Transition from ITG to MTM linear instabilities near pedestals of high density plasmas

Investigation of linear gyrokinetic ion-scale modes ( k(?)?(s) = 0.3) finds that a transition from ion temperature gradient to microtearing mode (MTM) dominance occurs as the density is increased near the pedestal region of a parameterized DIII-D sized tokamak. H-modes profile densities, temperatures, and equilibria are parameterized utilizing the OMFIT PRO_create module. With these profiles, linear gyrokinetic ion-scale instabilities are predicted with CGYRO. This transition (n(MTM)) has a weak dependence on radial location in the region near the top of the pedestal ( ? = 0.7 - 0.9), which allows simulating single radii to examine the approximate 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 that the transport due to MTM may be less important for a reactor. Additionally, n(MTM) is sensitive to the pedestal temperature, the local electron and ion gradients, the ratio of ion to electron temperature T-i / T-e, and the current profile. MTMs are predicted to be the dominant instability in the core at similar Greenwald fractions for DIII-D, NSTX, and NSTX-U H-mode experiments, supporting the results of the parameterized study. Additionally, MTMs continue to be the dominant linear instability in a DIII-D L-mode after an H-L transition as the plasma approaches a density limit disruption despite the large change in plasma profiles.

Automated summary

Investigation on linear gyrokinetic ion-scale modes reveals a transition from ion temperature gradient to microtearing mode (MTM) dominance as density increases in the pedestal region of a parameterized DIII-D sized tokamak. Profiles of H-mode densities, temperatures, and equilibria are parameterized to analyze the linear gyrokinetic ion-scale instabilities using CGYRO. The critical n(MTM) scales with plasma current and can be reduced by increasing the minor radius and major radius. However, the relationship between n(MTM) and density limit physics remains unclear.