Micro-Tearing Modes in the DIII-D plasma edge

The Micro Tearing Model (MTM) [T Rafiq et al. Phys. Plasmas 23 (2016) 062507] in the Multi-Mode Model 8.2 [T Rafiq et al. Phys. Plasmas 20 (2013) 032506] is used in OMFIT to examine the dependence of the MTM linear growth rate on plasma parameters in DIII-D. In DIII-D, these modes might contribute to the development of the H-mode pedestal structure and play a role in the electron thermal transport at the H-mode pedestal top. In this study, analysis of recent DIII-D discharges shows that MTMs can be unstable near the H-mode pedestal top even in the discharges with small or moderate β. Electromagnetic in nature, these modes are driven by the electron temperature gradients. However, MTMs also have electrostatic components that become important in the discharges with relatively small β. For these discharges, large electron density gradients become a primary mechanism of MTM triggering. The role of different plasma parameters in driving the MTM modes in the DIII-D discharges with different β is investigated in this research. The ranges of temperature and density scale lengths as well as plasma collisionality that are favorable for triggering of MTMs in the DIII-D plasma edge are found in a set of scans.