Journal of Physics: Conference Series PAPER OPEN ACCESS Comparison of Trapped Field Characteristic of Bulk Magnet System Using Various Type Refrigerators To cite this article: K Yokoyama et al 2018 J. Phys.: Conf. Ser. 1054 012072 View the article online for updates and enhancements. This content was downloaded from IP address 148.251.232.83 on 17/10/2018 at 16:46
Comparison of Trapped Field Characteristic of Bulk Magnet System Using Various Type Refrigerators K Yokoyama 1, A Katsuki 2, A Miura 2 and T Oka 3 1 Division of Electrical and Electronic Engineering, Ashikaga University, 268-1 Omae-cho, Ashikaga, Tochigi 326-8558, Japan 2 Department of Electrical and Electronic Engineering, Graduate School of Engineering, Ashikaga University, 268-1 Omae-cho, Ashikaga, Tochigi 326-8558, Japan 3 Materials Science and Engineering, College of Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-Ward, Tokyo, 135-8548 Japan Abstract. We developed several type superconducting bulk magnets with the goal of their industrial application, and study to improve a magnetic field activated by pulsed field magnetization (PFM). When considering a practical use of bulk magnet, it is important problem to choose a suitable refrigerator to cool a bulk superconductor. This paper investigated trapped field characteristics when using several type refrigerators; one is a Stirling refrigerator with an ultimate temperature of 40 K at no-load condition and cooling capacity of 11 W at 77 K. The other is a dual-stage GM type refrigerator with the cooling ability of 13 K and 5 W at 20 K. Now, we developed a new bulk magnet system using a dual-stage GM refrigerator with the cooling ability of 12 K and 12 W at 20 K aiming to improve a trapped field by removing heat generated after a pulse application quickly. Using a GdBCO bulk with dimensions of 60 mm in diameter and 20 mm thick, magnetizing tests were carried out and the relationship between trapped field performance and cooling ability was investigated. Even the bulk magnet using Stirling refrigerator with relatively low cooling capacity could trap a high magnetic field of 3.0 T on the bulk surface and maximum total flux of 2.0 mwb. The trapped magnetic field of new bulk magnet system was improved greatly at low temperature; the maximum trapped field of 3.6 T and the maximum total flux of 2.9 mwb were achieved. 1. Introduction The application of REBCO bulk as magnetic field generator has attracted much attention because the high magnetic field over 2 T can be trapped with a compact system and low running cost, and accordingly, various industrial applications are considered [1-5]. From this background, many advanced studies about material fabrication [6,7] and magnetizing method [8-11] are progressing. However the relationship between trapped field performance and a cooling ability has received little attention, and thus, it's necessary to discuss the selection of refrigerator when considering a practical use of a bulk magnet system. We study to improve of trapped field of REBCO bulk magnet by exciting with pulsed field magnetization (PFM) aiming at an industrial application. In our previous research, the several bulk magnet systems using a dual-stage GM refrigerator with an air-cooled compressor [12] and a Stirling refrigerator without compressor [13] were developed based on the idea that the system was simplified by not using a chiller. On the other hand, a cooling ability may not always be enough. Now, we developed a new bulk magnet system using a dual-stage GM refrigerator with a water-cooled compressor aiming to enhance a trapped field by improving a cooling ability. The main specifications of the above magnet systems were listed in Table 1. The bulk system using a new refrigerator (referred Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by Ltd 1
to as 12WGM ) had more than 2 times of cooling ability of the bulk system using the existing GM refrigerator (referred to as 5WGM ). When magnetic flux penetrates into the sample, the bulk always generates heat due to pinning loss and viscous loss. Therefore, it is necessary to make the bulk trap magnetic flux while considering the relationship between heat generation and cooling. In this paper, we pay attention to the difference of cooling ability on different types of refrigerators. Then, the magnetizing characteristics are investigated by applying a single pulsed field to a GdBCO bulk 60 mm in diameter and 20 mm thick while varying the amplitude of applied fields and temperature in each bulk magnet system. Table 