Paper M1-D-08 Presented at CEC-ICMC 2003, Anchorage, Alaska, September 2003, accepted for publication in Advances in Cryogenic Engineering.

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1 Paper M1-D-08 Presented at CEC-ICMC 2003, Anchorage, Alaska, September 2003, accepted for publication in Advances in Cryogenic Engineering. PROGRESS ON THE USE OF INTERNAL FINS AS BARRIERS TO REDUCE MAGNETIZATION ON HIGH CURRENT DENSITY MONO ELEMENT INTERNAL TIN CONDUCTORS (MEIT) Bruce A. Zeitlin 1, Eric Gregory 1, Taeyoung Pyon 2, R. M. Scanlan 3, Anatolii A. Polyanskii 4, and Peter J. Lee 4 1 Supergenics LLC Sarasota, Fl 34242, USA 2 Outokumpu Advanced Superconductors Waterbury, CT, 06704, USA 3 Lawrence Berkeley National Laboratory Berkeley, CA, 94720, USA 4 University of Wisconsin-Madison, Applied Superconductivity Center Madison, WI, 53706, USA ABSTRACT A number of configurations of a mono element internal tin conductor (MEIT) were fabricated to explore the effect of internal fins on the effective filament size (D eff ) and its effect on wire processing. A current density of 2.85 x 10 9 A/m 2 (12 T) was achieved in a high tin, high Nb conductor. Wire lengths as long as 15.8 km at mm diameter with breaks averaging 3 per unit length were achieved. Magnetization measurements and Magneto-Optical (MO) images were taken of the finned and non-fin conductor which indicated the fins appeared to be effective. The D eff achieved in the fin conductor was 80 µm compared with an equivalent conductor without a fin of 165 µm. INTRODUCTION The need for higher energy accelerators than the Large Hadron Collider presently under construction at an acceptable cost to the High Energy Physics community is driving development of lower cost higher performance superconductors. The next generation accelerators such as the VLHC may require magnets with fields in the range of 12 to 15 Tesla [1]. Goals have been set to produce superconductors with a current density of 1

3,000 A/mm 2, at 12 T, an effective filament diameter of less than 40 µm, a piece length of greater than 10,000 meters, and at a cost of less than $1.50/kilo-Ampere meter (kam) [2].

Programs such as the Central Solenoid Model Coil (CSMC) for the ITER program and the Levitated Dipole Experiment have demonstrated its success in conductors of lower current densities [3,4].

2 3,000 A/mm 2, at 12 T, an effective filament diameter of less than 40 µm, a piece length of greater than 10,000 meters, and at a cost of less than $1.50/kilo-Ampere meter (kam) [2]. An Internal Tin conductor as manufactured in various forms is one of the prime candidates to meet these needs. Internal Tin has been used in large programs over the years. Programs such as the Central Solenoid Model Coil (CSMC) for the ITER program and the Levitated Dipole Experiment have demonstrated its success in conductors of lower current densities [3,4]. The Korea Superconducting Tokamak Advanced Research (KSTAR) will use 26 metric tons of Internal Tin manufactured by Outokumpu Advanced Superconductors and Mitsubishi [5]. Conductors delivered to these projects are far more expensive than the goals of the HEP community. The Internal Tin process in variations as manufactured by Outokumpu A.S. and Oxford Instruments Superconducting Technology offers some of the highest current densities with values ranging from 2,700 A/mm 2 to 3,000 A/mm 2 at 12 T [6,7]. This is primarily achieved through reaction of a high area fractions of Nb made possible by the large area of tin that can be introduced in the Internal Tin process. The Nb filaments of these conductors have to be spaced so closely that upon reaction the filaments expand and bridge, creating a very large D eff. Solutions to reducing D eff have mainly focused on reducing the sub-element size and hence increasing the number of sub-elements in the base conductor [6, 8]. Internal Tin as presently practiced has not yet been amenable to large-scale billet fabrication such as in NbTi manufacture. This is a result of the high breakage and hence lower yield experienced in the present manufacturing process. Three main factors are thought to contribute to this problem. The first is the total strain on the filament as a result of a two-stage reduction process required to achieve the sub-element size to meet D eff. The second is diffusion barrier integrity about each sub-element and the third is lack of a metallurgical bond in the early stage processing of the restack. The Mono Element Internal Tin (MEIT) process as first presented in 2000 is being developed to overcome these limitations [9]. The MEIT process uses just one multifilament element of the classic internal tin. This element can be fabricated in 305 mm diameter billets and processed to rod or wire. The wire is expected to be used in cable form with individual elements typically of 0.28 mm to yield a 0.8 mm cable. The effective filament size is designed to be reduced by the introduction of non-superconducting internal barriers (fins) which subdivide the bridged filaments into smaller units by acting as a reaction barrier [10]. Cost models have shown FIGURE 1. MEIT conductor BAZ6 at 3.25 mm with two Nb60wt%Ta fins segmenting the filaments. 2

TABLE 1. Key Parameters of Billet Matrix BAZ billets - 178 mm diameter by 305 mm length that the $1.50 /kam target can be met with margin [11]. FIGURE 1 illustrates a cross section of BAZ6 at 3.

