Advanced Nb3Sn Strands Research Articles

Electro-mechanical modeling of Nb 3Sn CICC performance degradation due to strand bending and inter-filament current transfer

Performance degradation of Nb 3Sn cable-in-conduit conductors (CICCs) is a critical issue in large-scale magnet design such as the International Thermonuclear Experimental Reactor (ITER) and the series-connected hybrid (SCH) magnets currently under development at the National High Magnetic Field Laboratory (NHMFL). Not only the critical current is significantly lower than expectations but also the voltage–current characteristic is observed to have a much broader transition from a single strand to a CICC cable. The variation of conductor voltage–current characteristic as a result of cable electromagnetic, mechanical and thermal interactions is challenging to model. In this paper, we use a new numerical model, called the Florida electro-mechanical cable model (FEMCAM) benchmarked against 40 different conductor tests, to study the influence of bending strain and current non-uniformity on the critical current and n-value of Nb 3Sn strands and CICC cables. The new model combines thermal bending effects during cool-down, electromagnetic bending effects during magnet operation and current transfer in strands with filament fracture. The n-value of a strand under bending is derived from integration of filament critical current over strand cross-section for full and no current transfer. The cable n-value is obtained from the power law relation of cable electric field and critical current curve. By comparing numerical results with measurements of advanced Nb 3Sn strands and various CICC cables, we demonstrate that FEMCAM is self-consistent in modeling inter-filament current transfer. The new model predicts that I c degradation of bent strands initially follows closely full current transfer but starts deviating and falls between full and no current transfer with an increasing bending strain. The results agree with recent TARSIS measurements for less than 1% bending strain mostly interested in practice. The strand n-value degradation from FEMCAM with no filament current transfer agrees better with measurements than that from full current transfer. Finally, FEMCAM simulated cable n-values are compared with various CICC measurements. The results imply that FEMCAM is a useful tool for the design of Nb 3Sn-based CICCs and both thermal bending and electromagnetic bending play important roles in CICC performance.

Manufacturing of the ITER TF Full Size Prototype Conductor

The experience gained in the past for the ITER toroidal field model coil conductor and the results obtained so far have led to the definition of an upgraded full size prototype conductor, based on advanced Nb3Sn strand, and entirely manufactured in the European Union (EU). Samples for the characterization in the Sultan facility have been prepared by Luvata (Italy) following the conductor layout defined by ITER. ENEA was responsible for conductor fabrication. Since the conductor layout was new, a full size copper dummy conductor has been preventively produced for the setting of the cabling and jacketing tools. Then, a total of four full size superconducting cables have been prepared by using Nb3Sn advanced strands produced by Oxford Instruments (OST) and European advanced superconductors (EAS), by internal tin and bronze technology, respectively. The details of manufacturing procedures will be described in this paper.

Influence of cable layout on the performance of ITER-type Nb3Sn conductors

In 2005-2006, the European Union tested in the SULTAN facility (CRPP Villigen, Switzerland) four ITER-type conductors assembled in two samples. Each conductor made use of an advanced Nb3Sn strand specially developed by European companies for the ITER Toroidal Field (TF) coil conductor. The cable layout of these conductors was the one of the ITER TF Model Coil conductor. In 2007, the European Union tested in SULTAN four other conductors within two other samples as prototypes of the updated ITER TF conductor. These conductors differed mainly from the former by their cable layout whereas identical or similar Nb3Sn strands were used. The paper reports on the current sharing temperature measurements of the second samples under ITER relevant operating conditions. The effect of cycling (1000 cycles) in current under full magnetic field was also investigated. The analysis of the test results is performed with respect to the strand properties using the usual smeared models of the cable. These test results were found to be significantly better than those obtained on the first conductors. The paper discusses the possible causes as well as the consequences of such a performance scattering.

Test Results of Two ITER TF Conductor Short Samples Using High Current Density Nb$_{3}$Sn Strands

Two short length samples have been prepared and tested in SULTAN to benchmark the performance of high current density, advanced Nb3Sn strands in the large cable-in-conduit conductors (CICC) for ITER. The cable pattern and jacket layout were identical to the toroidal field model coil conductor (TFMC), tested in 1999. The four conductor sections used strands from OST, EAS, OKSC and OCSI respectively. The Cu:non-Cu ratio was 1 for three of the new strands, compared to 1.5 in the TFMC strand. The conductors with OST and OKSC strands had one Cu wire for two Nb3Sn strands, as in TFMC. In the EAS and OCSI conductors, all the 1080 strands in the cable were Nb3Sn. A dc test under relevant load conditions and a thermal-hydraulic campaign was carried out in SULTAN. The CICC performance was strongly degraded compared to the strand for all the four conductors. The current sharing temperature at the ITER TF operating conditions (jop = 286 A/mm2, B = 11.15 T) was lower than requested by ITER.

Review of Nb 3Sn conductors for ITER

The ITER magnets experiencing high magnetic field (>11 T), the central solenoid (CS) and the toroidal field (TF) system, will make use of complex conductors using Nb 3Sn as superconducting material. This superconductor offers the required capability in temperature and magnetic field but shows sensitivity to strain. The two ITER model coils, the CS model coil (CSMC) and the TF model coil (TFMC) tested in 2000–2002 have given confidence in the industrial feasibility and the operating capability of the ITER magnets. However, careful analyses have revealed a degradation in terms of temperature margin. Taking advantage of the progress made by the superconductor industry since the model coil fabrications, the ITER Nb 3Sn conductors have been redesigned using higher performance (advanced) strands to recover the original designed temperature margin without changing the conductor envelopes. An R&D program was launched among the ITER participant teams (PTs) to understand the source of degradation which was found to be likely the bending of the strands inside the cable under the electromagnetic load. Meanwhile, new strands were ordered to industrial companies, to manufacture and to test full-size conductors. The European Union PT tested by mid-2006 two full-size conductor samples with advanced Nb 3Sn strands but using the TFMC conductor design, they showed performance much below expectation with regard to strand performance and to ITER TF conductor design. Overall performance is not much above the TFMC conductor sample performance. This poor performance has been attributed to more sensitivity to deformation of advanced strands. Nevertheless, tests of real ITER TF conductor samples are needed to draw final conclusions. The first samples should be tested at the end of 2006 and in 2007. However, one must already be prepared to redesign the ITER conductors as well as to modify and adapt the strand specifications, using the new tools (test facilities, mechanical models) developed since the model coil tests. In addition, the way to extrapolate strand and full-size short sample test results to ITER coils is not straightforward and has to be agreed among the ITER team and the ITER PTs.

An Extended Characterization of European Advanced<tex>$rm Nb_3rm Sn$</tex>Strands for ITER

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 Nb3Sn strands, to be used in the manufacturing of the ITER high field CS and TF magnets. We performed an extended characterization of the advanced Nb3Sn strands coming from different European companies, in terms of strand layout (diameter, thickness of Cr coating, Cu:non-Cu ratio), critical 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 non-Cu Jc of about 800 A/mm2, a Cu:non-Cu ratio of about 1, a strand diameter of 0.81 mm, and with non-Cu hysteresis losses limited to less than 1000 kJ/m3 on a plusmn3 T field cycle at 4.2 K