Oxford Superconducting Technology Research Articles

Impact of indentation on the performance of high Jc Nb3Sn strand

To meet the peak magnetic field requirements of the central solenoid (CS) and toroidal field (TF) magnets in the China Fusion Engineering Test Reactor (CFETR), cable-in-conduit conductors (CICC) that are composed of Nb3Sn strands with high critical current density (Jc) have been developed. The Short-Twist-Pitch (STP) design is adopted to avoid performance degradation caused by the electromagnetic cycles. But the STP design and tight cabling will cause severe indentation at the contact point between strands. Previous studies on Nb3Sn CICC show that the indentation has a significant effect on the critical current (Ic) and residual resistivity ratio (RRR) of the superconducting strand. Therefore, the same study was carried out for the high Jc Nb3Sn superconducting strand produced by Oxford Superconducting Technology (OST) and Kis wire Advanced Technology CO., Ltd. (KAT). In this paper, the Ic and RRR measurements of sample at different indentation depths were made, and the results were compared with the previous Nb3Sn indentation strands and flat rolled samples. Also, the hysteresis loss of the samples with different indentation depth was measured, and the change of effective filament diameter with indentation depth was obtained.

High-Performance Superconductors for Fusion Nuclear Science Facility

High-performance superconducting magnets play an important role in the design of the next step large-scale, high-field fusion reactors such as the fusion nuclear science facility (FNSF) and the spherical tokamak (ST) pilot plant beyond ITER. Princeton Plasma Physics Laboratory is currently leading the design studies of the FNSF and the ST pilot plant study. ITER, which is under construction in the south of France, utilizes the state-of-the-art low temperature superconducting magnet technology based on the cable-in-conduit conductor design, where over a thousand multifilament Nb 3 Sn superconducting strands are twisted together to form a high-current-carrying cable inserted into a steel jacket for coil windings. We present design options of the high-performance superconductors in the winding pack for the FNSF toroidal field magnet system based on the toroidal field radial build from the system code. For the low temperature superconductor options, the advanced J c Nb 3 Sn RRP strands (J c > 1000 A/mm 2 at 16 T, 4 K) from Oxford Superconducting Technology are under consideration. For the high-temperature superconductor options, the rectangular-shaped high-current HTS cable made of stacked YBCO tapes will be considered to validate feasibility of TF coil winding pack design for the ST-FNSF magnets.

Effects of filament size on critical current density in overpressure processed Bi-2212 round wire.

Bi2Sr2CaCu2Ox (Bi-2212) conductor is the only high temperature superconductor manufactured as a round wire and is a very promising conductor for very high field applications. One of the key design parameters of Bi-2212 wire is its filament size, which has been previously reported to affect the critical current density (Jc ) and ac losses. Work with 1 bar heat treatment showed that the optimal filament diameter was about 15 μm but it was not well understood at that time that gas bubbles were the main current limiting mechanism. Here we investigated a recent Bi-2212 wire with a 121×18 filament architecture with varying wire diameter (1.0 to 1.5 mm) using 50 bar overpressure processing. This wire is part of a 1.2 km piece length of 1.0 mm diameter made by Oxford Superconducting Technology. We found that Jc is independent of the filament size in the range from 9 to 14 μm, although the n value increased with increasing filament size. A new record Jc (4.2 K, 15 T) of 4200 A/mm2 and JE (4.2 K, 15 T) of 830 A/mm2 were achieved.

Progress in the Development of the HFML 45 T Hybrid Magnet

To extend its user's facilities, the High Field Magnet Laboratory (HFML-EMFL) at the Radboud University is in the process of building a 45-T hybrid magnet. The magnet system will consist of a 22 MW 32.7 T resistive insert and a 600-mm-bore 12.3 T superconducting outsert magnet, and the design was significantly adjusted after a thorough design revision in 2011. The HFML hybrid magnet will be operated with separate current sources for the superconducting and resistive coils (20 kA at 10 V and 40 kA at up to 550 V, respectively). The outsert coil is a solenoid layer wound with all-Nb 3 Sn/Cu cable-in-conduit conductor (CICC), cooled by a forced flow of supercritical helium and operated at 20 kA. Similar to the series-connected hybrids for the HZB (Berlin, Germany) and the NHMFL (Tallahassee, FL, USA), the HFML outsert coil contains three grades of conductor. All CICC grades are based on high-current density Nb3Sn strand produced by Oxford Superconducting Technology. The CICC production and qualification program has been completed successfully. The coil will be wound at the NHMFL and, after heat treatment and impregnation, sent to Nijmegen for integration into the cryostat. In this paper, the design choices and current status of the program are presented.

