Exploring trends in Faraday-effect polarimetric measurements of the DIII-D tokamak core magnetic fields for future diagnostic and control applications

Faraday-effect polarimetry can provide fast time resolution (<1 μs) measurements of tokamak core magnetic fields without dependence on neutral beams. Using measurements from the three chords of the DIII-D tokamak’s Radial Interferometer-Polarimeter (RIP) diagnostic along with simple analytic models, one can obtain real-time information of the on-axis current density (J₀) and position of the magnetic axis (Z₀), in addition to line-integrated density (all important parameters for device control). Additional equilibrium parameters (such as on-axis safety factor q₀) can be obtained by constraining the Grad-Shafranov equilibrium fitting code EFIT with RIP measurements. To explore the bounds of validity of these models, and to gain a foundation for further refinement of the models’ accuracy and speed, a survey of >500 DIII-D plasma discharges has been conducted, chosen based on data availability for Faraday rotation, electron density profiles, and Motional Stark Effect (MSE), so as to make comparisons between analytic models, Faraday-constrained EFIT, and MSE-constrained EFIT, and EFIT constrained only with external coil measurements. The ITER baseline scenario on DIII-D is an example scenario found to benefit from internal magnetic constraints (Faraday or MSE) for accurate calculation of J₀ and q₀, to show q₀ < 1 during periods of sawtooth crash activity. Plasma parameters are explored for their impact on Faraday constraints’ accuracy, such as the impact of electron density, toroidal magnetic field, and plasma shape on the influence of the Cotton-Mouton effect, in order to show that real-time measurement of the position of the magnetic axis and J₀ are possible, and to next steps for potential application of Faraday-effect polarimetry to real-time diagnosis and control.