Temporal and spatial analysis of fields generated by eddy currents in superconducting magnets: Optimization of corrections and quantitative characterization of magnet/gradient systems

TLDR

Eddy‑current fields expanded into spherical harmonics reveal higher‑order terms that cannot be corrected by standard pre‑emphasis devices. The study proposes methods for spatial and temporal characterization of time‑dependent magnetic fields generated by eddy currents after gradient switching. By extracting a time‑dependent frequency shift γΔBz(t) and decay constant k(t) from the unresolved signal of an extended sample, the authors enable interactive optimization of multiexponential pre‑emphasis and spectral volume selection, and they suggest an imaging sequence to map the spatial distribution of eddy‑current fields at a chosen post‑gradient time point. The numerical analysis provides a quantitative tool for characterizing any magnet/gradient system’s eddy‑current performance. © 1991 Academic Press, Inc.

Abstract

Abstract We propose methods for the spatial and temporal characterization of time‐dependent magnetic fields generated by eddy currents after switching gradients. For an on‐line determination of the temporal variations of the fields, we extract two terms from the unresolved signal of an extended sample, describing the time evolution of a frequency shift γΔ B z (t) and of a decay constant k (t). This procedure allows us to optimize interactively the multiexponential pre‐emphasis as well as any spectral volume selection method with respect to eddy currents, Additionally, we suggest an imaging sequence which allows us to determine the spatial distribution of eddy current fields at a chosen time‐point after any gradient sequence to be tested. Expansion of these eddy currents fields into spherical harmonic functions proves the existence of a higher order terms, which cannot be corrected by a standard pre‐emphasis device, where time constants and amplitudes are adjusted on the X, Y, Z and Z 0 coils. The proposed numerical analysis gives a tool to characterize any magnet/ gradient system quantitatively with respect to eddy current performance. © 1991 Academic Press, Inc.