Abstract
We present promising results of novel high-temperature superconducting (HTS) shim coil prototypes that circumvent the size and strength limitation of our earlier innovative HTS shim concept based on 46-mm wide REBCO tape. The HTS shim coil is placed inside the HTS magnet, mainly for ultra-high-field (> 1 GHz or 23.5 T) NMR magnets, and thus unaffected from the windings’ diamagnetic wall effects. One full-scale version will be applied to clean up Z1 and Z2 harmonic errors in the MIT 1.3-GHz high-resolution NMR magnet composed of an 835-MHz HTS insert, while another version for an MIT 1-GHz microcoil NMR magnet whose small-scale model we are currently building. The prototype sets were wound with a 2-pile, 1.03-mm wide, 0.30-mm thick REBCO conductor. Operated at 77 K, the Z1 shim set generated a 1st harmonic field strength of 179 kHz/cm at 70 A, while the Z2 shim set, composed of two pairs, Z21 and Z22, generated the 2nd harmonic field of 141 kHz/cm2 at 50 A. Together with discussion on technical challenges for this REBCO shim coil concept, we demonstrate its feasibility for the next generation of ultra-high-field (UHF) HTS NMR magnets.
Highlights
A nuclear magnetic resonance (NMR) magnet requires spatially homogeneous and temporally stable (≤ 0.01 ppm/hour) field over, generally, 1-cm diameter spherical volume (DSV) at the center where the sample is p laced[1]
We present the experimental results of a proof-of-concept study of our proposed Z1 and Z2 high-temperature superconducting (HTS) shim coils obtained with small-scale prototypes
We first show the experimental results of a diamagnetic wall effect with one of the HTS magnets used in H800
Summary
A nuclear magnetic resonance (NMR) magnet requires spatially homogeneous (typically ≤ 0.01 ppm) and temporally stable (≤ 0.01 ppm/hour) field over, generally, 1-cm diameter spherical volume (DSV) at the center where the sample is p laced[1]. Most HTS coils, of REBCO or Bi2223, are wound with a tape, monofilament or multifilaments, millimeters wide These sizes are not to be equated to an ideal diamagnetic wall, HTS coils do have attenuating and distorting effects of the diamagnetic w alls[21]. Most REBCO conductors have standard sizes of ≥ 4-mm width and ≤ 0.15-mm thickness, which may not be suitable for our shim coil application because of (1) additional SCF errors from wide REBCO tape; (2) less design freedom than narrower tape conductors; and (3) difficulty of continuous-multi-layer-winding. For the shim coil application in 1.3G, the operating current is limited to < 80 A because the maximum permissible stress of this 2-pile REBCO conductor is 350 MPa at 4.2 K 22
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