Early internal detection of tearing modes in high-q_min DIII-D plasmas and correlation with ideal and resistive stability calculations

Faraday-effect polarimetry via the Radial Interferometer Polarimeter (RIP) detects internal magnetic fluctuations in high-q_min DIII-D plasmas up to 300 ms before the edge magnetic sensing coils. In these q_min > 1.4 plasmas, the goal of achieving g high ??_N is often limited by the emergence of tearing modes (TM). RIP measures the line integral of magnetic fluctuations across the plasma center, enabling detection of n > 1, core-resonant modes whose narrow eigenfunctions render them challenging to detect at the edge. The DCON ideal stability code was used to calculate ideal-wall kink-mode ??_N limits for different n as a proxy for linear TM stability. For these plasmas, the ??_N limit for the n = 3 mode is generally the lowest, and is detected on RIP before the n = 2 or 1 modes. In one example, a 6/3 mode (resonant at the q = 2 surface) occurs initially without 4/2 or 2/1 modes. This might be consistent with the lower ??_N limit for the n = 3 kink mode. However, resistive DCON shows that Delta' for the 6/3 TM is initially near zero and only increases later, after the equilibrium becomes ideally unstable, indicating that at initial detection, the mode may not be a classical TM.