3 TABLE 1. Key Parameters of Billet Matrix BAZ billets mm diameter by 305 mm length that the $1.50 /kam target can be met with margin [11]. FIGURE 1 illustrates a cross section of BAZ6 at 3.25 mm a MEIT conductor with two reaction barrier fins of Nb60wt%Ta. The outer diffusion barrier is Nb. The fins extend partially about the barrier to segment the reaction of the barrier. Nb at% Sn at% Cu at% Local Area Ratio Nb/Sn Ratio Nb area % BAZ6, #1095 filament, Reference Metals, 2 Nb60Ta, Fins, Ti+Sn BAZ7, #1044 filament, RM, Ti+Sn BAZ8, #1027 Nb7.5Ta filament, Cabot, 2 Nb60Ta Fins APPROACH Three 178 mm diameter billets were fabricated under an SBIR phase II program to investigate the effect of the fins on the magnetization, drawing properties, and the substitution of NbTa for pure Nb filaments on the J c. BAZ7 was the control billet free of fins with pure Nb filaments and a Sn+Ti core. BAZ6 had two Nb60wt%Ta fins designed to lower D eff as illustrated in FIGURE 1. The third billet (BAZ8) also had two fins but used Nb7.5wt%Ta alloy to increase the critical field. The design details of the billets are given in TABLE 1. A fourth billet with three fins has also been processed in the MEIT configuration and will be included in table 1 for completeness [6]. Nb60wt%Ta was chosen for the fin as under the reaction conditions it is expected to be a poor superconductor with any superconducting properties quenched at low fields [12]. The alloy is also easier to fabricate than the more costly pure Ta. Nb60wt%Ta was used in BAZ8 as a number of problems emerged in BAZ6 and BAZ7 that indicated that the Ti addition to the Sn was not reaching the reaction layer. FIGURE 2b suggests that much of the Ti is tied up in the inner filament area and in precipitates as described by Suenaga [13]. In addition it appeared to limit reaction. Still the effective critical field (H*) of BAZ7 extrapolated to 26 T [9]. a) b) FIGURE 2. a) BAZ6 - SEM image (2300X) and b) Ti EDS X-ray map of reacted conductor showing strong local Ti concentrations in the conductor core compared with lower level in the Nb 3 Sn filament pack. 3

TABLE 2. Homogenization and Heat Treatment Time (hrs) Matrix for BAZ6, BAZ7 Billet and Sample# 185 ºC 340 ºC 575 ºC 650 ºC BAZ7 #662 0.152 mm 34 48 100 100 BAZ7 #414 0.

152 mm - FNAL 100 48 100 100 FABRICATION, HEAT TREATMENT, AND METALLURGICAL ANALYSIS Three 178 mm diameter by 355 mm length billets were assembled from copper clad Nb rods drawn to a 3.

4 TABLE 2. Homogenization and Heat Treatment Time (hrs) Matrix for BAZ6, BAZ7 Billet and Sample# 185 ºC 340 ºC 575 ºC 650 ºC BAZ7 # mm BAZ7 # mm BAZ6 # element restack - LBNL BAZ6 # mm - FNAL FABRICATION, HEAT TREATMENT, AND METALLURGICAL ANALYSIS Three 178 mm diameter by 355 mm length billets were assembled from copper clad Nb rods drawn to a 3.27 mm across the flat hex. BAZ6 and BAZ7 used Nb from Reference Metals while BAZ8 used Nb7.5Ta from Cabot Performance Metals. The billets were extruded at 800 ºC to 37.6 mm diameter. Samples of wire were drawn to mm, mm and twisted for testing. The mm wire diameter scales to mm if extruded from a commercial size 305 mm diameter billet. Final drawing below 1 mm was done at NEEW at 110 m/min. on a no slip multi-die machine using a full 20.6% area reduction die schedule meters in one piece of BAZ7 at mm diameter was delivered to LBL for evaluation. An additional 11.1 km in four pieces has been drawn to 0.38 mm from two cuts lengths. 13 km of the 22.5 km of BAZ6 at 0.33 mm diameter clad with an additional 30% copper was delivered to BNL in one piece. An additional 64 km was drawn from two cut lengths to mm to yield 9 pieces the longest being 15.8 km. Samples of BAZ8 were drawn to mm but have not yet been heat treated. The samples of BAZ6 and BAZ7 for heat treatment and testing were cabled with six copper strands of the same size with the superconductor being the central strand. This has been found in the past to aid testing through stabilization and improving the mechanical strength of the fine wire. Samples of BAZ6 after reaction were imaged and x-ray images of Sn, Ti, Nb, Cu and Ta, were taken with special attention to the fin. FIGURE 3 shows the x-ray image of Sn, which can be seen to be well distributed through the filaments but mainly absent in the Nb60Ta fin. FESEM images at the University of Wisconsin-Madison (UW) of the fin at thinner sections produced in re-stacks did indicate reaction.[14] In FIG 5 we show a a) b) FIGURE 3. X Ray (EDS) maps of a) for Sn and b) Ta of BAZ6 from the same SEM image as FIG 2a 2300X. 4

