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50 T pulsed field coils using multi-composite wire
ARTICLE IN PRESS
Physica B 346–347 (2004) 571–575
50 T pulsed field coils using multi-composite wire Kris Rosseela,*, Fritz Herlacha, Johan Vanackena,1, Alexander Lagutina,2, Yvan Bruynseraedea, Jan Van Humbeeckb b
a Laboratorium voor Vaste-Stoffysica en Magnetisme, Celestijnenlaan 200D, Leuven B-3001, Belgium Department of Metallurgy and Materials Engineering, Kasteelpark Arenberg 44, Leuven B-3001, Belgium
Abstract The multi-composite (MC) wire is a new concept in the pursuit of obtaining an ideal conductor for high-performance pulsed magnets. Prototype MC wires were made with an ultimate tensile strength of 1.2 and 1.7 GPa and a conductivity of 40% IACS at 300 K. With the 1.2 GPa wire, a series of three coils with 26 mm bore were wound that performed well above expectation. Using the 1.7 GPa wire, a coil was designed and manufactured with similar bore (18 mm) and pulse duration as our standard 50+ T user coils with internal reinforcement. This coil generated 53 T before failure, even without internal stress optimization; there is still much room for improvement. r 2004 Elsevier B.V. All rights reserved. PACS: 07.55.Db; 81.05.Zx; 83.60.Np; 83.80.Ab Keywords: Pulsed magnets; Composite materials
1. Introduction For a pulsed magnet, the maximum field is limited by the thermal and mechanical load that the coil structure can sustain [1]. Fields above 50 T require the use of high strength–low resistance wires and/or very strong reinforcement, as well as special construction techniques.
*Corresponding author. Tel.: +32-16-32-71-98; fax: +32-1632-79-83. E-mail address: [email protected] (K. Rosseel). 1 Current address: Laboratoire National des Champs Magne! tiques Puls!es, 143 Avenue de Rangueil, 31432 Toulouse Cedex 04, France. 2 Current address: Forschungszentrum Rossendorf, P.O. Box 510119, 01314 Dresden, Germany.
High-strength conductors (e.g., CuNb [2,3], CuAg [4] and copper encased in a stainless steel mantle [5]) have been around since the end of the 1980s. Strengths of order 1.2–1.4 GPa at a resistivity of 0.3–0.5 mO cm at 77 K have been achieved. 2 GPa at 77 K has been reported for CuNb [6] at the expense of cross-section and conductivity. This is a general trend: a trade-off has to be made between strength and conductivity. It is also difficult to wind these stiff wires with a large cross-section over small bores. To create a conductor with an ideal combination of strength, conductivity and flexibility for winding over a small diameter, the Multi-Composite (MC) wire was proposed [7]. The MC wire is a bundle of thin wires (soft copper as well as highstrength material) enclosed in a braided insulating sleeve. All voids between the wires are filled up
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.01.084
ARTICLE IN PRESS K. Rosseel et al. / Physica B 346–347 (2004) 571–575
572
with strong fibres (e.g., carbon, S2 glass, Zylons or M5, a new fibre developed by Magellan, with a targeted strength of 9.5 GPa [8]). The wire is flexible for winding and is impregnated afterwards to obtain a compact coil. Properties can be changed over a large range by changing the composition, even along the length of a continuous wire.
2. Experimental The equipment for the production of the MC wires and coils consists of three parts: a platter with bobbins that contain the core wires, the braiding and shaping stage and the coil winder. In Fig. 1, the braiding and shaping stage of the manufacturing process is shown for wire MC3 (see further). First, the core wires are guided through a PTFE matrix to fix the core geometry. A simple PTFE die is used to press the core into rectangular shape; when the braiding point is kept within the die, a quasi-rectangular wire is obtained. For the braiding, a 16-carrier vertical maypole braider is
4.3 mm
3.1 mm
Shaper
Matrix
Fig. 1. Braiding stage during production of the MC3 wire. After being fed through a hole matrix to fix their mutual positions, the core wires are encased in a braided glass-fibre sleeve. A PTFE shaper is used to produce a quasi rectangular MC wire (gray circles: copper wires, black: carbon yarns).
employed. An adjustable electronic gearbox synchronises the speed between coil winder and braider. Under the right conditions of tension and friction, the core wires adjust to the different winding radii inside the MC wire, resulting in a smooth winding. Thin (0.15 mm) E-glass cloth is inserted between the layers to enhance the interlayer insulation. After winding, the inner and outer leads are formed using AMP ‘‘W’’ crimp contacts and the coil is vacuum impregnated, using STYCAST W19 epoxy. The prototype MC wires manufactured thus far have a core of copper and Tenax IMS5131 carbon yarns (410 tex), with an ultimate tensile strength (UTS) of 5.8 GPa (or 1.25 kN per yarn) at 1.8% strain. The coils were designed using the PMDS program [9]; no additional internal reinforcement was applied.
