Events on Friday, August 8th, 2025
- Thesis Defense
- Characterization of Tearing Mode Dynamics with the Radial Interferometer-Polarimeter on DIII-D
- Time: 10:00 am - 12:00 pm
- Place: B343 Sterling or
- Speaker: Rachel Myers, Physics PhD Graduate Student
- Abstract: The Faraday-effect Radial Interferometer-Polarimeter (RIP) diagnostic on DIII-D measures line-integrated magnetohydrodynamic fluctuations, such as tearing modes, at all radii, including modes not detected with external sensing coils. RIP results are compared to a synthetic line-integrated eigenfunction, corresponding to the mode amplitude on coils, using the linear code TJ. Even modes are finite on RIP, where the cylindrical approximation shows them vanishing. It is confirmed that RIP measures odd sidebands generated by linear coupling to toroidicity, which scales with inverse aspect ratio, and shaping. Experimentally measured shaping effects on RIP and tearing stability in inner-wall limited plasmas, which have more shape flexibility, are presented. Stability is calculated with DCON, which relies on equilibrium reconstructions with kineticEFIT. Modes grow as they approach the ideal-wall stability limit, which is near the classical tearing limit. These low-safety factor plasmas were dominated by sawteeth, which did not trigger tearing modes as expected. Tearing modes were driven as the plasma approached the ideal-wall limit, as opposed to the triggered, pressure-driven neoclassical tearing modes found in higher-performance plasmas. Even modes were less attenuated than previously measured because low safety factor induces a strong, detectable odd sideband to the even mode. RIP amplitudes varied over time due to nonlinear effects. RIP also experimentally confirms nonlinear coupling between edge harmonic oscillation (EHO) components in quiescent H-mode (QH-mode), previously predicted by nonlinear MHD simulations. We observe with RIP, but not sensing coils, nonlinear coupling of the EHO to core-resonant tearing modes, which interrupts the EHO and prevents access to long-lived QH-mode. This is concerning because QH-mode has an H-mode pedestal without edge localized modes (ELMs), which is attractive for ITER or a fusion reactor. Bicoherence values above 99% significance reveal that the first three EHO components are coupled throughout the QH phase, reflecting the cascade that sustains the EHO. In a case where QH-mode ceases and ELMy H-mode arises, we detect with RIP, but not with the coils, an intermittently returning EHO nonlinearly coupled to two core-resonant tearing modes. Coupling to tearing modes has caused the coupling between EHO components to vanish. The EHO becomes intermittent, while ELMs persist.
- Host: John Sarff/Brett Chapman