Implementing Faraday effect measurement constraints into the Grad–Shafranov equilibrium fitting code EFIT

Abstract

A new tool for the exploration and diagnosis of the internal magnetic field of plasmas in the DIII-D tokamak in the form of a constraint on the EFIT (Equilibrium Fitting) Grad–Shafranov code based on the Faraday-effect Radial Interferometer-Polarimeter (RIP) diagnostic is presented, including description, verification, and sample application. The physics underlying the diagnostic and its implementation into EFIT are discussed, and the results showing the verification of the model are given, and the model’s limitations are discussed. The influence of the diagnostic’s input on the resulting equilibrium parameters is characterized. The effect of electron density profile refinement is evaluated and found to be negligible. A sample application of the diagnostic is shown, indicating that the RIP constraint has similar effects on the equilibrium as motional Stark effect constraints do.

Core inductive electric field during sawtooth crashes on DIII-D

“…Grad-Shafranov [15,16] equilibria were computed using the Faraday-constrained [23] equilibrium fitting code EFIT [17] at two time resolutions: equilibria were computed for each of the nine shots from 2400 ms to 4400 ms in 1 ms increments, covering the entire sawtooth cycle. As a proxy for when the crash occurs, the peak of the derivative of the signal from a poloidal magnetic field probe (located on the outboard midplane +37• toroidally from the RIP chords) was used.…”