3. Results and discussion In Table 1, the coils and wires are listed that have been manufactured and tested so far. Wire MC1 consists of nine insulated soft copper wires and 10 carbon yarns. Insulated copper was used, as the initial quality of the braid was insufficient to guarantee adequate insulation. MC1 wires were manufactured with several core geometries and sleeve types. For the sleeve, S2 glass fibre (66 tex), Zylon HMs (110 tex) and a mixture of S2 and Zylon was used; (typical sleeve thickness: S2: 0.18 mm, Zylon: 0.26 mm, S2+Zylon: 0.22 mm). The initial wires showed a load at failure of 8 kN at 300 K, or 65% of the theoretical maximum [10]. Including the S2 sleeve, the wire had a UTS of 1 GPa and a conductivity of 38% of the International Annealed Copper Standard (IACS). For the core this was estimated as 1.2 GPa and 42%, respectively. In general, the MC wires show a virtually linear stress–strain curve during tensile tests due the presence of the carbon yarns [10]. Round MC1 wires with different sleeve types were used to manufacture a series of large bore coils. Based on the tensile test data, a field at failure of 35 T was anticipated. MCC1 was used to establish the manufacturing process and test the
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573
Table 1 Details of the coils discussed in this paper. Coil MCC1 was used for impregnation tests; coil MCC7 awaits testing. Awire is the crosssection over the MC wire including the sleeve; fwire is the area fraction of MC wire and fCu is the fraction of bare copper in the coils. For coils MCC1 to MCC6, round MC wires were used; for coil MCC7, the die shaper was used, which leads to a quasi rectangular MC wire and a higher filling factor fwire. For coils MCC5 to MCC7, two wire cross-sections are listed, as the strength of the MC wire was reduced (by reducing the number of carbon yarns) in the outer layers of these coils in favour of conductivity Coil
Bore (mm)
L (mH)
Rise (ms)
Pulse (ms)
Bcalc (T)
Bexp (T)
# windings (layers)
fwire
fCu
Wire type
Awire (mm2)
Sleeve type
MCC1 MCC2 MCC3 MCC4 MCC5 MCC6 MCC7
26 26 26 26 18 18 18
— 867 804 951 297 726 1094
— 4.0 3.7 4.3 2.1 3.6 —
— 20 19 22 8 17 —
— 35 35 35 52 54 57
— 43 37 38 49 53 —
220 (10) 222 (10) 209 (10) 230 (10) 168 (8) 215 (10) 249 (12)
0.76 0.77 0.73 0.66 0.76 0.83 0.88
0.24 0.24 0.21 0.20 0.29 0.30 0.31
MC1 MC1 MC1 MC1 MC3 MC3 MC3
8.6 8.6 9.2 8.9 12.7/8.1 12.7/8.3 12.7/9.7
S2 S2 Zylon Zylon+S2 S2 S2 S2
quality of the vacuum impregnation. MCC2 was a remake of MCC1 and failed at 43 T, a substantially higher field than expected. For these coils, estimating the core loading at a given field is not straightforward. To model the coil properties, a wire is used in the PMDS code that has the same core dimensions but an insulation thickness that reproduces the actual layer thickness and number of windings per layer. For coil MCC2, the code then calculates a core strength of 1.5 GPa. For MCC3 and MCC4, we obtain considerably lower values: 1.3 and 1.2 GPa, respectively. To investigate the influence of the sleeve, the MC1 wires as used in the coils were reproduced. From tensile tests, a maximum load of around 10.5 kN at 1.7% strain was obtained at 300 K. The core strength derived is close to 1.6 GPa. This increase to 85% of the maximum possible load can be attributed to a more balanced loading of the carbon yarns. As the load at failure is quasi the same for the different sleeve types, it can be concluded that the sleeve does not contribute to the wire strength in this type of experiments. Differences in coil performance may be related to the quality of impregnation. Microscopy data show a partially impregnated core and sleeve for the samples with the Zylon sleeve. For the S2+Zylon sleeve, only the core and S2 yarns are completely impregnated. For the pure S2 sleeve, both sleeve and core were completely impregnated. Although Zylon has a higher strength than S2, our
