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Description
A portable nuclear magnetic resonance (NMR) superconducting magnet with conduction-cooled cryostat system was under development with central field strength 7 T. The designed diameter of spherical volume (DSV) of the magnet is 0.05 m and the peak-peak homogeneity is 8 ppm. After shimming, the field homogeneity will be improved to 0.1 ppm over a DSV 0.01 m. The magnet is actively-shielded with a 5 Gauss line 0.65 m at the longitudinal direction and 0.40 m at the radial direction. The magnet coils have a diameter 0.27 m and length 0.36 m. The fabricated magnet will have a standard warm bore diameter 0.054 m.
The magnet design was based on NbTi superconducting wire, which consumes wire length 16.6 km and wire volume 0.005 m3. There are 12 magnet coils in total including 5 primary coils, 4 compensating coils and 3 shielding coils and superconducting wires with several gauges were applied to reduce the integral wire usage. The operating current for the target field strength is 110.59 A, accompanying a maximum hoop stress 122 MPa. The current margin and temperature margin for the magnet are 78.1% and 1.49 K at 4.2 K, respectively. The coil inductance is 14.8 H and the magnetic energy is 90.6 MJ.
Four bobbins were designed for the magnet coils support and another one between the compensating coils and shielding coils was used for shim coils attachment. The cold heads of cryocooler were mounted on the metal bobbins, which cooled the magnet coils temperature much lower than critical temperature.
The conduction-cooled NMR magnet largely reduced the dependence of superconducting magnet on liquid helium, which not only well copes with the scarcity of liquid helium market in the future, but also reduce the overall cost of the magnet system in a long run.