FIGURE 4. MO images (Polyanskii-UW) of a) BAZ6 at 0.149 mm and b) BAZ7 in a restack at 0.02 mt and 11.

$FESEM fractograph of the fin area in the 36 sub-element restack of BAZ 6 after reaction, with the brittle cleavage of the reacted fin clearly indicated.$

5 FIGURE 4. MO images (Polyanskii-UW) of a) BAZ6 at mm and b) BAZ7 in a restack at 0.02 mt and 11.6 K showing penetration of the flux into the core of BAZ6 but not into the sub-element cores of BAZ7. FESEM fractograph of the fin area in the 36 sub-element restack of BAZ 6 after reaction, with the brittle cleavage of the reacted fin clearly indicated. Magneto-optical, MO, characterization was performed by Polyanskii at the UW using a 5 µm thick iron garnet indicator film with in-plane magnetization. The sample was placed on a cooling finger of a continuous flow optical cryostat located on the X-Y stage of a polarized optical microscope in reflective mode. The garnet indicator was placed on the polished surface of superconductors to register the normal components of the magnetic flux. The details of this method have been reported earlier [15,16]. MAGNETIZATION AND J c VS B RESULTS Magneto-optical images were made of BAZ6 in the MEIT form and of BAZ7 in a restack form to compare the flux penetration between conductors with and with out the fin. The measurements were performed at 11.6 K and at 0.02 mt hence the effectiveness at 4.2 K is not determined. As can be seen in FIG 4a the white core indicates the flux has penetrated through the fins in conductor BAZ6 while in the BAZ7 conductor (FIG 4a) the FIGURE 5. FESEM fractograph of the 36 sub-element restack BAZ 6 (Fin) at 0.5 mm (FNAL HT). 5

6 Magnetization (ka/m) Magnetization (ka/m) a) Applied Field (T) b) Applied Field (T) FIGURE 6. Magnetization per total sample volume for mm diameter wire of a) BAZ6 and b) BAZ7 at 2.68 K for heat treatment of 100 hrs at 650 ºC. continuous dark contrast across the sub-elements indicates that there has been no flux penetration in the sub-element cores. In FIG 5 we show a FESEM fractograph of the fin area in BAZ after reaction, with the brittle cleavage of the reacted fin clearly indicated, showing the extent of the A15 reaction. The T c of Nb(Ta) 3 Sn decreases with increasing Ta content [e.g. 17]. Additionally Ta40 wt.% Nb diffusion barriers have been used successfully in the past as a sub-element barrier material for low hysteresis loss strand, and the hysteresis loss due to the A15 formed as a layer on the Ta40 wt.% Nb diffusion barrier has been reported as minimal [18]. McKinnell et al. [18] also suggest that low temperature (650 C) heat treatment is also important in maintaining a low T c in their Nb(Ta) 3 Sn. Magnetization measurements were made by Ron Goldfarb at NIST of mm diameter BAZ6 and BAZ7 heat treated for 100 hrs at 650 ºC. Significant flux jumping occurred at 4.2 K. Measurements at 2.6 K significantly reduced the effect as can be seen in FIGURE 6. J c for calculation of D eff at 5 T were estimated by extrapolating J c from a Kramer plot and then adjusting for the temperature using the model of L.T. Summers et al [19]. J c of the sample of BAZ7 was 2.66 x10 9 A/m 2 at 12 T. BAZ6 J c testing has proven J c 10e 9 (A/m²) BAZ7.152mm # hr, 650 C BAZ7.152 mm # hr, 650 C BAZ6, mm # hr, 650 C BAZ6.152mm # hr, 650 C B (Tesla) FIGURE 7. Conductor BAZ6 and 7 J c vs. B (T) for 650 C heat treatments in MEIT and one 18 element restack. 6