results thus indicate S2-glass as the material of choice for the sleeve. For wire MC3, we used 20 carbon yarns and 20 hard drawn copper wires of 0.5 mm diameter for the core. As nearly 100% S2 braid coverage could be achieved, non-insulated copper was used. Details of the shaping and braiding stage of this wire are shown in Fig. 1, together with the design layout and dimensions. Cross-sections of the wire after impregnation show a fairly regular stacking of copper wires in the core embedded in a quasi homogeneous carbon–epoxy matrix. The first tensile tests show a load at failure of 17.5 kN at 300 K, which yields a core strength of 1.7 GPa and an overall strength of 1.2 GPa at 1.5% strain. The core conductivity is 38% IACS. Assuming that the same loading levels in the carbon composite are possible as for the MC1 wire, strengths of order 2 GPa for the core and 1.7 GPa overall at room temperature are to be expected. With the MC3 wire, we decided to build coils that would mimic a standard 50 T user coil. The fields at failure listed in Table 1 were calculated using 1.7 GPa as the maximum core strength. With MCC5, a lot of technical problems were encountered. This led to the omission of the die shaper and to eight layers instead of 10 for this coil. The coil failed at a somewhat lower field than anticipated. Most of the problems were solved for the next coil, MCC6, although also in this case the die shaper had to be omitted. After further
ARTICLE IN PRESS K. Rosseel et al. / Physica B 346–347 (2004) 571–575
574
60 hoop radial axial v_Mises
4 3
Induced voltage (V)
50
1.0
40 Field (T)
Stress (GPa)
1.5
0.5
30
1 0 -1 -2 -3 -4
20
0.0
2
0
5
10
15
Time (ms)
10 -0.5 0
10
20
30
40
50
r (mm) Fig. 2. Stress distribution at 53 T in coil MCC6 as wound. For layers 7–10, the number of carbon yarns was reduced from 20 to 8 in favour of increased conductivity.
modification of the manufacturing set-up, we managed to incorporate the shaping die for the production of coil MCC7. As MCC7 still awaits testing, we will concentrate in the following on coil MCC6. In Fig. 2, the stress distribution for coil MCC6 is shown at 53 T, the field at failure. The calculations show that the four inner layers will separate. Further calculations indicate that a thin fibre wound shell—either from Zylon or carbon— between layers 2–4 will increase the mechanical load capacity of the coil to 60 T. For coil MCC6, we reduced the number of carbon yarns for the last four layers from 20 to 8 in favour of increased conductivity. In Fig. 3 the evolution of the pulse shape for coil MCC6 is shown during testing using our 10 kV bank at C=8.9 mF. The change in pulse shape indicates substantial heating in this coil. From the change in resistance we estimate that the coil reaches an average temperature of 350 K after a 52 T pulse. Despite the homogeneous distribution of the carbon inside the core, our results suggest that the thermal loading is mainly carried by the copper conductors. This is probably due to the low heat conductivity of the carbon–epoxy composite. A more intimate mixing of carbon and copper may
0 0
5
10
15
Time (s) Fig. 3. Pulse shape for coil MCC6 during coil testing. The inset shows the dB/dt signal at failure.
be required to enhance the heat diffusion in the MC core. In Fig. 4 the evolution of the self-inductance L is shown for all the tested coils. For a standard soft copper coil with internal reinforcement, the difference between the initial L value and that obtained at the end of coil testing is typically around 1%. For the MC coils however, the changes are much smaller, typical of order 0.25%. Also during a field pulse the changes in L are small, indicating a limited deformation of the coil structure during a pulse. This confirms the largely linear stress–strain behaviour of the MC wires as observed in tensile tests. All coils tested so far have shown a very gentle failure mode. As with soft copper coils with S2 internal reinforcement, the only sign of immanent failure seems to be a decrease of the selfinductance. Tensile tests and microscopy analysis of wire fragments after coil failure show that the braided sleeve tolerates a large amount of deformation before it fails. Even when the core fails, the wire will not snap immediately: part of the energy released at failure will be absorbed in the destruction of the braid.
