Speaker: Rachel Myers, Department of Physics Graduate Student
Abstract: Internal, resistive MHD instabilities called tearing modes (TMs) significantly contribute to disruptions and confinement degradation in tokamaks. Measuring nonlinear TM coupling with high m/n near the magnetic axis can aid in understanding neoclassical TM seeding, growth, and decay. For this tracking, we use the Radial Interferometer-Polarimeter (RIP), which is sensitive to core-resonant magnetic fluctuations on DIII-D and has detected TMs well before they appear on edge magnetic sensing coils [Pandya, DPP invited talk 2021]. Here we employ RIP to analyze the impact on pre-existing TMs of multiple, emerging MHD modes that are never detected by the coils. In one example, RIP detects a higher-frequency mode coupled to both an n=2 mode and a lower-frequency mode that later diminishes. All three modes are resonant near the magnetic axis, and their interaction is only visible using RIP. In this talk we will also discuss tearing mode structure and its potential effects on RIP-measured tearing mode amplitudes.