A Pulsed HTS Coil Cooled by Heat Pipes as Part of a Pre-Polarizer Magnet System

Future magnetic-resonance imaging (MRI) systems for medical diagnosis may work on the principle of pre-polarization. This requires a strong local magnetic field which can be ramped up and down in milliseconds. Such fields can be generated efficiently by compact coils made with high-temperature superconductors (HTS). Copper conductors would be too bulky; low-
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superconductors would give high AC loss at 4 K. We have designed, constructed and tested an HTS double-pancake coil which generates 0.3 T of pulsed field in a 200 mm inner bore. An operating temperature of 30 K was chosen as a compromise between efficient cooling and optimum use of the HTS tape. The coil is cooled by a single GM refrigerator via a heat-pipe system filled with liquid Neon. The heat pipes and supporting structure were designed for minimum eddy-current loss in the pulsed field. The design process had to account also for AC loss in the superconductor, heat transfer from the coil to the cooling fluid and Lorentz forces on the HTS tape. Initial experiments validated the winding process for the mechanically weak Bi-2223/Ag tape, and the glued connections under thermal contraction of the materials. The production process of the coil aimed at ensuring optimum thermal contact between winding and heat pipes. The critical current at 77 K proved that there was no degradation of the HTS material. In tests at 30 K, the coil carried 250 A, 1 Hz pulsed current during up to 20 minutes. Thus the coil fulfilled the design specs.