1. Specifications of the bulk magnet systems Label 12WGM 5WGM Stirling Hall sensor bulk sample Photo 548 Thermo-controller Stirling refrigerator Power-controller Refrigerator Dual-stage GM (RM20,ULVAC) Dual-stage GM (RS271,AISIN) Stirling (CryoTel CT, UNPOWER) Lowest temperature (no load) 12 K 13 K 40 K Cooling capacity 12 W@20 K (2nd stage) 35 W@77 K (1st stage) 5 W@20 K (2nd stage) 19 W@80 K (1st stage) 11 W@77 K Power consumption 5.0 kw / 3 200 V 1.6 kw / 3 200 V 160 W / 1 100 V Compressor Water-cooled Air-cooled Unnecessary 2. Experimental A GdBCO bulk 60 mm in diameter and 20 mm thick with the high trapped field performance of 2 T at 77 K (Nippon Steel & Sumitomo Metal Corporation), which was reinforced mechanically with a stainless steel ring 2 mm thick (SUS316L), was set to the sample stage connected to a cold head of the refrigerator using a sample holder made of a stainless (SUS304). A Hall sensor (BHT-921, F.W.BELL) was adhered at the center of bulk surface with Kapton tape to measure the time variation of the magnetic flux during magnetization, and a Cernox temperature sensor (CX-1030-SD-HT-1.4L/J, LakeShore) was also adhered near the Hall sensor with the GE7031 varnish to measure the time variation of temperature. After a vacuum vessel was covered and evacuated by a diffusion pump, the bulk was cooled to 20, 30, 40, and 50 K. A single pulsed field was applied while changing the amplitudes from 3.1 to 7.0 T for each temperature, where the rising time and fall time of the pulse were 10 ms and 100 ms, respectively. During magnetization, the magnetic flux density and temperature were measured in an interval of 100 s and 100 ms, respectively. After magnetization, the trapped field map at 7 mm above the bulk surface was measured using the three-dimensional Hall sensor (BH-703, F.W.BELL) with a 2-mm pitch and the total magnetic flux was calculated using the two-dimensional distribution. 3. Results and discussion Figure 1 shows magnetic field density at the center of bulk surface, B z, as a function of applied field, B app, for 12WGM, 5WGM and Stirling refrigerators at 20 K and 50 K, where there is the result of Stirling refrigerator only at 50 K. At 20 K, a maximum value of B z was reached at B app =5.4 T in both 12WGM and 5WGM, and the maximum values were 3.6 T and 2.6 T, respectively. The maximum trapped field of 12WGM was approximately 1.4 times as high as that of 5WGM, indicating that a cooling ability has a strong influence on trapped field property. In higher applied fields, the B z value 2
was decreased due to flux flow caused by heat generation for both systems. At 50 K, the peak of B z shifted toward lower applied field in 12WGM and 5WGM because J c was decreased with increasing temperature, resulting that magnetic flux was able to penetrate the bulk easily. Then, the maximum values were 2.8 T for B app =4.6 T in 12WGM and 2.4 T for B app =3.9 T in 5WGM. On the other hand, the B z value of Stirling system was increased with applied field, reaching a peak of 3.0 T for B app =6.2 T. This means that even the bulk magnet using Stirling refrigerator with relatively low cooling capacity can trap a high magnetic field though the peak value was smaller than that of the refrigerator with high cooling capacity. Figure 1. Applied field dependence of magnetic flux density at the center of bulk surface at 20 K and 50 K Figure 2. Comparison of maximum temperature rise on the bulk surface at 20 K and 50 K (a) 20 K (b) 50 K Figure 3. Time variations of normalized temperature rise by the peak value at 20 K and 50 K 3
Figure 2 compares the maximum temperature rise between both GM systems at 20 K and 50 K. Since a critical current density is increased with a decrease in temperature, the magnetic shield becomes high. Then, heat generation caused by pinning loss is increased at lower temperature, and therefore, the temperature rise at 20 K was higher than that at 50 K. Also, all data increased with applied fields. At 20 K, both curves were almost same, and the maximum T was 31.9 K in 12WGM and 32.4 K in 5WGM for B app =7.0 T. At 50 K, on the other hand, the T value of 12WGM was about 1.2 times higher than that of 5WGM, and the maximum values were 16.0 K and 16.6 K, respectively. The difference of trapped field shown in Figure 1 cannot be explained in terms of the amplitude of temperature rise shown in Figure 2. Then, the measured time variation of temperature was normalized by the peak value as illustrated in Figure 3. The decrease of T after reaching a peak was faster in 12WGM as compared with 