kappa^{-1.24} 0.38^{\delta} \beta_{\theta,\mathrm{ped}}^{1.05}$ for the wide-pedestal branch with pedestal width $\Delta_{\mathrm{ped}}$, aspect-ratio $A$, elongation $\kappa$, triangularity $\delta$, and normalized pedestal height $\beta_{\theta,\mathrm{ped}}$. A width-transport scaling is found to vary significantly if pedestal height is varied either with fixed density or fixed temperature, showing how fueling and heating sources affect the pedestal density and temperature profiles for the kinetic-ballooning-mode (KBM) limited profiles. For an NSTX equilibrium, at fixed density, the wide-branch is $\Delta_{\mathrm{ped} } = 0.028 \left(q_e/\Gamma_e - 1.7 \right)^{1.5} \sim \eta_e ^{1.5}$ and at fixed temperature $\Delta_{\mathrm{ped} } = 0.31 \left(q_e/\Gamma_e - 4.7 \right)^{0.85} \sim \eta_e ^{0.85}$ where $q_e$ and $\Gamma_e$ are turbulent electron heat and particle fluxes and $\eta_e = \nabla \ln T_e / \nabla \ln n_e$ for electron temperature $T_e$ and density $n_e$. Pedestals close to the KBM limit are shown to have modified turbulent transport coefficients 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 lithiated wide pedestal discharge. Finally, the stability, transport, and flow-shear constraints are combined and examined for a NSTX experiment.