Nb3Sn RRP Strand and Rutherford Cable Development for a 15 T Dipole Demonstrator

Keystoned Rutherford cables made of 28 strands and with a stainless steel core were developed and manufactured using 1-mm $\text{Nb}_{3}\text{Sn}$ composite wires produced by Oxford Superconducting Technology with 127 and 169 restacks using the Restacked-Rod-Process. The performance and properties of these cables were studied to evaluate possible candidates for 15-T accelerator magnets.

Influence of the heat-treatment conditions, microchemistry, and microstructure on the irreversible strain limit of a selection of Ti-doped internal-tin Nb3Sn ITER wires*

Systematic studies of the intrinsic irreversible strain limit εirr,0, microstructure, and microchemistry were made on several internal-tin Nb3Sn pre-production wires, fabricated for the domestic agencies of the USA and China participating in the International Thermonuclear Experimental Reactor. These wires were produced by Luvata, Oxford Superconducting Technology (OST), and Western Superconducting Technologies (WST), and were intended for the tokamak’s toroidal-field coils. The results of this study show that, for a final heat-treatment at 650 °C to form the A15 phase, both εirr,0 and the de-pinning field Bc2* improved by increasing heat-treatment duration beyond 100 h for the Luvata wires. On the other hand, we saw no improvement in these two parameters as a function of heat-treatment duration in the OST wires. Furthermore, micro-chemical analysis of OST wires revealed that some Nb3Sn filaments have a Sn- and Ti-rich phase at the interface between Cu(Sn) matrix and Nb3Sn in the form of a shell around individual filaments. This phase is far less prominent in the Luvata and WST conductors, and could inhibit diffusion of Sn and Ti into Nb3Sn filaments during the reaction and may potentially be the reason for the lack of noticeable change in Bc2* with heat-treatment duration in the OST wires. The increase of εirr,0 and Bc2* with heat-treatment duration in the Luvata wires and the lack of increase in the OST wires may suggest a possible correlation between εirr,0 and the stoichiometry of the A15 composition. Investigation of the samples’ microstructure revealed only a small number of cracked Nb3Sn filaments despite the significant and permanent degradation of their critical current Ic when subjected to longitudinal tensile strain ε beyond εirr,0. The scarcity of cracks indicate that Ic(ε) measurements are highly sensitive to crack formation in Nb3Sn filaments, especially at low electric-field criteria ≦̸0.1 μV cm−1, even when the sizes of the individual filaments are only few micrometers. All the strands contained substantial Kirkendall porosity, but we found that the quantity and distribution of the Kirkendall voids vary significantly with strand design. Luvata wires have the least porosity, followed by WST wires, and then by OST strands. However, even though the presence of cracks in the Nb3Sn filaments that are in close proximity to Kirkendall voids suggest a correlation between crack initiation and the proximity of the filaments to these voids, the porosity investigation established no definitive relationship between porosity and εirr,0 in the wires studied.

Bi-2212 Round Wire Development for High Field Applications

Oxford Superconducting Technology continuously improves Bi-2212 round wire performance because this product has a unique property for ultra high-field magnet ( 25 T) applications. Our recent results on improving the engineering current density by filament densification and reducing trace carbon and hydrogen contamination are presented. The swaging, isostatic pressing, and over-pressure heat treatment processes have been demonstrated to effectively increase the Bi-2212 filament mass density in the final wire and results in high performance over long wire length. Engineering current density values exceeding 550 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 4.2 K, 15 T have been achieved on 1-m-long barrel samples. Several configurations have been developed to meet different operating current requirements with optimum filament size ( ~ 15 micron) on critical current density, Jc. The round wire has also been twisted down to 12 mm in twist pitch length without performance degradation.

