Graduate School of Frontier Sciences, The University of Tokyo, Japan1
Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo, Japan2
A superconducting magnetic bearing (SMB) [1] has been used as a contactless rotation mechanism for low heat dissipation application. One of the examples is to employ SMB at the temperature of 4 K in a satellite platform by LiteBIRD [2]: millimeter wave telescope to test the theory of cosmic inflation by measuring Cosmic Microwave Background (CMB). The SMB consists of a ring-shaped High Temperature Superconductor (HTS) YBCO bulk as the stator and a ring shaped SmCo permanent magnet (PM) as the rotor.
When SMB is to be used for a space application, one of the standard requirements is to minimize the mass. YBCO has a density of 6.3 g/cm3, and thus it can take a significant resource in the mass budget. This raises a question if a typical bulk YBCO thickness, 20-30 mm, can be trimmed down and yet such a thinner bulk YBCO can maintain its performance, i.e. the same trapped field under the field cooling.
We carried out an experiment using four cylindrical YBCO bulk samples with different thicknesses (6, 10, 13 and 16 mm). Each sample was magnetized separately by a SmCo PM placed 5 mm above it and field-cooled by submerging the experimental platform in LN2. The pinned magnetic fields were measured by scanning in one direction using a Hall Sensor and compared for each thickness. The results showed no strong correlation between the thickness range used in this experiment with the magnetic field. Additionally, it is of interest to recreate the measurements at a lower temperature (~ <10 K) close to the actual operational temperature, 4 K, to estimate any changes in the current density. The measurements are carried in a cryostat that is optimized to fit the experimental platform, that is set for measuring the magnetic field of two YBCOs at a time, once with the presence of the PM and once without to remove any interference with the YBCO trapped field.
We report the results of the measurements and also discuss its implication by using the finite element modelling software JMAG to extend the experimental results into a model representing the SMB.
[1] Hull, J. R. (2000). Superconducting bearings. Superconductor Science and Technology, 13(2), R1.
[2] Matsumura, T., Akiba, Y., Borrill, J., Chinone, Y., Dobbs, M., Fuke, H., ... & Hazumi, M. (2014). Mission design of LiteBIRD. Journal of Low Temperature Physics, 176(5-6), 733-740.
Keywords: Superconducting magnetic bearing (SMB), Cosmic inflation, YBCO bulk, Hall sensor