7 difficult as most testing has been done at mm. A J c of 2.0x10 9 A/m 2 at 12 T of mm diameter sample #606 was used to calculate the D eff for BAZ6. This may overestimate the J c at mm as reaction is incomplete and the filament size is larger. Using these J c values results in a D eff of 165 µm for BAZ7 and 80 µm for BAZ6 was estimated. The calculated diameter of the core (d o ) of BAZ7 is 209 µm and BAZ6 is 207 µm. Flux jumping in BAZ7 lowers the magnetization and hence understates D eff in this measurement. Work by M. D. Sumption et al. indicates that D eff can actually be greater than D o and presents experiment and theory on the internal fin barrier [20]. J c vs. B Results Testing of BAZ7 was performed at the National High Magnetic Field Laboratory in Tallahassee. Testing of BAZ6 in MEIT form was carried out by E. Barzi at FNAL. FIG 7 illustrates the J c vs B for the best results on BAZ7, 2.85 x 10 9 A/m 2 at 12 T. The best of BAZ6 yielded 2.0 x 10 9 A/m 2 at 12 T. BAZ6 tested at mm and as a composite #6101 composed of 18 sub-elements of was not fully reacted [8]. Testing has been difficult on these fragile samples using the standard ITER mandrel. The Sn to Nb ratio is estimated to be 3.09 with the Nb in the fins included, insufficient for full reaction. This lower ratio, combined with blocking of the diffusion paths by the Ti precipitates [14], should result in a both reduced volume of Nb 3 Sn and a lower critical current density in the Nb 3 Sn layer [15]. CONCLUSION MEIT conductors both with and without fins can be fabricated to yield long lengths with the longest to date being 15.8 km at mm diameter in a production environment. Success in drawing to mm diameter shows that the process can be scaled to 305 mm diameter billets. High J c at 12 T (2.85x10 9 A/m 2 ) can be achieved and further improvements using Nb7.5Ta and additional optimization should yield higher current densities. The fin as a means to reduce D eff appears to work but needs more analysis and testing. The use of Ta in the MEIT process could remove any uncertainty though it may affect wire piece length. The fin may be useful in other processes such as the Hot Extruded Rod under development by OI-ST as it reduces the number of sub-elements required. The thickness of the fin may have to be adjusted to compensate for reaction. Ta if fabricable in other high total strain Internal Tin processes would be another alternative to Nb60wt%Ta and eliminate concerns of a superconducting layer penetrating through the fin. ACKNOWLEDGEMENTS The authors would like to thank at Outokumpu A. S., M. Dormandy, and M. Vincenzi, for their efforts on the wire. We would also like to thank E. Barzi for measurements and helpful discussions. B. Zeitlin would especially like to thank T Carneiro of Reference Metals Co. for contribution of the niobium ingot. P. J. Lee and A. A. Polyanskii would like to thank Bill Starch at the UW for metallographic sample fabrication. This work was supported by a U.S. Dept. of Energy SBIR phase II grant DEFG0299ER82899 and additional work was supported at the University of Wisconsin-Madison by the U.S. Dept. of Energy, Division of High Energy Physics (DE-FG02-91ER40643). 7