ARTICLE IN PRESS
Inductance change after pulse(%)
K. Rosseel et al. / Physica B 346–347 (2004) 571–575
575
Much room for improvement is still available for optimisation of the core layout and composition of the MC wire as well as for the internal reinforcement and filling factor of the coil. The potential of the promising M5 fibre for application in the MC core as well as for the sleeve will be explored.
MCC2 MCC3 MCC4 MCC5 MCC6
Acknowledgements
0.3 %
10
20
30
40
50
60
Inductance change during pulse(%)
Field (T)
MCC2 MCC3 MCC4 MCC5 MCC6
References [1] [2] [3] [4]
0.3 %
10
20
30
40
50
The authors thank F. Gentens and P. Muylaert for expert technical assistance. The Belgian IUAP, the Flemish GOA and FWO are supporting this work. K.R. acknowledges financial support by FWO project G.0387.98. A.L. acknowledges financial support by EC project HPRI-199950011 (‘ARMS’).
60
Field (T)
Fig. 4. Evolution of the self-inductance during and after each field pulse for all the coils wound with MC wires. For clarity the curves have been shifted by 0.3%.
[5] [6] [7] [8]
4. Conclusions and outlook It has been demonstrated that coils using multicomposite wires can reach fields well above 50 T.
[9] [10]
F. Herlach, Rep. Prog. Phys. 62 (1999) 859. S. Foner, Appl. Phys. Lett. 46 (1986) 982. V. Pantsyrnyi, et al., Physica B 294-295 (2001) 669. Y. Sakai, K. Inoue, T. Asano, H. Wada, H. Maeda, Appl. Phys. Lett. 59 (1991) 2965. H. Jones, et al., IEEE Trans. Magn. MAG-24 (1988) 1055. L. Thilly, F. Lecouturier, J. von Stebut, Acta Mater 50 (2002) 5049. K. Rosseel, F. Herlach, W. Boon, Y. Bruynseraede, Physica B 294-295 (2001) 679. D.J. Sikkema, Polymer 39 (1998) 5981–5986.www.M5fiber.com. J. Vanacken, Li Liang, K. Rosseel, W. Boon, F. Herlach, Physica B 294&295 (2001) 674. K. Rosseel, et al., IEEE Trans. Appl. Supercond. 12 (2002) 707.
Physica B 346–347 (2004) 571–575
50 T pulsed field coils using multi-composite wire Kris Rosseela,*, Fritz Herlacha, Johan Vanackena,1, Alexander Lagutina,2, Yvan Bruynseraedea, Jan Van Humbeeckb b
a Laboratorium voor Vaste-Stoffysica en Magnetisme, Celestijnenlaan 200D, Leuven B-3001, Belgium Department of Metallurgy and Materials Engineering, Kasteelpark Arenberg 44, Leuven B-3001, Belgium
Abstract The multi-composite (MC) wire is a new concept in the pursuit of obtaining an ideal conductor for high-performance pulsed magnets. Prototype MC wires were made with an ultimate tensile strength of 1.2 and 1.7 GPa and a conductivity of 40% IACS at 300 K. With the 1.2 GPa wire, a series of three coils with 26 mm bore were wound that performed well above expectation. Using the 1.7 GPa wire, a coil was designed and manufactured with similar bore (18 mm) and pulse duration as our standard 50+ T user coils with internal reinforcement. This coil generated 53 T before failure, even without internal stress optimization; there is still much room for improvement. r 2004 Elsevier B.V. All rights reserved. PACS: 07.55.Db; 81.05.Zx; 83.60.Np; 83.80.Ab Keywords: Pulsed magnets; Composite materials
1. Introduction For a pulsed magnet, the maximum field is limited by the thermal and mechanical load that the coil structure can sustain [1]. Fields above 50 T require the use of high strength–low resistance wires and/or very strong reinforcement, as well as special construction techniques.
*Corresponding author. Tel.: +32-16-32-71-98; fax: +32-1632-79-83. E-mail address: [email protected] (K. Rosseel). 1 Current address: Laboratoire National des Champs Magne! tiques Puls!es, 143 Avenue de Rangueil, 31432 Toulouse Cedex 04, France. 2 Current address: Forschungszentrum Rossendorf, P.O. Box 510119, 01314 Dresden, Germany.
