Development of fast cooling pulsed magnets at the Wuhan National High Magnetic Field Center Tao Peng, Quqin Sun, Jianlong Zhao, Fan Jiang, Liang Li, Qiang Xu, and Fritz Herlach Citation: Review of Scientific Instruments 84, 125112 (2013); doi: 10.1063/1.4849195 View online: http://dx.doi.org/10.1063/1.4849195 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/84/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Efficient continuous-duty Bitter-type electromagnets for cold atom experiments Rev. Sci. Instrum. 84, 104706 (2013); 10.1063/1.4826498 A current-carrying coil design with improved liquid cooling arrangement Rev. Sci. Instrum. 84, 065115 (2013); 10.1063/1.4811666 Nuclear magnetic resonance apparatus for pulsed high magnetic fields Rev. Sci. Instrum. 83, 083113 (2012); 10.1063/1.4746988 Development of a High Field Superconducting Magnet Cooled by a 2 K Cryocooler (1) — Magnet and Cooling System AIP Conf. Proc. 710, 734 (2004); 10.1063/1.1774749 50 T pulsed magnetic fields in microcoils J. Appl. Phys. 87, 1996 (2000); 10.1063/1.372126

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REVIEW OF SCIENTIFIC INSTRUMENTS 84, 125112 (2013)

Development of fast cooling pulsed magnets at the Wuhan National High Magnetic Field Center Tao Peng,1 Quqin Sun,1 Jianlong Zhao,1 Fan Jiang,1 Liang Li,1 Qiang Xu,1 and Fritz Herlach2 1 2

Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan, China Department of Physics, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium

(Received 11 October 2013; accepted 28 November 2013; published online 26 December 2013) Pulsed magnets with fast cooling channels have been developed at the Wuhan National High Magnetic Field Center. Between the inner and outer sections of a coil wound with a continuous length of CuNb wire, G10 rods with cross section 4 mm × 5 mm were inserted as spacers around the entire circumference, parallel to the coil axis. The free space between adjacent rods is 6 mm. The liquid nitrogen flows freely in the channels between these rods, and in the direction perpendicular to the rods through grooves provided in the rods. For a typical 60 T pulsed magnetic field with pulse duration of 40 ms, the cooling time between subsequent pulses is reduced from 160 min to 35 min. Subsequently, the same technology was applied to a 50 T magnet with 300 ms pulse duration. The cooling time of this magnet was reduced from 480 min to 65 min. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4849195] I. INTRODUCTION

Pulsed magnets are important tools for high field science because they can generate much higher magnetic fields than DC magnets. High performance pulsed magnet coils are usually wound with an optimized combination of conductor wire and high strength fiber composite such as Zylon impregnated with epoxy. The high strength fiber confines efficiently the huge magnetic force exerted on the wire. Most pulsed magnets are pre-cooled to 77 K in liquid nitrogen to increase the capacity for absorbing Joule heating. After each pulse, it takes quite some time until the magnet cools down again to 77 K. Complete cool-down is important because even a slightly higher temperature at the beginning of the pulse will degrade the electrical performance. The high strength fiber is generally a poor conductor of heat. For this reason, the Joule heating in the wire is transferred to the liquid nitrogen at very low efficiency. The cooling time between subsequent pulses for a typical 60 T magnet with pulse duration of the order millisecond to a few hundreds of ms is about 2–4 h.1, 2 For a long-pulse magnet with pulse duration of more than 100 ms, this can even amount to more than 8 h.3 Reduction of the cooling time is of great benefit for experiments, as it permits more experiments to be done in a given time. It is then a challenge for the magnet designer to increase the total number of pulses that the magnet can sustain. Different cooling methods have been studied in a few laboratories. At the NHMFL (Los Alamos), the fast cooling pulsed magnet consists of an inner and an outer coil.4 These two coils are fabricated individually. On the outer surface of the inner coil, a thick layer of glass fiber with epoxy is wound and axial grooves are cut in the glass fiber layer to create channels for the liquid nitrogen. The inner and outer coils are assembled coaxially and they are electrically connected in series. The cooling time between pulses for 60 T is approximately 30 min. This method is efficient for cooling, but the fabrication procedure is quite complicated. At the HLD 0034-6748/2013/84(12)/125112/4/$30.00

