Performances of a Compact Shielded Superconducting Magnet for Continuous Nuclear Demagnetization Refrigerator
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Performances of a Compact Shielded Superconducting Magnet for Continuous Nuclear Demagnetization Refrigerator S. Takimoto1 · R. Toda1 · S. Murakawa1 · Hiroshi Fukuyama1,2 Received: 30 August 2019 / Accepted: 27 December 2019 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract We have successfully developed and tested a compact shielded superconducting (SSC) magnet with a FeCoV magnetic shield. This was developed for the PrNi5-based nuclear demagnetization refrigerator which can keep temperatures below 1 mK continuously (CNDR) (Toda et al. in J Phys Conf Ser 969:012093, 2018). The clear bore diameter, outer diameter, and total length of the SSC magnet are 22, 42, and 169 mm, respectively, and it produces the maximum field of 1.38 T at an electric current of 6 A. In order to realize both the compactness and the high shielding performance, we carefully chose material and optimized design of the magnetic shield by numerical simulations of the field distribution based on typical magnetization curves of several candidate materials with high permeability. We also measured the heat generated by sweeping the SSC magnet in vacuum to be 248 mJ per field cycle. This value agrees very well with an estimation from the measured magnetic hysteresis of the superconducting wire used to wind the magnet. Keywords Superconducting magnet · Nuclear demagnetization refrigerator · Magnetic shield · Magnetic hysteresis
1 Introduction The sub-mK temperature environment is important for studies of unique quantum phases and phase transitions in liquid and solid 3 He and other condensed matters. It is also useful in even broader fields, for example, those in which higher sensitivities * S. Takimoto [email protected]‑tokyo.ac.jp Hiroshi Fukuyama [email protected]‑tokyo.ac.jp 1
Cryogenic Research Center, The University of Tokyo, 2‑11‑16 Yayoi, Bunkyo‑ku, Tokyo, Japan
2
Department of Physics, The University of Tokyo, 7‑3‑1 Hongo, Bunkyo‑ku, Tokyo, Japan
13
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Journal of Low Temperature Physics
in detection of X-ray or microwave are demanded, because thermal noises and heat capacities are extremely small at such temperatures. In order to realize much easier access to sub-mK temperatures for non-experts, we are developing a compact and continuous nuclear demagnetization refrigerator (CNDR) [1] using PrNi5 , a hyperfine enhanced nuclear magnet [2], as a coolant. CNDR has two independent PrNi5 refrigeration units which are connected in series between the sample stage and the mixing chamber of dilution refrigerator through two superconducting heat switches. It can keep a constant temperature below 1 mK continuously with a cooling power larger than 10 nW. To take full advantage of the design concept of CNDR, it is crucial to develop a compact, low heat dissipation, and high-performance shielded superconducting (SSC) magnet. Specifications required for the SSC magnet of our CNDR are the followings: (i) maximum magnetic field produced by a current of I = 6 A: Bmax ≥ 1.2 T, (ii) clear bore diameter
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