Effect of Pre-magnetization on Quasistatic Force Characteristics in a Space Superconducting Interface Structure Adopting High T c Superconductors

Transcription

1 Effect of Pre-magnetization on Quasistatic Force Characteristics in a Space Superconducting Interface Structure Adopting High T c Superconductors Wenjiang Yang, Long Yu, Weijia Yuan & Yu Liu Journal of Superconductivity and Novel Magnetism Incorporating Novel Magnetism ISSN Volume 27 Number 1 J Supercond Nov Magn (2014) 27: DOI /s

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3 J Supercond Nov Magn (2014) 27: DOI /s ORIGINAL PAPER Effect of Pre-magnetization on Quasi-static Force Characteristics in a Space Superconducting Interface Structure Adopting High T c Superconductors Wenjiang Yang Long Yu Weijia Yuan Yu Liu Received: 21 May 2013 / Accepted: 23 May 2013 / Published online: 9 June 2013 Springer Science+Business Media New York 2013 Abstract Flux-pinned interaction between high T c superconductors (HTSs) and an applied magnetic field provides a new, no-contact interface approach that can be used in docking and assembling process of space module systems. Unlike operations on the Earth, the magnetization of the HTS happens in orbit which differs from the traditional field cooling (FC) magnetization, and the additional field has to be used to magnetize the HTS in advance and make it produce a self-stable force in the interacting process with the interfacing magnet. This paper presents a type of superconducting interface structure configuration consisting of bulk HTSs, actuation electromagnets and interfacing magnets, and discusses the effects of different magnetization conditions on the quasi-static force interaction between the HTS and the interfacing magnet. Primary experiments show that the HTS after pre-magnetization can show self-stable force behavior, which often happens in the traditional FC magnetization, and the self-stable force is further enhanced with the increase of the pre-magnetizing current. Multi-pulse field magnetization after the pre-magnetization is also applied to raise the trapped field strength (B T ) of the superconductor. The results show that B T is added with the increasing number of the pulsed field, and the corresponding selfstable force properties are also improved. Therefore, the premagnetization combined with the pulsed field magnetization W. Yang ( ) L. Yu Y. Liu 403, School of Astronautics, Beihang University, No. 37 Xueyuan Road, Haidian District, Beijing , People s Republic of China yangwjbuaa@sina.com W. Yuan Department of Electronic and Electrical Engineering, University of Bath, Bath, BA2 7AY, UK can enhance the flux trapping in the HTS and bring more stable force for the superconducting interface structure. Keywords High temperature superconductor Flux pinning Superconducting device Spacecraft interface 1 Introduction The magnetic interaction between type II HTSs and applied magnetic fields is a special self-stable phenomenon, which brings about an action-at-a-distance force but is not subject to the limitations described by Earnshaw s Theorem [1, 2]. Based on the interaction, bulk HTSs have caused many applications, such as maglev trains [3], superconducting bearings [4], flywheel storage systems [5], and other stable force devices [6]. Recently, researchers in Cornell University suggested that the superconducting interaction may have extended applications in station-keeping, formation, and reconfiguration of modular space systems, and named the interaction a superconducting flux-pinned interface (FPI) [7]. FPI is hoped to provide a new type of stable mechanism for spacecraft formations which formerly would rely upon active control techniques, and lots of static and dynamic experiments have been done to validate mechanical properties of the FPI [8]. However, because the superconductors working on orbit had difficulties in achieving field cooling (FC) magnetization which is usually used in ground levitating devices, the magnetic flux cannot be trapped effectively by superconductors to provide self-stable flux-pinned force for the interfacing system. The unstable interfacing phenomenon can be observed in a low gravity flight experiment for the FPI structure in 2010 [9]. Therefore, how to adopt reasonable method to increase the trapped flux in superconductors becomes the

