Microtearing Stability and Turbulence in the Pedestal: Linear Gyrokinetics, Reduced Models, and Nonlinear Turbulent Transport.

Magnetic confinement fusion research relies on accurate prediction of turbulent transport in tokamak edge plasma to optimize device operation. This work focuses on understanding electron heat transport driven by microtearing modes (MTMs). Analysis of conventional tokamak plasmas suggests that small-scale instabilities localized near the rational surface, such as microtearing modes, have a significant effect on confinement in tokamak.

MT draws on the electron temperature gradient as a free-energy source and rearranges magnetic topology through the creation ion-Larmor-radius-scale magnetic islands and thereby playing some role in determining the pedestal characteristics. In order to understand the causes and the evolution of the electron heat transport in tokamak discharges, a reduced kinetic transport model based on electromagnetic quasilinear theory has been implemented in the eigenvalue code Solve-AP.

This code is based on a current sheet model, combining the Fokker-Planck and Maxwell equations and evaluating them inside the resistive layer to obtain a system of equations linking the

vector potential and the electric potential. This system of equations is solved numerically as an eigenvalue problem. A reduced

transport model based on this code is tested and compared with gyrokinetic simulations using JET experimental data showing a good agreement. Analysis of nonlinear

gyrokinetic simulations shows that a quasilinear transport model for microtearing transport reproduces nonlinear trends for a variety of parameter regimes.

To achieve faster prediction of heat fluxes, a database using Solve-AP simulations has been developed. This database will be used to train a machine learning model, allowing for faster and prediction of heat electromagnetic heat fluxes in the future, particularly in the critical tokamak pedestal region.