Modeling electron temperature profiles in the pedestal with simple formulas for ETG transport

TL;DR

This paper refines simple formulas for electron heat transport driven by ETG turbulence in the pedestal, accounting for geometry and shape, and tests their applicability across different tokamak configurations and experimental scenarios.

Contribution

The paper improves existing ETG transport formulas by enhancing parameterization and geometry considerations, extending their applicability to spherical tokamaks and various experimental conditions.

Findings

Model accurately predicts electron temperature profiles in steep gradient regions.

Pedestal temperature is highly sensitive to separatrix temperature and density.

Formulas are less accurate in low gradient, toroidal instability regimes.

Abstract

This paper reports on the refinement (building on Ref.~\cite{hatch_22}) and application of simple formulas for electron heat transport from electron temperature gradient (ETG) driven turbulence in the pedestal. The formulas are improved by (1) improving the parameterization for certain key parameters and (2) carefully accounting for the impact of geometry and shaping in the underlying gyrokinetic simulation database. Comparisons with nonlinear gyrokinetic simulations of ETG transport in the MAST pedestal demonstrate the model's applicability to spherical tokamaks in addition to standard aspect ratio tokamaks. We identify bounds for model applicability: the model is accurate in the steep gradient region, where the ETG turbulence is largely slab-like, but accuracy decreases as the temperature gradient becomes weaker in the pedestal top and the instabilities become increasingly toroidal in nature. We use the formula to model the electron temperature profile in the pedestal for four experimental scenarios while extensively varying input parameters to represent uncertainties. In all cases, the predicted electron temperature pedestal exhibits extreme sensitivity to separatrix temperature and density, which has implications for core-edge integration. The model reproduces the electron temperature profile for high $\eta_e = L_{ne}/L_{Te}$ scenarios but not for low $\eta_e$ scenarios in which microtearing modes have been identified. We develop a proof-of-concept model for MTM transport and explore the relative roles of ETG and MTM in setting the electron temperature profile. We propose that pedestal scenarios predicted for future devices should be tested for compatibility with ETG transport.