High-strength conductors (e.g., CuNb [2,3], CuAg [4] and copper encased in a stainless steel mantle [5]) have been around since the end of the 1980s. Strengths of order 1.2–1.4 GPa at a resistivity of 0.3–0.5 mO cm at 77 K have been achieved. 2 GPa at 77 K has been reported for CuNb [6] at the expense of cross-section and conductivity. This is a general trend: a trade-off has to be made between strength and conductivity. It is also difficult to wind these stiff wires with a large cross-section over small bores. To create a conductor with an ideal combination of strength, conductivity and flexibility for winding over a small diameter, the Multi-Composite (MC) wire was proposed [7]. The MC wire is a bundle of thin wires (soft copper as well as highstrength material) enclosed in a braided insulating sleeve. All voids between the wires are filled up
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.01.084
ARTICLE IN PRESS K. Rosseel et al. / Physica B 346–347 (2004) 571–575
572
with strong fibres (e.g., carbon, S2 glass, Zylons or M5, a new fibre developed by Magellan, with a targeted strength of 9.5 GPa [8]). The wire is flexible for winding and is impregnated afterwards to obtain a compact coil. Properties can be changed over a large range by changing the composition, even along the length of a continuous wire.
2. Experimental The equipment for the production of the MC wires and coils consists of three parts: a platter with bobbins that contain the core wires, the braiding and shaping stage and the coil winder. In Fig. 1, the braiding and shaping stage of the manufacturing process is shown for wire MC3 (see further). First, the core wires are guided through a PTFE matrix to fix the core geometry. A simple PTFE die is used to press the core into rectangular shape; when the braiding point is kept within the die, a quasi-rectangular wire is obtained. For the braiding, a 16-carrier vertical maypole braider is
4.3 mm
3.1 mm
Shaper
Matrix
Fig. 1. Braiding stage during production of the MC3 wire. After being fed through a hole matrix to fix their mutual positions, the core wires are encased in a braided glass-fibre sleeve. A PTFE shaper is used to produce a quasi rectangular MC wire (gray circles: copper wires, black: carbon yarns).
employed. An adjustable electronic gearbox synchronises the speed between coil winder and braider. Under the right conditions of tension and friction, the core wires adjust to the different winding radii inside the MC wire, resulting in a smooth winding. Thin (0.15 mm) E-glass cloth is inserted between the layers to enhance the interlayer insulation. After winding, the inner and outer leads are formed using AMP ‘‘W’’ crimp contacts and the coil is vacuum impregnated, using STYCAST W19 epoxy. The prototype MC wires manufactured thus far have a core of copper and Tenax IMS5131 carbon yarns (410 tex), with an ultimate tensile strength (UTS) of 5.8 GPa (or 1.25 kN per yarn) at 1.8% strain. The coils were designed using the PMDS program [9]; no additional internal reinforcement was applied.
3. Results and discussion In Table 1, the coils and wires are listed that have been manufactured and tested so far. Wire MC1 consists of nine insulated soft copper wires and 10 carbon yarns. Insulated copper was used, as the initial quality of the braid was insufficient to guarantee adequate insulation. MC1 wires were manufactured with several core geometries and sleeve types. For the sleeve, S2 glass fibre (66 tex), Zylon HMs (110 tex) and a mixture of S2 and Zylon was used; (typical sleeve thickness: S2: 0.18 mm, Zylon: 0.26 mm, S2+Zylon: 0.22 mm). The initial wires showed a load at failure of 8 kN at 300 K, or 65% of the theoretical maximum [10]. Including the S2 sleeve, the wire had a UTS of 1 GPa and a conductivity of 38% of the International Annealed Copper Standard (IACS). For the core this was estimated as 1.2 GPa and 42%, respectively. In general, the MC wires show a virtually linear stress–strain curve during tensile tests due the presence of the carbon yarns [10]. Round MC1 wires with different sleeve types were used to manufacture a series of large bore coils. Based on the tensile test data, a field at failure of 35 T was anticipated. MCC1 was used to establish the manufacturing process and test the
ARTICLE IN PRESS K. Rosseel et al. / Physica B 346–347 (2004) 571–575
573
