Direct Identification of Microtearing Modes in the H-mode Pedestal via Current Profile Perturbations

We present a direct, time-dependent experimental identification of low-n microtearing modes (MTMs) in the H-mode pedestal. In a series of dedicated discharges on DIII-D, we decouple the resonant location and the instability drive of pedestal-localized MTMs by exposing the plasma to large, fast vertical oscillations which induce substantial edge current perturbations. These transient events restructure the edge q profile and modify the inter-ELM evolution of the pedestal ω_*,e profile, changing the dynamic frequency evolution of magnetic fluctuations from up-chirping to down-chirping and switching the mode numbers of pedestal instabilities. Clear and distinct changes in the mode response are both qualitatively and quantitatively explained by analytic profile-based predictions, providing a compelling experimental validation of the MTM model. Additional evidence for the MTM identification is given by measurements of propagation direction, gradient saturation and transport fingerprints. Since MTMs primarily drive electron heat flux through the pedestal region, these robust experimental results suggest that, in order to predict electron temperature profile effects, reduced pedestal models should include electromagnetic terms and accurate calculations of MTM stability.