An Extended Characterization of European Advanced ... - IEEE Xplore

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 16, NO. 2, JUNE 2006

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An Extended Characterization of European Advanced Nb3Sn Strands for ITER L. Muzzi, S. Chiarelli, A. della Corte, A. Di Zenobio, M. Moroni, A. Rufoloni, A. Vannozzi, E. Salpietro, and A. Vostner

Abstract—Within the framework of ITER-related projects, new tasks have been recently launched by EFDA CSU Garching (European Fusion Development Agreement Close Support Unit Garching), for the definition and production on industrial scale of advanced strands, to be used in the manufacturing of 3 the ITER high field CS and TF magnets. We performed an exstrands coming tended characterization of the advanced 3 from different European companies, in terms of strand layout :noncoating, ratio), critical (diameter, thickness of transport current, RRR, and hysteresis losses. The results of the measurement campaign show that the upgraded strands meet the latest ITER requirements, with an overall critical transport current of at least 200 A (at 12 T, 4.2 K), equivalent to a non2 ,a :nonratio of about 1, a strand Jc of about 800 diameter of 0.81 mm, and with nonhysteresis losses limited to 3 on a 3 T field cycle at 4.2 K. less than 1000

Nb Sn

Cr

Index materials.

TABLE I MAIN FEATURES OF THE ANALYZED Nb SN STRAND SAMPLES: ONLY THE LAST TEMPERATURE STEP OF THE HEAT TREATMENT (HT) HAS BEEN INDICATED

Nb Sn Cu Cu

A mm Cu Cu Cu kJ m Terms—ITER, Nb3 Sn strand,

Cu

superconducting

III. EXPERIMENTAL CHARACTERIZATIONS I. INTRODUCTION HE CONTINUOUS interest shown in from both fusion and high energy physics communities has challenged suppliers in producing multifilamentary strands with higher and higher performances. EFDA has stimulated several European industries to optimize their production in order to meet the latest ITER design criteria and requirements, in terms of both critical current and hysteresis losses [1]. ENEA has been involved in the study and characterization of the structural and superconducting properties of such European “advanced” strands, whose main results are presented here.

T

II. SAMPLES strand samples analyzed in this work have been The supplied by two different European companies: Oxford Superconducting Technology (OST) [2], and Outokumpu Copper Superconductors Italy (OCSI), both producing an Internal Tin type strand. The main features of the characterized strands are reported in Table I. Heat treatments (HT) have been performed at ENEA, in some case with slightly different temperature or time steps with respect to those recommended by suppliers, as explicitly indicated in the table.

Manuscript received September 19, 2005. L. Muzzi, S. Chiarelli, A. della Corte, A. Di Zenobio, M. Moroni, A. Rufoloni, and A. Vannozzi are with EURATOM-ENEA Association, ENEA C.R. Frascati, 00044 Frascati (Rome), Italy (e-mail: [email protected]). E. Salpietro and A. Vostner are with EFDA, CSU Garching, D-85748 Garching, Germany. Digital Object Identifier 10.1109/TASC.2006.870802

An extended characterization of the strands has been performed. Both structural properties (strands diameter, thickness :nonratio) have been of external coating, if present, and investigated, as well as superconducting performance, in terms of electrical properties (critical current density and RRR), and magnetic properties (hysteresis losses in 3 T magnetic field cycles). A. SEM Analysis The samples cross sections have been analyzed by Scanning Electron Microscope (SEM), both before and after having performed the heat treatments. From SEM micrographs the strand diameter has been measured, by both measuring the diameter of the best fitting circumference and by direct measurement, as well as the thickness of the outer layer. Moreover, starting :nonratio from the same SEM cross sections, also the of the different samples has been determined by a gray scale contrast method, in which the digital image is binarized and white and black sections, respectively representing the copper and noncopper areas, are counted. The results are reported in Table II. As one can see, in the case of the OST strands, the measured strand diameters after heat treatment are slightly higher than the ones measured before the heat treatment, owing to the volumetric change during reaction to A15 phase . Two examples of the acquired SEM cross-sections are reported in Figs. 1 and 2 for the OST-Type I strand and the OCSI strand, respectively: the different layout of the strands produced by the two suppliers is clearly visible. In particular, the strand

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TABLE II RESULTS OF THE MEASUREMENTS PERFORMED USING THE SEM

Fig. 1. SEM image of the OST-Type I internal tin strand, after heat treatment. The external Ta diffusion barrier is clearly visible. One of the 19 strand subelements is also shown in enlarged view (right).

