Ultra-sensitive SQUID Systems for Pulsed Fields—Degaussing Superconducting Pick-Up Coils

Abstract

SQUID systems for ultra-low-field magnetic resonance (ULF MR) feature superconducting pick-up coils which must tolerate exposure to pulsed fields of up to 100 mT. Using type-II superconductor niobium (Nb) field distortions due to trapped vortices in the wire result. In addition, their rearrangement after quick removal of the pulsed field leads to excess low frequency noise which limits the signal-to-noise ratio. In contrast, type I superconductors, such as lead (Pb), do not exhibit vortices but form an intermediate state with the coexistence of normal and superconducting domains.We measured the magnetization loops of superconducting wire samples of Nb and Pb together with their noise behavior after pulsed fields. Pb also exhibits significant excess low frequency noise once the wire has been driven into the intermediate state.To avoid this problem, we removed the field not abruptly but in a linearly decaying sinusoidal manner thereby degaussing the wire. After application of 57 mT, we found that Nb can be degaussed within at least 50 ms, the shortest time used in this study. Pb can also be degaussed, albeit within 100 ms and a more complex dependency on the degaussing parameters. After successful degaussing, negligible excess low frequency noise is observed.

Superconducting receiver arrays for magnetic resonance imaging

“…Practical considerations include how to fabricate such an array and what materials to use. For instance, wirewound Type-I superconducting pickup coils have shown some favorable properties [57,58] in pulsed systems, and exploiting the dynamics of superconductor-penetrating flux [52,53,59] has been promising. However, existing techniques are not suitable for helmet configurations with overlapping coils.…”

Evaluating the Performance of Ultra-Low-Field MRI for in-vivo 3D Current Density Imaging of the Human Head

“…Such an increase in the polarizing current benefits the image SNR and the SNR of the field estimates by the same factor. However, approaching such high fields will cause flux trapping in the sensor [18,29,30] and the superconducting filaments of the coil [17,31], which has to be dealt with. Also larger currents required for the compensation of the field transient [32] cause excessive heating in the compensation coils, requiring more sophisticated techniques [33].…”

Evaluating the Performance of Ultra-Low-Field MRI for In-vivo 3D Current Density Imaging of the Human Head

“…Such an increase in the polarizing current benefits the image SNR and the SNR of the field estimates by the same factor. However, approaching such high fields will cause flux trapping in the sensor [29,30,18] and the superconducting filaments of the coil [31,17], which has to be dealt with. Also larger currents required for the compensation of the field transient [32] can cause excessive heating in the compensation coils, requiring more sophisticated techniques [33].…”