The Gyrokinetic Critical Pedestal Constraint

We present a new framework that augments existing pedestal models by using gyrokinetic simulations to determine a stability boundary analogous to the ballooning critical pedestal (BCP) constraint in an EPED-like approach [1]. By incorporating the critical linear temperature and density gradients for dominant micro instabilities, we find a new pedestal pressure gradient constraint — the Gyrokinetic Critical Pedestal (GCP) constraint — for NSTX discharges. Local, linear gyrokinetic stability analysis is performed in CGYRO [2] by varying the experimental equilibrium self-consistently, which is then used to predict pedestal width and height. This self-consistent equilibrium calculation rescales temperature and density gradients starting from the experimental point. Since our model distinguishes between density and temperature profiles, we characterise how stability thresholds — and therefore pedestal evolution — are affected by varying temperature and density profiles. This analysis is performed at multiple time intervals in the inter-ELM buildup, which shows how turbulent transport evolves during the pedestal ELM cycle.

Calculating the GCP constraint not only provides stability information about kinetic ballooning modes (KBMs) --- which are similar to the ballooning modes captured by the BCP --- but also shows other microstabilities present during the pedestal evolution. Therefore, our model captures both stability and transport properties as the pedestal evolves.