Table 1 Details of the coils discussed in this paper. Coil MCC1 was used for impregnation tests; coil MCC7 awaits testing. Awire is the crosssection over the MC wire including the sleeve; fwire is the area fraction of MC wire and fCu is the fraction of bare copper in the coils. For coils MCC1 to MCC6, round MC wires were used; for coil MCC7, the die shaper was used, which leads to a quasi rectangular MC wire and a higher filling factor fwire. For coils MCC5 to MCC7, two wire cross-sections are listed, as the strength of the MC wire was reduced (by reducing the number of carbon yarns) in the outer layers of these coils in favour of conductivity Coil
Bore (mm)
L (mH)
Rise (ms)
Pulse (ms)
Bcalc (T)
Bexp (T)
# windings (layers)
fwire
fCu
Wire type
Awire (mm2)
Sleeve type
MCC1 MCC2 MCC3 MCC4 MCC5 MCC6 MCC7
26 26 26 26 18 18 18
— 867 804 951 297 726 1094
— 4.0 3.7 4.3 2.1 3.6 —
— 20 19 22 8 17 —
— 35 35 35 52 54 57
— 43 37 38 49 53 —
220 (10) 222 (10) 209 (10) 230 (10) 168 (8) 215 (10) 249 (12)
0.76 0.77 0.73 0.66 0.76 0.83 0.88
0.24 0.24 0.21 0.20 0.29 0.30 0.31
MC1 MC1 MC1 MC1 MC3 MC3 MC3
8.6 8.6 9.2 8.9 12.7/8.1 12.7/8.3 12.7/9.7
S2 S2 Zylon Zylon+S2 S2 S2 S2
quality of the vacuum impregnation. MCC2 was a remake of MCC1 and failed at 43 T, a substantially higher field than expected. For these coils, estimating the core loading at a given field is not straightforward. To model the coil properties, a wire is used in the PMDS code that has the same core dimensions but an insulation thickness that reproduces the actual layer thickness and number of windings per layer. For coil MCC2, the code then calculates a core strength of 1.5 GPa. For MCC3 and MCC4, we obtain considerably lower values: 1.3 and 1.2 GPa, respectively. To investigate the influence of the sleeve, the MC1 wires as used in the coils were reproduced. From tensile tests, a maximum load of around 10.5 kN at 1.7% strain was obtained at 300 K. The core strength derived is close to 1.6 GPa. This increase to 85% of the maximum possible load can be attributed to a more balanced loading of the carbon yarns. As the load at failure is quasi the same for the different sleeve types, it can be concluded that the sleeve does not contribute to the wire strength in this type of experiments. Differences in coil performance may be related to the quality of impregnation. Microscopy data show a partially impregnated core and sleeve for the samples with the Zylon sleeve. For the S2+Zylon sleeve, only the core and S2 yarns are completely impregnated. For the pure S2 sleeve, both sleeve and core were completely impregnated. Although Zylon has a higher strength than S2, our
results thus indicate S2-glass as the material of choice for the sleeve. For wire MC3, we used 20 carbon yarns and 20 hard drawn copper wires of 0.5 mm diameter for the core. As nearly 100% S2 braid coverage could be achieved, non-insulated copper was used. Details of the shaping and braiding stage of this wire are shown in Fig. 1, together with the design layout and dimensions. Cross-sections of the wire after impregnation show a fairly regular stacking of copper wires in the core embedded in a quasi homogeneous carbon–epoxy matrix. The first tensile tests show a load at failure of 17.5 kN at 300 K, which yields a core strength of 1.7 GPa and an overall strength of 1.2 GPa at 1.5% strain. The core conductivity is 38% IACS. Assuming that the same loading levels in the carbon composite are possible as for the MC1 wire, strengths of order 2 GPa for the core and 1.7 GPa overall at room temperature are to be expected. With the MC3 wire, we decided to build coils that would mimic a standard 50 T user coil. The fields at failure listed in Table 1 were calculated using 1.7 GPa as the maximum core strength. With MCC5, a lot of technical problems were encountered. This led to the omission of the die shaper and to eight layers instead of 10 for this coil. The coil failed at a somewhat lower field than anticipated. Most of the problems were solved for the next coil, MCC6, although also in this case the die shaper had to be omitted. After further
ARTICLE IN PRESS K. Rosseel et al. / Physica B 346–347 (2004) 571–575
574
60 hoop radial axial v_Mises
4 3
Induced voltage (V)
50
1.0
40 Field (T)
Stress (GPa)
1.5
0.5
30
1 0 -1 -2 -3 -4
20
0.0
2
0
5
10
15
Time (ms)
10 -0.5 0
10
20
30
40
50
r (mm) Fig. 2. Stress distribution at 53 T in coil MCC6 as wound. For layers 7–10, the number of carbon yarns was reduced from 20 to 8 in favour of increased conductivity.
