Identification of Multimode Interactions of Magnetic Fluctuations by Faraday-effect Polarimetry in DIII-D QH-mode Plasmas*

Measuring nonlinear tearing mode (TM) coupling with high m/n near the magnetic axis can aid in understanding neoclassical TM seeding, growth, and decay mechanisms. 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 the edge 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. We focus on plasmas in QH-mode, where infrequent ELMs make TMs more clearly identifiable. In one example, RIP detects an n=3 mode coupled to both an n=2 mode and a lower-frequency intermittent mode. The n=3 mode loses up to 98 percent of its energy during the coupling, while the other two modes grow. When the intermittent mode disappears, the n=3 mode begins to grow robustly. All three modes are resonant near the magnetic axis, and their interaction is only visible using RIP. In another example, the edge coils show an n=2 mode apparently dissipating in the absence of other modes, but RIP shows instead that it loses energy through coupling to higher-order modes.