Fig. 4. EDX analysis of the OST-type II strand, after the heat treatment. The white area represents the spatial distribution of each element. Upper left: Cu; upper right: Nb; lower left: Sn; lower right: Ta.

Fig. 2. SEM image of the OCSI internal tin strand, after heat treatment (left). One of the 26 strand sub-elements is also shown in enlarged view (right), evidencing the distributed Ta-Nb barrier, surrounding each “island”.

Fig. 5. EDX analysis of the OCSI strand, after the heat treatment. The white area represents the spatial distribution of each element. Upper left: Cu; upper right: Nb; lower left: Sn; lower right: Ta.

of the OST type II strand, before and after the HT respectively; the distribution of the selected elements has been evidenced in the analysis and, in each image, the white area correspond to: ; upper left ; lower right ; lower left . upper right Fig. 5 reports the result of the same kind of analysis, performed on the OCSI strand, after the heat treatment, which evidences barrier. the layout of the distributed Fig. 3. EDX analysis of the OST-type II strand, before the heat treatment. The white area represents the spatial distribution of each element. Upper left: Cu; upper right: Nb; lower left: Sn; lower right: Ta.

produced by OST is characterized by a single diffusion barrier, while the OCSI strand is designed with a distributed diffusion barrier. In addition, Energy Dispersive X-ray (EDX) spectroscopy has been performed on all the samples, to identify the distribution of the different elements within the strand cross-section and, by comparison between the results taken before and after the HT, to observe the effect of the diffusion reaction between barriers. As an example, Figs. 3 and 4 report the EDX analyses

B. Electrical Characterizations All samples have been wound and heat treated on standard Ti6Al4V ITER barrels. For the measurement of the RRR each sample has been then transferred by hand on an adapted ITER barrel, from which the coiled strand is separated by a thin electrically insulating paper. A thermometer is mounted on the end section of the sample holder and a copper cap encloses the whole system, in order to keep the temperature uniform over the strand length. The sample resistance is measured by a 4-points technique, using a supply current of 30 mA, from room temperature down to 4.2 K. RRR is determined from the resistance ratio between 300 K and a temperature of about 18.5 K, just above the superconductor transition.

MUZZI et al.: AN EXTENDED CHARACTERIZATION OF EUROPEAN ADVANCED

STRANDS FOR ITER

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TABLE III RESULTS OF ELECTRICAL AND MAGNETIC CHARACTERIZATIONS

Fig. 7. Transport critical current at 0.1 V=cm, measured on the different samples at 4.2 K. The different curve symbols are explained in the figure inset.

Fig. 6. I-V curves measured at 4.2 K and 12 T, for some Nb Sn strand samples; the different curve symbols are explained in the figure inset.

For what concerns transport critical current, following the usual procedures, the strands have been measured at 4.2 K on the same standard ITER barrels used for the heat treatments. Owing to the expected high critical current values, in order to avoid sample heating at the terminations, for each sample one full turn from an additional length of the same strand is soldered, at the holder terminations, in parallel to the wire to be measured. electric field. The Critical current is determined at 0.1 result of the reference 12 T point for all the strands is reported in Table III. As one can see, all the suppliers have succeeded in producing strands that meet the 200 A minimum ITER requirement. For the strands produced by OCSI, beside a new layout strand that has been recently proposed, with good results in terms of critical current, HT optimization is still underway: for the strand, the layout of which is shown in Fig. 2, the critical current increases with increasing HT duration, up to a maximum, below which an grains probably causes a decrease excessive growth of of the pinning force and consequently of the critical current [3]. The critical current has been measured at fields between 10 T and 14 T. For comparison, the I-V curves at 12 T are shown in Fig. 6, for some of the samples. The OST type II strand showed an instability behavior just before reaching its critical current electric field), while it was not possible to measure (0.1 it at fields below 12 T, owing to the occurring of strand breakage as soon as a measurable electric field developed.