4 96 J Supercond Nov Magn (2014) 27: key problem that decides if the FPI structure can be applied to the docking mechanism. In our previous article [10], the study showed that the interface force properties were seriously affected by the magnetization process of the HTS. We investigated the combining interaction of the actuation electromagnet and the HTS with the interfacing magnet under different actuating currents, and found lots of strong self-stable features in the superconducting interface structure along the axial direction. However, the actuating field dependence of magnetic properties of the HTS itself in the magnetization process has not been discussed clearly. Therefore, in this paper, we describe a superconducting interface structure configuration considering pre-magnetization, and then analyze the effects of different magnetization conditions including zero FC (ZFC), FC, and pulsed field magnetization on the quasi-static force properties of the superconducting interface structure. Fig. 1 Principle description of induced current and trapped flux distribution in superconductor under three different magnetizing conditions. (a) Zero field cooling (ZFC) condition. (b) Field cooling (FC) condition. (c) Pre-magnetization condition using an actuation magnet 2 A Superconducting Interface Structure Configuration Considering Pre-magnetization As is well known, the coupling interaction of shielding current (Meissner current) force and flux-pinned force between a bulk superconductor and an applied field provides basis for the self-stable levitation of the superconductor [2]. Figure 1 gives the possible distribution of induced current and trapped flux when the magnet approaches the HTS under different magnetization conditions. In Fig. 1(a), under ZFC condition, the superconductor has no trapped magnetic flux before magnetic interaction so that the interacting force of the magnet with the HTS is dominated by the Meissner force in approaching process, and then causes unstable levitation. In Fig. 1(b), the superconductor having experienced the FC operation can trap lots of magnetic flux in its interior, and then resist the Meissner force by the flux-pinned force to provide a possible self-stable levitation for the magnet. Similar to the levitation application of superconductor, by placing superconductors on one spacecraft and magnets on the other spacecraft, we hope non-touching electromagnetic forces can be provided between superconductors and magnets to cause possible self-stable interfacing process for the two spacecrafts. However, the space docking and interfacing process has to happen from far distance to the end, and the superconductor cannot fulfill the normal FC operation to capture magnetic flux from the far interfacing magnet in the process, so the interacting force between them will show unstable force behavior similar to the situation in ZFC condition. If we add an actuation electromagnet near the superconductor to magnetize the superconductor and make it trap lots of flux, the flux-trapped superconductor should give the self- Fig. 2 A non-contacting interface configuration between two spacecrafts using superconducting modules magnetized by actuation magnets stable interacting force when it closes in onto the interfacing magnet. Figure 1(c) describes a type of configuration depending on the pre-magnetization given by the actuation magnet. The HTS after the pre-magnetization shows different induced current distribution including clockwise current due to the demagnetization of the actuation magnet and anticlockwise current led by the approaching magnet. Meanwhile, a quantity of magnetic flux is also trapped in the superconductor by the pre-magnetization process. Figure 2 shows an example interface configuration depending on such non-contacting superconducting modules. Each module consists of a bulk superconductor, an actuation magnet and an interfacing magnet. However, whether the superconductor after the pre-magnetization can appear self-stable force properties similar to the FC condition? This is the main problem we want to study in the paper, and the effects of different magnetization conditions on the interfacing force are also investigated.

5 J Supercond Nov Magn (2014) 27: Here, the magnetic force interaction in the superconducting module was studied by measuring the axial and lateral forces of a typical magnet superconductor pair. A measuring setup [11] in our group was adopted and adjusted to fulfill the task, asshowninfig.3. In the measuring setup, a copper coil magnet with an inner diameter of 30 mm and outer diameter of 90 mm is used to provide the pre-magnetization condition, which can produce 0.2 T axial field strength when 40 A current is supplied. An YBaCuO bulk superconductor with 30 mm diameter is placed into the center of the coil magnet, and then it is fixed on the central bottom in a cryostat. An NdFeB permanent magnet (PM) is clamped to a smooth shaft end of the setup to perform the function of the interfacing magnet. In a general test, we first cool down the coil magnet by adding liquid nitrogen into the cryostat and make the electromagnet produce a stable magnetic field when it enters the thermal equilibrium under a given current. Secondly, the bulk superconductor is placed into the core of the electromagnet and experiences the magnetization with the reduction of the bulk temperature. Then, the coil magnet current is reduced to zero to finish the pre-magnetization process. The PM is driven vertically from a 105 mm gap between the bottom of the PM and the top surface of the bulk (indicating little influence of the PM field on the superconductor) to the small one of 5 mm and returned the original position to finish a vertical moving cycle. When the gap returns to the 5 mm again, the cryostat mounted on a lateral motorized stage is driven horizontally with a range of ±10 mm relative to the axis of the shaft to finish a lateral moving cycle. During these vertical and lateral moving cycles, two force transducers interacting with the smooth shaft separately measure the axial and lateral forces between the PM and the premagnetized superconductor. A Hall sensor and a set of chromel constantan thermocouple are separately fixed on the bulk top surface (Fig. 3(b)) to observe the trapped magnetic field and working temperature of the bulk superconductor in the pre-magnetization process. The Hall sensor is positioned on top surface center, and the thermocouple has a distance of 3 mm from the Hall sensor. 4 Results Analysis and Discussion Fig. 3 Sketch map of a quasi-static force measuring setup used for studying the superconducting interface module. (a) The front view. (b) Positions of the Hall sensor and the thermocouple on the bulk top surface 3 Experimental Setup 4.1 Comparing the Pre-magnetization with the Traditional FC Magnetization In order to analyze effect of pre-magnetization on the selfstable properties, we first compared the quasi-static forces in the pre-magnetization with ones in the traditional FC magnetization. In the FC magnetization procedure, the PM was first decreased to a position with a small gap above the bulk surface (the first one was 20 mm, named by FC20mm, and the second one was FC10mm), and then the superconductor was cooled by the liquid N 2 under the field environment of the PM, finally the axial and lateral forces between the bulk and the PM were measured using the same measuring cycles. In the two kinds of magnetization processes, the maximum applied field strength (B app ) and the trapped field strength (B T ) measured by the Hall sensor on the bulk top surface are recorded and shown in Table 1. It can be seen that, with the decrease of the FC position or the increase of the pre-magnetization (PreM) current, the increasing B app led to the added trapped field strength. Furthermore, compared to the FC magnetization, the pre-magnetization can obtain the bigger B T /B app ratio which may come from the better position of the HTS in the actuating field.

