Abstract Body

Many large-scale engineering applications involve low-frequency magnetic fields of high amplitude extending over wide surfaces. When equipment sensitive to magnetic fields (e.g., a cryocooler) is placed near such devices, the large stray magnetic field must be screened. Superconductors can act as excellent screens thanks to the current loops appearing when they are subjected to a time-varying applied field. Unlike ferromagnetic materials, superconductors are able to screen efficiently well above the tesla range: they are thus well suited for high-field applications.

Among high-temperature superconductors (HTS), bulk materials and coated conductors (CCs) can be considered for magnetic screening. High-quality HTS bulks exhibit excellent screening properties but they are hardly scalable over ~10 cm². By contrast, structures made of CCs can easily be made larger than 10 cm² but provide limited screening at their centre. In a previous work [1], we demonstrated that hybrid screens, combining a disk-shaped bulk with closed-loop CCs, significantly surpass HTS bulks in terms of screening efficiency and extension of the screened region. In order to create a closed loop, a slit is milled in the middle of a CC segment and both halves are separated around a cylindrical holder. The resulting “eye-shaped” structure is asymmetric and there exists a vertical offset between both halves. The typical geometry of a hybrid screen is shown in Fig. 1.

In this work, we describe and demonstrate experimentally how to extend the size of such hybrid HTS screens in practice so that they can be scaled up efficiently while using a bulk superconductor of typical size.

Experiments are carried out using a 30 mm-diameter disk-shaped bulk made of GdBCO/Ag from CAN Superconductors and closed-loop CCs made out of second-generation GdBCO tapes from Shanghai Superconductor Technology. The screens are cooled down using liquid nitrogen. They are subjected to a DC inhomogeneous field reaching ~100 mT at the bottom surface of the bulk. A bespoke 3-axis cryogenic Hall probe [2] attached to a 3D displacement system is used to measure B_x,B_y,B_z above the screen.

Understanding physically the behaviour of hybrid screens is the first step towards obtaining large screened regions in practice. As explained in [1], provided that the superconductors are weakly penetrated, the flux lines need to meander around them to reach the region above the screen. The flux lines are therefore diverted either all around the loops or are forced to go near the superconducting loops. The point is that loops can oppose at best the flux lines passing close to them because the opposite field generated by the induced currents in the loops is maximal close to their surface. Hence, superconducting screens acting over wide surfaces can be designed by adding several closed-loop CCs of increasing sizes around the bulk. If the spacing between the loops is sufficiently small, increasing the number of loops enables less flux lines to meander all around the screen. This process of adding larger and larger loops can be extended easily.

We build a wide screen experimentally by adding 45, 60, 75 and 90 mm-diameter loops around the 30 mm-diameter disk-shaped bulk. In this case, the screened surface, for which the flux density at 3.7 mm above the screen is attenuated by a factor 2 or more, is roughly a 92 mm-diameter circle. In comparison, the screened surface, at the same height, for the bulk alone is equivalent to a 32 mm-diameter circle.

The second part of this work focuses on the detrimental effect of the asymmetry of the loops on their screening ability. This asymmetry induces a transverse component which strongly limits the maximum attenuation of the field. Experimentally, this limitation can be solved by creating a hybrid screen whose one loop out of two is reversed. In other words, the upper part of the loops is alternatively on one side of the bulk then on the other side. For a screen with 45 and 60 mm-diameter loops around the bulk, both configurations are illustrated in Fig. 2. Measurements show that the maximum screening factor SF = at 3.7 mm above the screen increases from 38 to 66. Additionally, the spatial distribution of SF is much less asymmetric as shown in Fig. 3. In comparison, the maximum measured SF for the bulk alone at this height is 18.

In conclusion, this work presents and explains two important design rules that should be applied to build practical hybrid superconducting screens for large surfaces. First, adding loops of increasing size around the bulk allows the screened region to be significantly extended while keeping the dimensions of the bulk unchanged. Second, symmetrizing the design by alternating the orientation of neighbouring loops largely improves the field attenuation.