Fig. 8. Critical current density of the different samples, normalized to the non-Cu section, as measured by SEM analysis. The different curve symbols are explained in the figure inset.

vs. B curves for the different samples are reported in Fig. 7. The 200 A level of the ITER minimum requirement for the total transport current at 12 T is also evidenced in the plot. content within the strand cross section is not Since the the same for all the samples (cfr. Table II), critical currents have been normalized to the noncopper area of each strand, in order critical current density. The reto compare the effective sult is reported in Fig. 8, in terms of critical current density as function of the applied magnetic field. Here, the 800 level of the ITER minimum requirement at 12 T is evidenced. :nonAs one can see, due to its sensibly higher value of ratio, the OCSI_b strand is well above the ITER criteria in terms critical current density. of nonC. Magnetization Measurements Hysteresis losses have been determined from magnetization cycles measured by Vibrating Sample Magnetometer (VSM). The measurements have been performed at 4.2 K in 3 T cycles, with a field variation rate of 0.1 T/min ( 3.3 mT/s) and using a vibrating amplitude and frequency of, respectively, 0.2 mm and 55 Hz. Samples for losses measurements are wound on a 5 mm diameter screw; after heat treatment 1 turn, out of the coiled length, is cut and its magnetization is measured. The calibration of the sample magnetic moment is performed by comparison with a high purity Ni sample with same geometry. Fig. 9 shows the measured magnetization cycles for some of the samples, while the results of the hysteresis AC losses, referred

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thus be characterized by lower hysteresis losses, due to smaller A15 grain size [3]. This will be verified by further experimental investigations. IV. COMMENTS AND CONCLUSIONS Structural and superconducting properties of “advanced” strands produced by different European industries for ITER application have been studied by means of morphological (SEM) and compositional analyses (EDX), as well as electrical and magnetic characterizations. Results have demonstrated that the developed production techniques on industrial scale meet the ITER requirements, even if in some case further optimization in terms of strand layout and reaction heat treatment is still underway. Fig. 9. Magnetization cycles measured at 4.2 K, with a magnetic field ramp of 3.3 mT/s. The different curve symbols are explained in the figure inset.

to the noncopper section, as calculated from the magnetization loop areas are reported in Table III, to be compared with the ITER requirement for the Toroidal Field (TF) coils internal tin non- . For the OCSI strand, only the strands of 1000 sample OCSI_d has been measured, showing slightly excessive hysteresis losses: on the contrary, the sample OCSI_b, which shows higher critical current, has undergone a much shorter heat treatment with respect to the OCSI_d(cfr. Table I) and should

REFERENCES [1] A. Vostner and E. Salpietro, “Enhanced critical current densities in Nb Sn superconductors for large magnets,” presented at the EUCAS’05 Conf. Vienna, Sep. 2005, submitted for publication to Supercond. Sci. Technol.. [2] J. A. Parrell, Y. Zhang, M. B. Field, P. Cisek, and S. Hong, “High field Nb Sn conductor development at Oxford superconducting technology,” IEEE Trans. Appl. Supercond., vol. 13, p. 2, 2003. [3] C. M. Fischer, P. J. Lee, and D. C. Larbalestier, “Irreversibility field and critical current density as a function of heat treatment time and temperature for a pure niobium powder-in-tube Nb Sn conductor,” in Adv. Cryo. Eng.: Proc. Int. Cryogenic Materials Conf.—ICMC, B. Balachandran, Ed., 2002, vol. 48, p. 1008.