Reduced predictive models for Micro-tearing modes in the pedestal*

This talk will present a global reduced model for microtearing modes (MTM) in the H-mode pedestal, which reproduces distinctive features of experimentally observed magnetic fluctuations, such as chirping and discrete frequency bands with definite mode numbers. Our model, importantly, includes the global variation of the q profile and diamagnetic frequencies.  In contrast, the local flux tube approach fails to reproduce these distinctive features.  Our model includes a recently recognized feature for MTM stability; the MTM is enabled by the alignment of a rational surface with the peak in the profile of the diamagnetic frequency. Conversely, MTMs are strongly stabilized for toroidal mode numbers for which these quantities are misaligned.  This feature explains the discrete fluctuation bands in several DIII-D and JET discharges.  Gyrokinetic global simulations of those discharges using the GENE code have also demonstrated these concepts. In this talk, we present the application of the model to many representative experimental pedestal conditions from DIII-D, by comparing the experimental spectrogram with the global reduced model and GENE simulations. The experimentally observed frequency bands overlap with the predicted frequencies of the global reduced model. The growth rate calculated by such a reduced model has a qualitative agreement with GENE global linear simulations. We also characterize the parameter regimes and scenarios in which the MTM is expected to be active. Based on recent combined experimental, theoretical, and simulation studies, it is well established that MTMs are one of the prime candidates for electron heat transport in the pedestal. A fast yet accurate reduced model for MTMs enables rapid interpretation of magnetic fluctuation data from a wide range of experimental conditions to help assess the role of MTM in the pedestal.