Error field estimation using variations to MHD mode frequency caused by EM torque

Changes to instantaneous mode frequency of rapidly-rotating MHD modes (f » 1/τ_wall) due to electromagnetic torque from 3d magnetic fields—including error fields—are used to estimate the relative amplitude and phase of the underlying fields. This work builds upon recent work [1] that similarly relies on the effect of EM torques from 3d magnetic fields on magnetic island dynamics. Whereas the previous work uses the torque balance of entrained modes at low to moderate rotation frequency (f < 100 Hz), this work uses magnetic measurements to calculate the changes in instantaneous mode frequency of rapidly-rotating modes (f > 1 kHz) for error field estimation. Presented here is an overview of the technique, theoretical considerations, and comparison to measurements in DIII-D plasmas. These initial results suggest a novel method for error field estimation that would be passive, non-disruptive, and capable of informing n > 1 error fields.



Work supported by US DOE under DE-FC02-04ER54698.

[1] Strait et al. Accepted to Plasma Physics and Controlled Fusion. 2025.