(Dresden) and the NKMFL (Oxford), sapphire and copper were tried to increase the thermal contact between the coil and the liquid nitrogen bath;5 the experimental results were not satisfying. At the LNCMI (Toulouse), nitrogen channels were created by stacking one sub-coil onto another with G10 spacers between them.5 The cooling was improved by a factor around 3.2. Following up on this scheme, we have developed a simple and more efficient cooling technology for pulsed magnets. The magnet fabrication and experimental results will be discussed in this paper.

II. MAGNET DESIGN

In a wire-wound coil, there is in the radial direction an inner and an outer region with different deformation behavior. This can be illustrated with the example of a simple coil with equidistant layers of wire. In this case, the magnetic force on a layer with radius r is proportional to r(ra − r), where ra is the outer radius of the winding. This is an inverted parabola with its peak at ra /2. In the inner part of the coil (r < ra /2), stress and deformation increase with radius, thus the deformation of each subsequent layer will be larger and it will separate from the layer underneath, unless they are glued together (separation can be facilitated by a thin layer of Teflon). In the outer part of the coil, the decrease of the force with radius results in the opposite situation where each subsequent layer is deformed less and thus they are pressed together. The most efficient system of containing the magnetic force is optimized internal reinforcement by fiber composites: on top of each layer of wire, a layer of fiber composite is wound, with the thickness of the layer adjusted to obtain the same stress in each layer.6 In the inner region where layers separate, each layer of wire forms an independent unit together with its reinforcement. The unequal distance between layers results in distortion of the parabola with the maximum shifted towards the outer radius. In the outer region, stress is transmitted from

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one unit to the next and optimization is not feasible; the stress transmitted towards the outside can be contained by additional outer reinforcement. The layers of fiber composite between layers of wire contribute to the containment but these are kept thin for electrical efficiency while thick enough for additional insulation. A gap that permits the free flow of liquid nitrogen can be placed without causing interference between any of the units that would separate during the pulse. Ideally, the radius of the gap would be chosen such that both the inner and the outer part of the coil are cooled most efficiently. The cooling of the coil is a complicated process that is difficult to calculate; this would require calculation of the heat transfer across several boundaries in both the radial and axial direction. A first reasonable guess is to place the gap at a radius that divides the coil into two regions that equally share the total heat. This must take into account the increased heating of the inner layers due to the skin effect. This can be done with the in-house developed software PMDS (Pulsed Magnet Design Software) which calculates the temperature of each layer both at peak field and after the pulse.7 When this is done for the actual 60 T design, the heat in the outer section is calculated to be 1.28 times larger than in the inner. This is a reasonable value because the outer section may cool faster as it has a larger contact surface with the nitrogen bath even though there is some insulating material in between. Some hints can be taken from the cooling of a pulsed magnet coil without a cooling gap that was investigated experimentally.8 Eventually, a similar experiment could be done for a coil with cooling channels. Our first objective for the design of a fast cooling user magnet was a peak field of 60 T in a bore size of 21 mm. This magnet is designed on basis of our conventional 60 T magnet.9 The coil consists of 10 layers of CuNb wire. The stress is calculated by PMDS,7 which has been successfully used for magnet design at the Wuhan National High Magnetic Field Center (WHMFC). The stress distribution at the mid-plane of the pulsed magnet is shown in Fig. 1. In the inner section, 6 units are free-standing. The radial stress at the interface of these units and at the cooling channels is zero. The cooling channels result in reducing the filling factor of

the coil; this could be expected to result in a general increase of the stress. Simulations with PMDS show that the same design without the cooling channels gives a highest stress of 2.17 GPa at 60 T, while the insertion of cooling channels increases the stress only by the small amount of 0.01 GPa. This is because the insertion of the cooling channels has no direct influence on the inner layers that are independent units. With a view to the reported strength of 3.5 GPa for Zylon/epoxy composite,10, 11 this increase is negligible of course. With the 1.6 MJ (at 25 kV) capacitor bank at the WHMFC, 60 T peak field can be obtained with a charging voltage of 20 kV.