5WGM in all data, indicating that the quick removal of heat by high cooling ability of a refrigerator leads to improvement of the trapped field. On the other hand, the rise time of 12WGM was slightly faster than that of 5WGM. However, the rise time of applied field was not changed because the same magnetizing equipment was used in both magnetizing experiments. We will consider the reason for a difference in rise time of temperature in detail. Figure 4 shows the trapped field distributions at 20 K and 50 K. At 20 K, the magnetic flux did not penetrate the center of bulk for B app =3.9 T, however the distribution became concentric for B app =5.4 T and the area of high trapped field over 1.4 T appeared widely. For B app =7.0 T, a decrease of trapped field in 5WGM was larger than that of 12WGM. At 50 K, the distribution was concentric even for low applied field of 3.9 T in both GM system, and the amplitude of magnetic flux was reduced with increasing an applied field. In Stirling system, on the other hand, the magnetic flux did not reach the center of bulk for B app =3.9 T, while the area of high trapped filed over 1.4 T for B app =7.0 T as almost same as that for B app =5.4 T. Using these distribution maps, the total magnetic flux,, and the maximum magnetic flux density on the pole surface, B z, were calculated, where the maximum B z was not always the value at the center of magnetic pole. Figure 4 compares the total magnetic flux, (bar (a) 12WGM (b) 5WGM (c) Stirling Figure 4. Trapped field distributions in 12WGM, 5WGM and Stirling systems at 20 K and 50 K. 4
(a) 20 K Fig. 5. Total magnetic flux as a function of applied field (b) 50 K Figure 5. Comparisons of total magnetic flux (bar graph, left axis) and maximum flux density on the pole surface (line graph, right axis) as a function of applied field at 20 K and 50 K. graph, left axis), and the maximum trapped flux density, B z (line graph, right axis), as functions of the applied field at 20 K and 50 K. At 20 K, the maximum and B z were 2.9 mwb and 2.3 T for B app =5.4 T in 12WGM, and 2.5 mwb and 2.0 T in 5WGM. At 50 K, they were 2.0 mwb and 1.7 T for B app =4.6 T in 12WGM, and 1.6 mwb and 1.2 T for B app =3.9 T in 5WGM. In Stirling system, on the other hand, they were 2.0 mwb for B app =5.4 T and 1.7 T for B app =6.2 T, respectively. When comparing between 12WGM system and Stirling system, the former can trap a higher magnetic field at lower applied field. These results suggest that the magnetizing system can be downsized and this is a major advantage for a practical use of bulk magnet system. 4. Conclusions We investigated the influence of the cooling ability to the trapped field performance using several type bulk magnet systems using different refrigerators; a new dual-stage GM refrigerator with the cooling capacity of 12 W at 20 K, an existing dual-stage GM refrigerator with5 W at 20 K and 13 K and a Stirling refrigerator with 11 W at 77K and 40 K. The time variations of magnetic flux density and temperature on the bulk surface and magnetic field distributions on the pole surface were measured by applying a single pulsed field was applied while varying an amplitude and temperature using a GdBCO bulk 60 mm in diameter and 20 mm thick. The experimental results indicated the followings; even the bulk magnet using Stirling refrigerator with relatively low cooling capacity could trap a high magnetic field of 3.0 T on the bulk surface and maximum total flux of 2.0 mwb. Moreover, the trapped magnetic field of 12WGM was improved greatly at low temperature; the maximum trapped field of 3.6 T and the maximum total flux of 2.9 mwb were achieved. We will make the data which is the guideline to select a refrigerator for industrial application of bulk magnets by performing the more detailed magnetizing test in the above bulk magnet systems. Acknowledgments This work was supported by JSPS KAKENHI Grant Number JP15K05951. References [1] Ohsaki H, Sekino M, Suzuki T and Terao Y 2009 2009 Int. Conf. Clean Electrical Power 479 [2] Felder B, Miki M, Zigang D, Tsuzuki K, Shinohara N, Izumi M and Hayakawa H 2011 IEEE Trans. Appl. Supercond. 21 2213 [3] Nakamura T, Tamada D, Yanagi Y, Itoh Y, Nemoto T, Utumi H and Kose K 2015 J. Magn. Reson. 259 68 [4] Nakagawa K, Mishima F, Akiyama Y and Nishijima S 2012 IEEE Trans. Appl. Supercond. 22 4903804 [5] Oka T, Takayanagi Y, Machida S, Ichiju K, Fukui S, Ogawa J, Sato T, Ooizumi M, Tsujimura M and Yokoyama K 2016 IEEE Trans. Appl. Supercond. 26 3700204 5
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