6 98 J Supercond Nov Magn (2014) 27: Table 1 B app and B T in pre-magnetization conditions and FC magnetization conditions Conditions FC10mm FC20mm 8 A-PreM 12 A-PreM B app (mt) B T (mt) Fig. 5 Lateral force comparison in pre-magnetization conditions with traditional FC magnetization conditions Fig. 4 Axial force comparison in pre-magnetization conditions with traditional FC magnetization conditions The axial and lateral forces in the two magnetization conditions were measured and plotted in Figs. 4 and 5, respectively. In general, the self-stable interacting force depending on the FC magnetization becomes stronger when the cooling field strength is bigger. In Fig. 4, compared to FC20mm condition, the FC10mm condition can provide bigger attracting force ( 3.1 N vs. 1.2 N, meaning the minimum value on the force curves) which benefits from flux trapping at the low field cooling position and is important for the superconducting interface module to make spacecrafts close to each other. On the other hand, the superconductor after premagnetization (8 A-PreM or 12 A-PreM) also gives an attracting force at a small vertical gap range, and the attracting force adds with the increase of the pre-magnetization current. In Fig. 5, the lateral force in the FC20mm condition does not show to be attractive but repulsive, indicating the magnetic flux is little trapped in the condition and the effective flux-pinned force cannot be produced to resist the lateral motion. As for FC10mm, the lateral force increases linearly with the negative rising of the horizontal displacement, which resists the horizontal movement and presents good self-stable properties. Compared to the FC conditions, the superconductor after 8 A or 12 A pre-magnetization also gives self-stable lateral forces with the moving of the horizontal position. Especially for the 12 A-PreM condition, the lateral force value is close to one in the FC10mm condition, indicating the pre-magnetization has the potential ability to provide good lateral stability. Therefore, through Figs. 4 and 5, the superconductor after pre-magnetization can show the same self-stable force behavior as the traditional FC magnetization when it interacts with the interfacing magnet although it has not been magnetized by the magnet. 4.2 Comparison in Different Pre-magnetization Conditions with Multi-pulse Field As shown in Fig. 6, four magnetic pulses named first to fourth in order were applied to magnetize the premagnetized bulk. The rise and decreasing time of the pulse were both about 1 s, and the pulsed field was applied iteratively following a procedure similar to IMRA method [12]. The first and fourth pulsed current values were both 30 A, and the second and third ones were 40 A with each 2 min time interval. Figure 7 gives the trapped field strength measured by the Hall sensor after each pulsed field, and shows the relation between B T and the applied pulsed field value. It shows B T increases with the increasing numbers of pulsed field, though the incensement becomes small gradually. In the pulsed field (PF) magnetization process, a small temperature rise about 1 K was observed and then disappeared in 2min.