FIG. 1. Calculated stress distribution at the mid-plane at 60 T.

FIG. 2. Illustrative drawing of the fast cooling magnet.

III. MAGNET FABRICATION

The fabrication procedure of this magnet is similar to that of the conventional magnets developed at the WHMFC. Each of the conductor layers in the inner section is reinforced by a layer of Zylon/epoxy composite with optimized thickness; the composite is made by wet winding. Between all the conductor layers in the outer section, a layer of 1.5 mm Zylon/epoxy composite is wound for additional insulation and reinforcement. The entire coil is inserted into a stainless steel cylinder with 3 mm wall thickness. On the outside of the cylinder a layer of carbon fiber composite with thickness 10 mm is wound as outer reinforcement. The cylinder is believed to provide stabilization in the axial direction. Compared to the conventional magnet, the only difference is the insertion of the cooling channels between the inner and the outer sections. Fig. 2 shows the elements of the fast cooling magnet. Fig. 3 illustrates the winding procedure. The coil is wound between two winding flanges with radial grooves. At the radius where the cooling channels are inserted, G10

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FIG. 5. Photograph after the outer surface of the coil is machined.

IV. EXPERIMENTAL RESULTS

FIG. 3. Coil winding procedure (upper frame) and G10 spacers (lower frame).

spacers are placed at a distance of 6 mm on the surface of the finished windings. The G10 spacers have a cross section of 4 mm × 5 mm and the same length as the coil. On the spacers, grooves are designed to allow liquid nitrogen to flow in not only the axial direction but also in the circumferential direction. On the outer side of the G10 spacers, a layer of glass cloth is wound and then the coil winding procedure continues. In order to prevent the cooling channels from being filled with epoxy in the process of wet winding, G10 plugs are placed in the grooves of the two winding flanges. A small G10 piece with an oblique groove is used to guide the wire for the layer to layer transition, as shown in Fig. 4. After the winding is finished, a thin layer of glass fiber is wound with epoxy on the outer surface. After the epoxy hardens, the glass fiber layer is machined, as shown in Fig. 5. Finally, the G10 plugs are taken away and the entire coil is inserted into the stainless steel cylinder. On both ends of the cylinder, “U” shape openings are designed, so that the cooling channels are connected to the liquid nitrogen bath via the grooves in the winding flanges. When the magnet is immersed in the liquid nitrogen, it has been observed that plenty of nitrogen gas bubbles emerge from the grooves at the top winding flange. We can deduce that the liquid nitrogen flows into the cooling channels through the grooves of the bottom winding flange and vaporizes in the channels.

FIG. 4. Wire transition at the cooling channels (left) and G10 piece with oblique groove for the layer to layer transition (right).

The magnet was tested with the 1.6 MJ capacitor bank, which has a capacitance of 5.12 mF. The magnetic field was measured with a calibrated pick-up coil. The resistance of the magnet was measured with the 4-wire method; this provides a measure of the average temperature of the windings. The test proceeded by increasing the voltage step by step in a sequence of pulses. After each pulse, the resistance measurement device was connected to the magnet and the resistance change was recorded as a function of time. The magnetic field waveforms are shown in Fig. 6 and the cooling curves in Fig. 7. Peak field of 61 T has been obtained at the charge voltage of 20 kV. The cooling curves show that the cooling time for 61 T is only 35 min. Compared with the cooling time of a similar magnet without cooling channels, the cooling efficiency is more than 4 times higher. Since June 2013, this magnet has been in service for use in scientific experiments. It has so far sustained 551 pulses in total; 141 of these at 60 T. Further experiments will be carried out to check the long term reliability of the technology adopted in the fast cooling magnet. Inspired by the successful experiment, we envisaged improving the cooling performance of the long pulse magnet for the dilution refrigerator. This pulsed magnet was wound

FIG. 6. Measured pulsed field waveform.

