Calculating an extended-MHD pedestal structure constraint

We present developments towards a higher fidelity model to consistently predict edge-localized mode (ELM) onset and the limiting pedestal structure in spherical torus (ST) configurations. It is well known that in reactor-scale tokamaks ELMs can severely damage plasma-facing components, and thus either need to be avoided or mitigated. One way to discover ELM-free operation and design scenarios for future operation is to predict the limiting edge pedestal structure, i.e. pedestal width, height and gradients that lead to ELM onset. The macroscopic stability constraint, which determines the onset of peeling-ballooning modes is often modeled within the ideal-MHD framework. However, it was recently shown that in NSTX and JET-ILW resistive effects can crucially shift the stability threshold. To incorporate extended-MHD effects and thus capture macroscopic edge stability limits more accurately we develop a new model consisting of an equilibrium variation that scales the pedestal width and height with self-consistent bootstrap current and different scaling laws for the kinetic profiles. These equilibria are then used as initial conditions for linear stability simulations with M3D-C1, which enables us to obtain a higher-fidelity macroscopic pedestal stability constraint. We present this new tool and simulation results for ELMing NSTX discharges as well as for spherical tokamak model equilibria.