Bi-2212 round wire development for high field applications

Oxford Superconducting Technology (OST) has been continuously improving Bi-2212 round wire performance because of its potential for application in high-field magnets (> 25 T). We focused on Bi-2212 wire configuration design, filament densification and reducing carbon and hydrogen contamination to improve the engineering critical current density (JE). Several wire configurations have been developed to meet different wire diameter and operating current requirements. The swaging, cold isostatic pressing (CIP) and over-pressure heat treatment processes have been demonstrated to effectively increase Bi-2212 filament mass density in the final wire and result in high performance over long length. The JE values exceeding 550 A/mm2 at 4.2 K, 15 T have been achieved on the CIPed 1 m long sample using a 10 bar over-pressure (OP) heat treatment. The twisted Bi-2212 wire significantly reduced ac loss without the critical current degradation.

Progress in &lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;$\hbox{Nb}_{3}\hbox{Sn}$&lt;/tex&gt;&lt;/formula&gt; RRP Strand Studies and Rutherford Cable Development at FNAL

A strong push to Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductor development in the U.S. as well as in Europe has been driven by the development of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn dipoles and quadrupoles for the Large Hadron Collider (LHC) luminosity upgrades. Rutherford cables with high aspect ratio are used for these magnets to achieve large fields and gradients at relatively low currents. At Fermilab National Accelerator Laboratory (FNAL), 40-strand keystoned cables with and without a stainless steel core were developed and produced using 0.7-mm Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands made by Oxford Superconducting Technology with 127 and 169 restacks using the Restacked Rod Process with either NbTa alloy or Ti doping. The performance and properties of such strands and cables were studied to evaluate possible candidates for the production magnets of the LHC upgrades. The electrical performance was first compared for wires under flat-rolling deformation and then in cables made with different processes and geometries. The round wires are also compared under tensile and compressive strain using a Walters' spring variable-temperature probe that was recently commissioned at FNAL. Finally, cable test results obtained with a 14 T/16 T Rutherford cable test facility with a bifilar sample and a superconducting transformer are shown.

Magnetization Measurements of High-$J_{\rm c}$$\hbox{Nb}_{3}\hbox{Sn}$ Strands

High critical current density Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> > 2500 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 4.2 K and 12 T) are the conductors considered for next-generation accelerator magnets. At present, the large magnetization of these strands is a concern within the scientific community because of the impact it might have on the magnet field quality. In order to characterize the magnetic behavior of these wires, an extensive campaign of magnetization measurements was launched at CERN. Powder-in-tube strands by Bruker-EAS and Restacked Rod Process strands by Oxford Superconducting Technology were measured between 0 and 10.5 T at different temperatures (ranging from 1.9 to 14.5 K). The samples, based on strands with different subelements dimensions (35 to 80 μm), were measured with a vibrating sample magnetometer. The experimental data were analyzed to: (1) calculate the effective filament size and the optimal parameters for the pinning force scaling law and (2) define the field-temperature region where there are flux jumps. It was found that the flux-jump can limit the maximum magnetization of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires and that the maximum magnetization at higher temperatures can be larger than the one at lower temperatures. In this paper, the experimental results and the analysis are reported and discussed.

Superconducting Strand and Cable Development for the LHC Upgrades and Beyond

Fermilab and CERN have started the development of 11 T Nb3Sn dipoles to replace a number of Large Hadron Collider (LHC) NbTi dipole magnets and free space for the additional collimators anticipated for the LHC luminosity upgrades. An essential step in the design of these magnets is the development of the 40-strand, high aspect ratio cable needed to achieve the nominal field of 11 T at the LHC operating current of 11.85 kA. To investigate conductors suited for this and other high-field magnet applications, a larger Superconducting Strand and Cable R&D lab was established at FNAL's Technical Division. Keystoned cables with and without a stainless steel core were developed and produced using 0.7 mm Nb3Sn strands made by Oxford Superconducting Technology with 127 (baseline) and 169 (advanced) restacks using the Restacked-Rod-Process. The electrical performance of these two strands is compared in cables made with different processes and geometries. Some of the effects of a cross-over in the cable were measured. Finally, it is shown how finite element modeling can be used as an aid in Rutherford-type cable design.

