Stability and Transport of Gyrokinetic Critical Pedestals

A gyrokinetic threshold model for pedestal width-height scaling prediction is applied to multiple devices. A shaping and aspect ratio scan is performed on National Spherical Torus Experiment (NSTX) equilibria, finding Delta(ped)=0.92A(1.04)kappa(-1.24)0.38(delta)beta(1.05)(theta,ped) for the wide-pedestal branch with pedestal width Delta(ped) , aspect ratio A, elongation kappa, triangularity delta, and normalized pedestal height beta(theta,ped). The width-transport scaling is found to vary significantly if the pedestal height is varied either with a 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(ped)=0.028(q(e)/Gamma(e)-1.7)(1.5)similar to eta(1.5)(e) and at fixed temperature Delta(ped)=0.31(q(e)/Gamma(e)-4.7)(0.85) similar to eta(0.85)(e), where q(e) and Gamma(e) are turbulent electron heat and particle fluxes and eta(e)=del lnT(e)/del lnn(e) for an electron temperature Te and density ne . Pedestals close to the KBM limit are shown to have modified turbulent transport coefficients compared to the 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 an NSTX experiment.