Impact of Faraday-effect-constrained equilibrium reconstruction on predicting instability onset in DIII-D

Understanding the behavior of equilibrium and fluctuating magnetic fields inside fusion plasmas is critical to maintaining control and stability and thus to enabling net fusion power generation. The radial Faraday-effect polarimeter on the DIII-D tokamak measures line-integrated internal magnetic fields at Z=0 and +/-13.5 cm with high temporal resolution up to the megahertz range for the full duration of plasma shots, making it useful for equilibrium reconstruction, especially when Motional Stark Effect measurements are not available, and for fluctuation evaluations. By using Faraday-effect-constrained Grad-Shafranov equilibrium reconstruction, a preliminary exploration of its impact is presented. The present status of the Faraday-effect constraint on EFIT is discussed, including its verification and sensitivity tests. The constraint is applied to questions concerning tokamak core physics: correlations between internal magnetic field and sawtooth crashes are investigated; current profile evolution during runaway electron plateaus are sought and characterized; correlation between current profile and tearing mode evolution is also investigated, particularly between current profile peaking and n=1 mode onset.