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the cooling time was reduced to one fourth or even less. For user magnets, cooling time of 35 min is more than acceptable. The 65 min cooling time for the long pulse magnet marks a most substantial progress but experimenters still would prefer it to be shorter. One possible solution is to provide two rings of cooling channels at suitable radii. However, inserting cooling channels in the region where radial stress is transmitted may be nontrivial. Experimental and theoretical studies of the performance of cooling channels depending on their position as well as dimension and shape would be helpful for further optimization. ACKNOWLEDGMENTS

FIG. 7. Measured resistance of the magnet after each pulse.

from 16 layers of soft copper wire. The bore is 33 mm and the height 200 mm. The outer diameter is 260 mm. It is driven by eleven 1 MJ bank modules in parallel. The total pulse duration is 300 ms. This is a big magnet; the cooling time between subsequent 50 T pulses is 370 min. A duplicate of the magnet was manufactured with cooling channels similar to the 60 T design. Experiments show that the cooling time for a 50 T pulse is only 65 min. The gain in cooling efficiency is a surprising factor of 7.4. V. CONCLUSION AND OUTLOOK

Pulsed magnets with optimized fast cooling technology have been successfully developed at the WHMFC. An essential feature of the new design is ease of fabrication. Experiments show that these fast cooling magnets have excellent cooling performance. Compared to the conventional magnet,

This work was supported by the National Natural Science Foundation of China (51177064), the Program for New Century Excellent Talents in University (NCET-12-0220), and the Fundamental Research Funds for the Central Universities (18131058), HUST. 1 The

LNCMP team, Physica B 346–347, 668 (2004). A. Swenson, W. S. Marshall, E. L. Miller, K. W. Gavrilin, K. Han, and H. J. Schneider-Muntau, IEEE Trans. Appl. Supercond. 14, 1233 (2004). 3 F. Weickert, B. Meier, S. Zherlitsyne, T. Herrmannsdeerfer, R. Daou, M. Nicklas, J. Haase, F. Steglich, and J. Wosnitza, Meas. Sci. Technol. 23, 105001 (2012). 4 C. A. Swenson, D. G. Rickel, and J. Sims, IEEE Trans. Appl. Supercond. 18, 604 (2008). 5 P. Frings, H. Witte, H. Jones, J. Béard, and T. Hermannsdoerfer, IEEE Trans. Appl. Supercond. 18, 612 (2008). 6 L. Li and F. Herlach, Meas. Sci. Technol. 6,1035 (1995). 7 T. Peng, L. Li, J. Vanacken, and F. Herlach, IEEE Trans. Appl. Supercond. 18, 1509 (2008). 8 F. Herlach, T. Peng, and J. Vanacken, IEEE Trans. Appl. Supercond. 16, 1689 (2006). 9 L. Li, T. Peng, H. X. Xiao, Y. L. Lv, Y. Pan, and F. Herlach, IEEE Trans. Appl. Supercond. 22, 4300304 (2012). 10 R. P. Walsh and C. A. Swenson, IEEE Trans. Appl. Supercond. 16, 1761 (2006). 11 T. Peng, Q. Q. Sun, X. Zhang, Q. Xu, H. X. Xiao, F. Herlach, Y. Pan, and L. Li, J. Low Temp. Phys. 170, 463 (2013). 2 C.

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Development of fast cooling pulsed magnets at the Wuhan National High Magnetic Field Center.

Pulsed magnets with fast cooling channels have been developed at the Wuhan National High Magnetic Field Center. Between the inner and outer sections o...
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