modification of the manufacturing set-up, we managed to incorporate the shaping die for the production of coil MCC7. As MCC7 still awaits testing, we will concentrate in the following on coil MCC6. In Fig. 2, the stress distribution for coil MCC6 is shown at 53 T, the field at failure. The calculations show that the four inner layers will separate. Further calculations indicate that a thin fibre wound shell—either from Zylon or carbon— between layers 2–4 will increase the mechanical load capacity of the coil to 60 T. For coil MCC6, we reduced the number of carbon yarns for the last four layers from 20 to 8 in favour of increased conductivity. In Fig. 3 the evolution of the pulse shape for coil MCC6 is shown during testing using our 10 kV bank at C=8.9 mF. The change in pulse shape indicates substantial heating in this coil. From the change in resistance we estimate that the coil reaches an average temperature of 350 K after a 52 T pulse. Despite the homogeneous distribution of the carbon inside the core, our results suggest that the thermal loading is mainly carried by the copper conductors. This is probably due to the low heat conductivity of the carbon–epoxy composite. A more intimate mixing of carbon and copper may
0 0
5
10
15
Time (s) Fig. 3. Pulse shape for coil MCC6 during coil testing. The inset shows the dB/dt signal at failure.
be required to enhance the heat diffusion in the MC core. In Fig. 4 the evolution of the self-inductance L is shown for all the tested coils. For a standard soft copper coil with internal reinforcement, the difference between the initial L value and that obtained at the end of coil testing is typically around 1%. For the MC coils however, the changes are much smaller, typical of order 0.25%. Also during a field pulse the changes in L are small, indicating a limited deformation of the coil structure during a pulse. This confirms the largely linear stress–strain behaviour of the MC wires as observed in tensile tests. All coils tested so far have shown a very gentle failure mode. As with soft copper coils with S2 internal reinforcement, the only sign of immanent failure seems to be a decrease of the selfinductance. Tensile tests and microscopy analysis of wire fragments after coil failure show that the braided sleeve tolerates a large amount of deformation before it fails. Even when the core fails, the wire will not snap immediately: part of the energy released at failure will be absorbed in the destruction of the braid.
ARTICLE IN PRESS
Inductance change after pulse(%)
K. Rosseel et al. / Physica B 346–347 (2004) 571–575
575
Much room for improvement is still available for optimisation of the core layout and composition of the MC wire as well as for the internal reinforcement and filling factor of the coil. The potential of the promising M5 fibre for application in the MC core as well as for the sleeve will be explored.
MCC2 MCC3 MCC4 MCC5 MCC6
Acknowledgements
0.3 %
10
20
30
40
50
60
Inductance change during pulse(%)
Field (T)
MCC2 MCC3 MCC4 MCC5 MCC6
References [1] [2] [3] [4]
0.3 %
10
20
30
40
50
The authors thank F. Gentens and P. Muylaert for expert technical assistance. The Belgian IUAP, the Flemish GOA and FWO are supporting this work. K.R. acknowledges financial support by FWO project G.0387.98. A.L. acknowledges financial support by EC project HPRI-199950011 (‘ARMS’).
60
Field (T)
Fig. 4. Evolution of the self-inductance during and after each field pulse for all the coils wound with MC wires. For clarity the curves have been shifted by 0.3%.
[5] [6] [7] [8]
4. Conclusions and outlook It has been demonstrated that coils using multicomposite wires can reach fields well above 50 T.
[9] [10]
F. Herlach, Rep. Prog. Phys. 62 (1999) 859. S. Foner, Appl. Phys. Lett. 46 (1986) 982. V. Pantsyrnyi, et al., Physica B 294-295 (2001) 669. Y. Sakai, K. Inoue, T. Asano, H. Wada, H. Maeda, Appl. Phys. Lett. 59 (1991) 2965. H. Jones, et al., IEEE Trans. Magn. MAG-24 (1988) 1055. L. Thilly, F. Lecouturier, J. von Stebut, Acta Mater 50 (2002) 5049. K. Rosseel, F. Herlach, W. Boon, Y. Bruynseraede, Physica B 294-295 (2001) 679. D.J. Sikkema, Polymer 39 (1998) 5981–5986.www.M5fiber.com. J. Vanacken, Li Liang, K. Rosseel, W. Boon, F. Herlach, Physica B 294&295 (2001) 674. K. Rosseel, et al., IEEE Trans. Appl. Supercond. 12 (2002) 707.