7 J Supercond Nov Magn (2014) 27: Fig. 6 The time dependence of the applied current in the coil magnet in the multi-pulse field process Fig. 8 Axial force curves of the pair in 0 A pre-magnetization condition after different PF magnetization Fig. 7 The actuating pulsed field dependence of the trapped field strength in the multi-pulse field process Fig. 9 Lateral force curves of the pair in 0 A pre-magnetization condition after different pulsed field magnetization (PFM) Figures 8 and 9 give the vertical and lateral force values in the 0 A-PreM condition before and after different pulsed field magnetization. Obviously, through the PF magnetization, the pre-magnetized superconductor shows bigger attracting force (seen in Fig. 8) and better lateral stability (seen in Fig. 9). The result indicates the PF magnetization is the effective method to add flux trapping and improve the stable force properties, even when the pre-magnetization condition happens at 0 A current (0 A-PreM) which is a similar situation to ZFC. The experimental data including 0 A-PreM and 8 A- PreM after 2nd PF magnetization can be seen in Figs. 10 and 11. Compared to 0 A-PreM condition, 8 A-PreM shows better axial and lateral stability. Especially after the second PF magnetization, the 8 A-PreM provides significant increase in the lateral stable force, indicating better influence of the PF magnetization on the lateral stability. Therefore, by combining the pre-magnetization with the PF magnetization, the larger self-stable quasi-static force can be achieved in the superconducting interface module, which is obviously helpful of the interface process.

8 100 J Supercond Nov Magn (2014) 27: of different magnetization conditions on the self-stable features. 5 Conclusion Fig. 10 Axial force curves of the pair in 0 A and 8 A pre-magnetization conditions after the second PF magnetization During the docking process of spacecrafts using superconducting interfacing modules, the superconductor is first premagnetized by the actuation magnet mounted on the same position on the target spacecraft to cause magnetic flux trapping as much as possible in orbit, and then the fluxpinned superconductor interacts with the interfacing magnet on the approaching spacecraft to provide the non-contacting self-stable force to assist the final interface stage for the two spacecrafts. The pre-magnetization experiments showed that the initial cooling field strength given by the actuation magnet was important for the mechanical properties in the superconductor magnet pair, and the trapped field strength and the self-stable force increased with the increasing of the PreM current. Sometimes the pre-magnetization can provide similar and even better stable force properties than the traditional FC magnetization. The pulsed field magnetization after the pre-magnetization can improve the stable force properties further. With the increasing pulsed field numbers, the trapped field was added slowly and then the self-stable force was also increased. We believe that the pre-magnetization combined with the PF magnetization is an effective method to increase the quasi-static self-stable force in the superconducting interface structure. Acknowledgements This work was supported by the Beijing Cooperation Construction Project for Beijing Universities (YB ) and National Natural Science Foundation of China ( ). References Fig. 11 Lateral force curves of the pair in 0 A and 8 A pre-magnetization conditions after the second PF magnetization Limited by the present design of the actuation magnet, the cryostat, and the power supply system, there is difficulty in continuing to increase the pulsed field strength and the trapped field strength by the experimental setup, so the effect of the PF magnetization with bigger field magnitude on the trapped field and the force ability in the superconducting interface structure has not been studied in this paper. Furthermore, besides the quasi-static force properties, dynamic force behavior should be investigated to validate the effects 1. Brandt, E.H.: Science 243, 349 (1989) 2. Brandt, E.H.: Am. J. Phys. 58, 43 (1990) 3. Wang, J.S., Wang, S.Y., Zeng, Y.W., et al.: Physica C , 809 (2002) 4. Werfel, F.N., Floegel, D.U., Rothfeld, R., et al.: Supercond. Sci. Technol. 25, (2012) 5. Strasik, M., Hull, J.R., Mittleider, J.A., et al.: Supercond. Sci. Technol. 23, (2010) 6. Ueda, H., Azumaya, S., Tsuchiya, S., et al.: IEEE Trans. Appl. Supercond. 16, 1092 (2006) 7. Shoer, J.P., Peck, M.A.: J. Astronaut. Sci. 57, 667 (2009) 8. Shoer, J.P., Peck, M.A.: J. Spacecr. Rockets 46, 466 (2009) 9. Shoer,J.,Wilson,W.,Jones, L.,et al.:j.spacecr.rockets 47, 1066 (2010) 10. Yang, W.J., Xu, J., Yu, L., et al.: Physica C 483, 173 (2012) 11. Yang, W.J., Qiu, M., Liu, Y., et al.: Supercond. Sci. Technol. 20, 281 (2007) 12. Fujishiro, H., Oka, T., Yokoyama, K., et al.: Supercond. Sci. Technol. 16, 809 (2003)