This presentation will address a unique cooling strategy of superconducting magnet for CADR (Continuous Adiabatic Demagnetization Refrigerator) applications. The magnet is to magnetize and demagnetize the magnetocaloric material in CADR up to a few Tesla during its refrigeration cycle period. The continuous operation of ADR (Adiabatic Demagnetization Refrigerator) is particularly challenging because the superconducting magnet inherently may generate significant AC (alternating current) loss during the fast ramping processes. In the CADR, the magnet is conduction-cooled by a typical two-stage Gifford-McMahon or pulse tube cryocooler that can keep the magnet temperature sufficiently low, but the cooling capacity of the cooler is always limited especially at 5 K range. For a stable ramping operation of the magnet, we proposed to insert the thermal drains that are made of Oxygen-Free High Conductivity (OFHC) copper in the winding pack for fast thermal diffusion to the heat sink. Each thermal drain aligned in an axial direction of the magnet has a width of 6 mm and a thickness of 50 μm with good thermal contact to the superconductor. The exemplary HTS solenoid is fabricated to produce the alternating 0 to 4 T magnetic field for the ADR and cooled at 5 K. The AC loss of the magnet (a solenoid with 112 layers each of which consists of 12 turns of the conductor in a layer-winding configuration) is estimated as 0.4 W with the ADR cycle period of 80 seconds and verified with the ramping rate of 0.1 T/s, which results in the successful operation without quench. The thermal analysis also ensures the temperature of the conductor to remain below 20 K during the experimental condition. Based on the results of the thermal analysis, we realize that it is also possible to design an LTS coil for CADR because the hysteresis AC loss of NbTi wire is smaller than that of typical 2G-coated HTS tape. In comparison to the 4-mm wide surface of HTS tape, a multifilamentary LTS wire with a few μm filament diameter is quite advantageous to reduce the overall hysteresis AC loss of the conduction-cooled magnet. This paper discusses three primary factors of the magnet design and its operation that determine the relative adequacy of HTS or LTS for CADR; heat generation due to AC loss, thermal contact consideration, and magnet volume with the safe current-sharing temperature. We will present the selection criteria of HTS or LTS magnet for CADR in terms of the safe magnet operating temperature for the efficient ADR cycle time period which determines the overall cooling power of the refrigerator. The stable fast-ramping operation of the conduction-cooled superconducting magnet is an enabling technology for developing an efficient sub-Kelvin CADR and ultimately a compact sub-Kelvin refrigerator.

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Fig. Fabricated HTS magnet for fast ramping operation; (a) Initial layer winding structure (b) Completed winding with copper strip thermal drains (c) Magnet assembly with final thermal drains anchored to the magnet flange.