8 REFERENCES 1. Gupta, R., Chow, K., Dietderich, D., Gourlay, S., Millos, G. McInturff, A. Scanlan, R., "A High Field Magnet Design for a Future Hadron Collider", IEEE Trans. on Applied Superconductivity, 9:2, pp (1999). 2. Scanlan, R. M., Dietderich, D. and Zeitlin, B. A., "Development of Cost-Effective Nb 3 Sn Conductors for the Next Generation Hadron Colliders", Advances in Cryogenic Engineering Materials, vol. 48B, edited by U. B. Balachandran et al., AIP, NY, 2001, pp Martovetksy, N., Michael, P., Takayasu, M., Ando, T., Isonon, T., Nakajima, H., Okuno, K., Mitchell, N., Bessette, D., "Central Solenoid Model Coil (CSMC) and CS Insert Coil Test Results," IEEE Trans. on Applied Superconductivity, 11:1, pp (2001). 4. Baang, S., et al., "The Test Facility for the KSTAR Superconducting Magnets at SAIY," IEEE Trans. on Applied Superconductivity, 10:1, pp (2000). 5. Smith, B. A. et al., "Design, Fabrication and Test of the React and Wind, Nb 3 Sn LDX Floating Coil", IEEE Trans. on Applied Superconductivity, 11:1, pp (2001). 6. Pyon, T., Somerkoski, J., Zeitlin B. A., "Improved Properties of Internal-tin Processed Nb 3 Sn Superconductor Developed for High Energy Physics", Paper M1-M-05, presented at CEC-ICMC, Anchorage Alaska, Sept 23-26, Parrell, J. A., Field, M. B., Zhang, Y., Hong, S., "Nb 3 Sn Conductor Developed for Fusion and Particle Accelerator Application", Paper M1-M-04, CEC/ICMC, Anchorage Alaska, Sept 23-26, Gregory, E. et al., "Attempts to Reduce A. C. Losses in High Current Density Internal-Tin Nb 3 Sn", Paper M1-D-09, presented at CEC-ICMC, Anchorage Alaska, Sept 23-26, Zeitlin, B. A., Gregory, E., Pyon, T. and Scanlan, R. M., "Continued Progress on a Low Cost High Current Density Mon Element Internal Tin Conductor (MEIT) with Integral Barriers", IEEE Trans. on Applied Superconductivity, 13:2, pp (2003). 10. Zeitlin, B. A., "A Method to Reduce Magnetization in High Current Density Conductors Formed by Reaction of Multi-component Elements in Filamentary Composite Superconductors" patent pending. 11. Zeitlin, B. A., Pyon, T., Gregory, E., Scanlan, R. M., "Progress on a High Current Density Low Cost Niobium3Tin Conductor Scalable to Modern Niobium Titanium Production ", Advances in Cryogenic Engineering Materials, vol. 48B, edited by U. B. Balachandran et al., 2001, pp McKinnell, J. C., O'Leary, P. M., Jablonski, P. D., and Siddall, M. B., "Tantalum-Niobium Alloy Diffusion Barriers for Superconducting Nb 3 Sn Wires in Fusion Applications", Advances in Cryogenic Engineering, vol. 42, edited by L. T. Summers, Plenum Press, 1996, pp Suenaga, M., "Tin Diffusion Effects in Variable Diameter Internal Sn Conductors", LTSW Napa Valley Marriott CA, Nov , Lee, P. J., and Larbalestier, D. C., "Advances in Superconducting Strands for Accelerator Magnet Application", paper TOAB003, Proceedings of the 2003 Particle Accelerator Conference to be published. 15. Polyanskii, A. A., Cai, X. Y., Feldmann, D. M., and Larbalestier, D. C., in Nano-crystalline and Thin Film Magnetic Oxides (NATO Science Series 3. High Technology-Vol. 72), edited by I. Nedkov and M. Ausloos, Kluwer Academic Publishers, 1999, pp Polyanskii, A. A., Feldmann, D. M., and Larbalestier, D. C., Magneto-Optical characterization Techniques, Chapter C3.4 of Handbook of Superconducting Materials, edited by D. Cardwell, and D. Ginley, Institute of Physics Publishing. Bristol, London, Philadelphia, St Petersburg USA, Suenaga, M., et al., "Superconducting properties of (Nb,Ta) 3 Sn wires fabricated by the bronze process, Advances in Cryogenic Engineering., vol. 26, edited by A. F. Clark and R. P. Reed, Plenum, NY, pp (1980). 18. McKinnell, J. C., O'Larey, P. M., Jablonski, P. D., and Siddall, M. B., Tantalum-niobium alloy diffusion barriers for superconducting Nb 3 Sn wires in fusion applications, Advances in Cryogenic Engineering (Materials), edited by L. T. Summers, Plenum, NY, vol. 42:2, 1997, pp Summers, L. T., Guinan, M. W., Miller, J. R., and Hahn, P. A., "A Model for the Prediction of Nb 3 Sn Critical Current as a Function of Field, Temperature, Strain, and Radiation Damage", IEEE Trans on Mag. 27:2, pp (1991). 20. Sumption, M. D., Lee, E., Peng. X., Wu, X., Collings, E. W., Gregory, E. "Magnetization and Effective Filament Diameter in High Energy Physics Relevant Rod-In-Tube Type Nb 3 Sn Strands", Paper M3-G-03, CEC/ICMC, Anchorage Alaska, Sept 23-26,

Development of Nb 3 Sn in Japan

Development of Nb 3 Sn in Japan T. Ogitsu, T. Nakamoto, M. Sugano KEK, S. Awaji Tohoku Univ., H. Oguro Tokai Univ., A. Vallarino, M. Benedict CERN, K. Miyashita, Y. Suzuki SH Copper Products., K. Saito,