Identifying small, emergent tearing modes in the plasma core using polarimetry and ECE data on DIII-D*

A comparison of line-integrated internal magnetic fluctuations due to tearing modes with local temperature fluctuations is made to identify small amplitude tearing modes. Because tearing modes can limit or even destroy confinement in a tokamak, detecting them early at small amplitude could facilitate control that limits their impact. Internal measurement of magnetic fluctuations via Faraday-effect polarimetry allows the detection of core-resonant tearing modes up to 300 ms before the edge sensing coils on the DIII-D tokamak[Pandya, DPP Invited 2021]. This is achieved by the Radial Interferometer Polarimeter (RIP), which measures the line-integrated fluctuating magnetic field, allowing the detection of tearing modes arising anywhere in the plasma. Here, we expand on previous work that correlated small-amplitude, core-localized magnetic fluctuations measured by RIP with electron temperature fluctuations observed locally via electron cyclotron emission (ECE). These temperature fluctuations likely correspond to an island, thus allowing us to identify the location of the mode resonant surface and estimate the island width. We initiate this work by benchmarking cases where the fluctuations are large on both RIP and ECE, followed by cases with small-amplitude fluctuations.