References

[1] Rotheudt N et al (2024) Supercond. Sci. Technol. 37 065008

[2] Rotheudt N et al (2023) Cryogenics 133 103693

Acknowledgment

N. Rotheudt is recipient of a research grant from F.R.S-FNRS. This work is supported by F.R.S.- FNRS grant CDR J.0184.23.

Keywords: Magnetic screening, hybrid superconducting screens, HTS, bulk, coated conductors, magnetic measurements.

Abstract Body

The ability of high-temperature superconductors to levitate in a magnetic field can be harnessed to create passively stable friction free rotating bearings. This is typically achieved using pieces of solid or “bulk” superconducting material. However, bulk superconductors come with size and quality limitations. In this work, we explored substituting the bulk by a series of non-insulated coils of REBCO superconducting wire. Measurements of the levitation force and stiffness properties were measured at cryogenic temperatures between 28 K and 77 K, on coils of various sizes and turn count. Excellent agreement was obtained between experimental data and H-formulation finite-element simulations employing a rotated anisotropic resistivity. This work successfully lays the groundwork for future research to fully realise the potential of no-insulation coils in superconducting bearings.

Acknowledgment

This work was funded by the New Zealand Ministry of Business, Innovation and Employment (MBIE) Advanced Energy technology Platform (AETP) program “High power electric motors for large scale transport” contract number RTVU2004.

Keywords: Magnetic levitation bearings, No-insulation coils

Abstract Body

We report the rotational instability of a superconducting magnetic bearing (SMB) system operating at about 10 K in a vacuum environment. An SMB has great potential in applications of low-energy loss systems. A typical SMB consists of a ring-shaped permanent magnet as a rotor that levitates over the high-temperature superconductor (HTS) array as a stator. We employ YBCO, a Type-II superconductor that pins the flux when it is cooled with the external magnetic field, so-called field-cooling. When the magnetic field of the rotor magnet has rotational symmetry about the axis of rotation, the HTS does not experience the change in the applied magnetic field. Thus, the rotor ring magnet rotates freely without any contact. This attractive feature enables us to construct a bearing with no mechanical contact, and thus no physical friction. As a result, the functionality of the SMB has low energy loss, and is capable of operating in low-temperature and vacuum environments, e.g., energy-storage flywheels [1-2], sensitive gravimeters [3], micro-force measurement [4], lunar telescopes [5], and polarimeters [6].

This attractive low-friction system can potentially add a challenge in the rotational kinetics. Any external torque in the rotational direction does not damp out immediately due to the low-loss system. As a result, the AC motor driven rotor can rotate with oscillation around the rotational frequency, and we investigated this effect.

We develop the rotational mechanism operating at the cryogenic temperature below 10 K. The rotational mechanism consists of the drive motor and the SMB. The rotor is held in place using three cryogenic holder mechanisms until the YBCO of SMB undergoes field cooling. The drive motor is based on a synchronous motor, the 72 alternative SmCo magnets, and the 18 sets of three-phase coils. The SMB consists of the 32 segmented SmCo magnets and the segmented bulk YBCO array. The inner diameter of the rotor magnet is 408 mm. The rotation rate of the rotor is reconstructed by an optical encoder, which consists of a disk with 128 slots on the rotor, the pair of LED and photodiode on the stator.

The test is conducted by placing the entire rotational mechanism in the cryostat cooled by a GM cooler, and the entire system is kept below the temperature of 10 K. The rotor is driven by the AC commanded by the motor driver placed outside the cryostat. The open-loop control drives this AC motor without any feedback for simplicity. One potential drawback is identified as the oscillation in rotation frequency around the target rotation frequency. Specifically, the rotor is accelerated from a stationary state to 0.8 Hz in the cryostat. The frequency is kept constant for several minutes, and then the rotation slows down and stops by disconnecting the motor driver cables. We reconstructed the rotation frequency by using the optical encoder signal. From the frequency data, we found that rotation frequency oscillates over time around the target frequency, and those oscillations were damped over time. The frequency amplitude of the oscillation is about 0.01 to 0.02 Hz, and the damping time constant is about 10 to 50 s. We also found that the frequency of the oscillation is roughly proportional to the square root of the drive current flowing through the coils.