Test Results and Analysis of LQS03 Third Long $ \hbox{Nb}_{3}\hbox{Sn}$ Quadrupole by LARP

With the first test of LQS03, the long quadrupole (LQ) R&D by LARP (the US LHC Accelerator Research Program, a collaboration of BNL, FNAL, LBNL, and SLAC) is approaching conclusion. LQS03 is the third 3.7-m-long quadrupole, with 90 mm aperture, using a full new set of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$ </tex></formula> coils. The LQS03 coils were made using 108/127 RRP strand (with 108 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$</tex></formula> subelements) produced by Oxford Superconducting Technology, whereas both previous models used 54/61 RRP strand (with 54 larger <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{Nb}_{3} \hbox{Sn}$</tex></formula> subelements). In this paper, LQS03 test results are presented and discussed. The test results are also compared with the performances of the previous models. Observations are made for the future use of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$</tex></formula> in accelerator magnets.

Progress in the Long ${\rm Nb}_{3}{\rm Sn}$ Quadrupole R&amp;D by LARP

After the successful test of the first long quadrupole (LQS01) the US LHC Accelerator Research Program (LARP, a collaboration of BNL, FNAL, LBNL and SLAC) is assessing training memory, reproducibility, and other accelerator quality features of long quadrupole magnets. LQS01b (a reassembly of LQS01 with more uniform and higher pre-stress) was subjected to a full thermal cycle and reached the previous plateau of 222 T/m at 4.5 K in two quenches. A new set of four coils, made of the same type of conductor used in LQS01 (RRP 54/61 by Oxford Superconducting Technology), was assembled in the LQS01 structure and tested at 4.5 K and lower temperatures. The new magnet (LQS02) reached the target gradient (200 T/m) only at 2.6 K and lower temperatures, at inter- mediate ramp rates. The preliminary test analysis, here reported, showed a higher instability in the limiting coil than in the other coils of LQS01 and LQS02.

Extensive Characterization of the 1 mm PIT &lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;${\rm Nb}_{3}{\rm Sn}$&lt;/tex&gt;&lt;/formula&gt; Strand for the 13-T FRESCA2 Magnet

In the framework of the EuCARD program, CERN is participating in the development of a 13 T 100-mm-aperture dipole magnet to upgrade the superconducting cable test facility FRESCA at CERN. The conductor candidates for building this magnet are two 1-mm Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands: the Powder In Tube (PIT) produced by Bruker-EAS and the 132/169 RRP by Oxford Superconducting Technology (OST). Recently the PIT strand has been extensively characterized by CERN in collaboration with the University of Geneva (UniGe). The critical current dependence on the magnetic field and on the axial strain has been measured at different temperatures. Furthermore, the strand magnetization has been measured at different temperature using a vibrating sample magnetometer. Finally the magneto-thermal stability of this strand was studied by measuring the quench current between 0 T and 12 T at 1.9 K and 4.3 K. The experimental results are compared with an optimized scaling law for the critical current of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands. In this paper the results obtained for the PIT strand are summarized and discussed.

Studies of ${\rm Nb}_{3}{\rm Sn}$ Strands Based on the Restacked-Rod Process for High Field Accelerator Magnets

A major thrust in Fermilab's accelerator magnet R&D program is the development of wires which meet target requirements for high field magnets, such as high critical current density, low effective filament size, and the capability to withstand the cabling process. The performance of a number of strands with 150/169 restack design produced by Oxford Superconducting Technology using the Restacked-Rod Process was studied for round and deformed wires. To optimize the maximum plastic strain, finite element modeling was also used as an aid in the design. Results of mechanical, transport and metallographic analyses are presented for round and deformed wires.

