Using the Radial Interferometer-Polarimeter on DIII-D to reconstruct magnetic field fluctuations of the (1,1) tearing mode

Magnetohydrodynamic (MHD) kink/tearing modes with (m,n) = (1,1) (m and n being the poloidal and toroidal mode numbers respectively) are ubiquitously observed in tokamak plasmas undergoing sawtoothing. These modes have been observed to generate a non-negligible current/emf via dynamo action in steady-state, sawtooth free discharges [1]. This is evidenced by the cross-phase between fluctuating radial magnetic field δB_r and, fluctuating electron temperature δT_e (diamagnetic dynamo) or fluctuating density field δn_e (MHD or single fluid dynamo) deviating from a value of π/2. In this work, we demonstrate a technique for reconstructing the space and time resolved δB_r in sawtoothing discharges with (1,1) tearing mode using the Radial Interferometer-Polarimeter (RIP) diagnostic on DIII-D. The amplitude of δB_r is obtained by fitting the power of the signals from three polarimetry chords and one poloidal Mirnov coil using a scatter-search algorithm. The fitting function involves three fitting parameters which are assumed to evolve on a timescale much slower than the mode rotation. The magnetic perturbation amplitude peaks in the core at 30 G and the phase of δB_r w.r.t. δT_e is obtained from the phase of the RIP polarimetry signals. The reconstructed δB_r can then be used to study the different dynamo mechanisms and their effects on sawteeth.

This work was supported by the U.S. D.O.E. contract DE-FC02-04ER54698 and DE-SC0019004





[1] Mao et al., PRR 5(2), L022047 (2023)