Nonlinear global simulations of microtearing transport in the pedestal

A thorough understanding of which processes set the residual turbulent transport in the pedestal region is a crucial step in determining the pedestal formation and its evolution. Recent linear gyrokinetic modeling indicates that Micro Tearing Modes (MTMs) reproduce key experimental features observed in several H-mode DIII-D pedestals. There are however no nonlinear simulations confirming these results due to the challenges associated with performing such runs.

We will present the first global nonlinear electromagnetic simulations (performed with the GENE code) of a DIII-D Elmy-H mode pedestal where, well within experimental error bars, the nonlinear electron heat transport is dominated at the ion scale by MTMs. Numerical results favorably compare with the experimental power balance as well as measured magnetic spectrograms. Electron scale contributions from ETG modes, also modeled with global gyrokinetic simulations, have a non-negligible contribution, suggesting that in the analyzed pedestal electron heat transport is regulated by a competition between these two transport mechanisms. Ion transport, in agreement with previous analysis, is dominated by neoclassical processes.

The kinetic ballooning limit is probed by varying the plasma profiles and plasma β , using both local and global simulations to assess the impact of the simulation model on the KBM threshold.