A theoretical model is presented that for the first time matches experimental measurements of the pedestal width-height Diallo scaling in the low-aspect-ratio high-β tokamak NSTX. Combining linear gyrokinetics with self-consistent pedestal equilibrium variation, kinetic-ballooning, rather than ideal-ballooning plasma instability, is shown to limit achievable confinement in spherical tokamak pedestals. Simulations are used to find the novel Gyrokinetic Critical Pedestal constraint, which determines the steepest pressure profile a pedestal can sustain subject to gyrokinetic instability. Gyrokinetic width-height scaling expressions for NSTX pedestals with varying density and temperature profiles are obtained. These scalings for STs depart significantly from that of conventional aspect ratio tokamaks.
This paper presents a theoretical model that matches experimental measurements of the pedestal width-height scaling in the spherical tokamak NSTX. It shows that kinetic-ballooning, rather than ideal-ballooning, plasma instability limits the achievable confinement in spherical tokamak pedestals. The study provides novel scaling expressions for pedestal width and height in NSTX, which differ from conventional tokamaks.