We modeled this rotational instability with the oscillating and damping terms in the rotational direction. The potential physical correspondence of these terms is the magnetic potential between the rotor and the stator of the synchronous motor, and the eddy current in the motor and SMB. We investigate the obtained effective spring constant and the damping time constant with respect to the independently obtained estimates.

In this presentation, we report our rotational mechanism containing the synchronous motor and SMB, the experimental setup, and the experimental results of the axial rotational instability. We finally discuss the physical interpretation of the source of the rotational instability.

References

[1] Bornemann, H. J., T. Ritter, C. Urban, O. Paitsev, K. Peber, and H. Rietschel. 1994. “Low Friction in a Flywheel System Pith Passive Superconducting Magnetic Bearings.” Applied Superconductivity 2 (7-8): 439–47. 

[2] Hull, John R., Thomas M. Mulcahp, Kenneth L. Uherka, Robert A. Erck, and Robert G. Abboud. 1994. “Flywheel Energy Storage Using Superconducting Magnetic Bearings.” Applied Superconductivity 2 (7-8): 449–55.

[3] Hull, J. R., and T. M. Mulcahy. 1999. “Gravimeter Using High-Temperature Superconducting Bearing.” IEEE Transactions on Applied Superconductivity: A Publication of the IEEE Superconductivity Committee 9 (2): 390–93.

[4] Lee, Eunjeong, Ki Bui Ma, T. L. Wilson, and Wei-Kan Chu. 1999. “Characterization of Superconducting Bearings for Lunar Telescopes.” IEEE Transactions on Applied Superconductivity: A Publication of the IEEE Superconductivity Committee 9 (2): 911–15.

[5] Yang, Wenjiang, Yu Ji, Mao Ye, and Haibin Tang. 2019. “A Micro-Force Measurement System Based on High-Temperature Superconducting Magnetic Levitation.” Measurement Science & Technology 30 (12): 125020.

[6] Kusaka, A., T. Essinger-Hileman, J. W. Appel, P. Gallardo, K. D. Irwin, N. Jarosik, M. R. Nolta, et al. 2014. “Modulation of Cosmic Microwave Background Polarization with a Warm Rapidly Rotating Half-Wave Plate on the Atacama B-Mode Search Instrument.” The Review of Scientific Instruments 85 (2): 024501.

Figure 1. One example of the oscillation and damping of rotation frequency.

Keywords: Rotational instability, Superconducting magnetic bearing, Synchronous motor, Oscillation, Damping

Abstract Body

Non-contact magnetic levitation is a unique property that was proposed early on when high-temperature superconductors were discovered. The magnetic field capture performance of HTS bulk magnets improves the lower the temperature they are cooled to, and the employing the cryocoolers improves the device structure simpler and operation of the device easier. Due to the decreasing specific heat in the cryogenic temperature ranges, the operating temperature of the bulk magnets does not need to be cryogenic, and a simple and easy-to-use refrigerator can be applicable in the certain devices, which leads the advantage of entire simple devices. Since the dimensions of the bulk magnet directly affect the magnetic levitation performance, the largest bulk magnet currently available industrially was selected, and a commercially available single-pole permanent magnet was nominated to evaluate its magnetic levitation performance. The drag force of 10 N was obtained in the space 10 mm above the magnetic pole with the bulk magnet installed. Based on these findings, application to a non-contact mixer has been proposed to be commercialized.

Keywords: Superconductor, Bulk magnet, Cryocooler, Levitation, Permanent magnet