Transition from ITG to MTM linear instabilities near pedestals of parameterized H-mode plasmas

Investigation of linear gyrokinetic ion-scale (k_θ ρ_s =0.3) most unstable modes near pedestals of parameterized H-mode plasmas using CGYRO finds a transition from Ion Temperature Gradient (ITG) to microtearing mode (MTM) dominance is predicted as the density is increased to slightly above the Greenwald density limit for DIII-D sized tokamak. This transition (n_MTM) has a weak dependence on radial location in the region near the top of the pedestal (ρ=0.7-0.9), which allows probing single radii to examine the scaling of n_MTM with global parameters. The critical n_MTM is found to scale with plasma current. Additionally, increasing the minor radius by decreasing the aspect ratio and increasing the major radius are found to reduce n_MTM. However, any relationship between n_MTM and density limit physics remains unclear as n_MTM increases relative to the Greenwald density with larger minor radius and with larger magnetic field, suggesting the MTM transport may be less important for a reactor. Additionally, n_MTM is sensitive to the pedestal temperature, local electron and ion gradients, and the current profile. The stability of MTMs is then examined in DIII-D and NSTX H-mode experiments. It is found that MTMs are predicted to become unstable deeper into the pedestal top on DIII-D when the density is increased and energy confinement is reduced relative to empirical scaling expectations, supporting the results of the parametrized study.