Study of Effects of Transverse Deformation in BSCCO-2212 Wires

Viability of BSCCO-2212 round wires for high field magnet applications is being investigated within the Very High Field Superconducting Magnet Collaboration (VHFSMC). Three BSCCO-2212 wires of different designs were homogeneously flat-rolled to decreasing sizes to study the effect of deformation as a function of the precursor used and of the architecture parameters of the wires. After heat treatment at Oxford Superconducting Technology (OST), size variations of round and deformed samples were measured, and their transport properties characterized at 4.2 K and at magnetic fields up to 14 T. The wire made with granulate precursor was the most homogenous. The wires made with powder precursor showed instead non-monotonic behavior in Ic and even more so in the n-value.

BSCCO-2212 Wire and Cable Studies

BSCCO-2212 round wires and cables are being studied for possible use in very high field magnets (>; 25 T) within the multi-Lab Very High Field Superconducting Magnet Collaboration (VHFSMC). Billets produced by Oxford Superconducting Technology were used to make Rutherford-type cables of various geometries. The effect of the cable parameters was measured on the extracted strand performance and deformation. Transport properties were measured at 4.2 K in fields up to 14 T for the extracted strands across the edges, and compared with as-received round wire. The same wire used in the cables was also deformed by flat rolling to reproduce the deformation seen by the round strand when formed into a Rutherford cable. After heat treatment, rolled samples were characterized for transport properties and results compared to those obtained for strands extracted from the cables. Finally cables were tested at 4.2 K and self-field using a superconducting transformer and results compared to those of the extracted strands.

Test and Analysis of China's First Short Conductor Sample for ITER Toroidal Field Coils

In the framework of the ITER qualification tests, the first China TF conductor sample (CNTF1) was tested at the SULTAN facility. The sample was made of two TF conductor sections manufactured from identical internal stannum strands provided by the Oxford Superconducting technology company (OST). In order to evaluate the conductor performance, the current sharing temperature (Tcs) was measured at specified electromagnetic load cycling steps. Both conductor sections of the CNTF1 sample showed identical performance. Tcs was 7.2 K before cycling loading, and 6.9 K even after 950 cycles, without significant degradation, which substantially exceeds the ITER requirement of 5.7 K. The tests of the CNTF1 conductor sample showed that the electromagnetic cyclic load exhibited a negligible effect on the conductor performance. The coupling time constant θ for AC loss was 214 ms and 71.52 ms before and after the cycling load, respectively. The test results of the sample are compared with the strand performance and parameter model analysis.

Heat Treatment and Characterization of $\hbox{Nb}_{3} \hbox{Sn}$ Strands for the Model Coil of the 40-T Hybrid Magnet

A model coil for the 40-T hybrid magnet is being built at the Chinese High Magnetic Field Laboratory, Chinese Academy of Sciences. /restack-rod-process (RRP) strands adopted for the model coil were heat-treated according to the heat treatment (HT) schedule recommended by the manufacturer, which is Oxford Superconducting Technology. The microstructure of the reacted strand was analyzed. Measurements of the critical current, the residual resistivity ratio and the hysteresis losses were also carried out. The results of critical-characteristic measurements confirmed that the performance of the heat-treated wire was good. All characteristics indicated that the HT of the /RRP strands was successfully completed.

Axial strain characterization of the Nb 3Sn strand used for China's TF conductor

Oxford Superconducting Technology (OST) has developed a type of Nb 3Sn strand which can be used in the ITER TF coils. The strand (billet number is OST 10424FE) is made by an internal tin process. The critical current ( I c) has been measured subjected to uniaxial strain in the Twente University. The strand is destined for the cable-in-conduit conductors (CICC) of the China first short conductor sample for ITER toroidal field coil. For the uniaxial strain characterization, the voltage–current characteristics were measured with an applied axial strain from −0.8% to +0.5%. The strand appears to be fully reversible in the compressive regime during the axial strain testing, while in the tensile regime, the behavior is already irreversibly degraded when reaching the maximum in the critical current versus strain characteristic. The parameters for the improved deviatoric strain description are derived from the I c data, giving the accuracy of the scaling with a standard deviation of 1.5 A, which is by far within the expected deviation for the large scale strand production of such a high I c strand. Analysis of the relation between I c, n index and the axial strain is reported.