Kinetic-ballooning-limited pedestals in spherical tokamak plasmas
J. F. Parisi, W. Guttenfelder, A. Nelson, Rahul Gaur, A. Kleiner, M. Lampert, G. Avdeeva, J.W. Berkery, C. Clauser, M. Curie, A. Diallo, W. Dorland, S. Kaye, J. McClenaghan, F. I. Parra
Abstract
Abstract 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.