Search

The high field superconducting magnets required for ongoing and planned upgrades to the Large Hadron Collider (LHC) will be wound with Nb3Sn Rutherford cables for which reason studies of Nb3Sn strand, cable, and magnet properties will continue to be needed. Of particular importance is field quality. The amplitudes of multipoles in the bore fields of dipole and quadrupole magnets, induced by ramp-rate-dependent coupling currents, are under the control of the interstrand contact resistances-crossing-strand, $R-{c}$, adjacent strand, $R-{a}$ , or a combination of them, $R-{{\text{eff}}}$. Although two decades ago it was agreed that for the LHC $R-{c}$ should be in the range 10-30 μ, more recent measurements of LHC quadrupoles have revealed $R-{c}$ values ranging from 95 to 230 μ. This paper discusses ways in which these values can be achieved. In a heavily compacted cable $R-{{\text{eff}}}$ can be tuned to some predictable value by varying the width of an included stainless steel (effectively 'insulating') core. But cables are no longer heavily compacted with the result that the crossing strands of the impregnated cable are separated by a thick epoxy layer that behaves like an insulating core. If a stainless steel core is actually present, $R-{{\text{eff}}}$ must be independent of core width. Since there is no guarantee that a fixed predetermined amount of interlayer separation could be reproduced from winding to winding it would be advisable to include a full width core.

Calorimetric measurement of coupling loss versus frequency has been measured on two sets of cored and uncored large Hadron Collider Accelerator Research Program high gradient quadrupole Nb3Sn Rutherford cables. Studied are the responses of the resulting interstrand contact resistances (ICR) to variation of stainless-steel (SS) core width and position and to variation of reaction-heat-treatment (RHT) condition. One pair of cables (an early HQ-series type) with and without core had received RHT under 20-MPa uniaxial face-on pressure. Another set of cables (recent QXF type) furnished with SS cores of various widths had received RHT under ambient pressure. The results were displayed as cable-cross-sectional micrographs and plots of ICR versus percent core coverage (W). The HQ cables were tightly compacted and produced results consistent with a previously expected continuous ICR versus W variation. On the other hand, the QXF cables were uncompacted such that their upper and lower layers were separated by what is referred to as a full-width 'pseudocore;' as a result, their ICRs were independent of the widths of the SS cores. Compaction versus noncompaction is discussed and future research directions are suggested.

The future fusion reactor devices built by Institute of Plasma Physics, Chinese Academy of Science will be a compact burning plasma facility to fill the gap between ITER and the China Fusion Engineering Test Reactor, with the mission of studying the deuterium–tritium plasma in steady-state operation. The winding-package (WP) of its toroidal field coil is graded into high-field WP and low-field WP, and the high critical current density (J c) Nb3Sn strand will be applied to the high-field WP based on the current design. The thermo-magnetic instability is the main issue in the application of high- J c Nb3Sn strand, which may induce premature quenching in the low field regions. This issue can be improved by reducing the effective filament diameter (d eff) to suppress flux jumps and increasing the residual resistivity ratio (RRR) to enhance the release of heat generated by flux jumps. However, in the conductor manufacturing process, local plastic deformations (indentation) of strands can impact the sub-element layout and degrade the RRR of the strand, which would increase again the risk on instability. In this study, magnetization measurements and V – I tests were performed on samples with different indentation depths. The hysteresis loss and d eff of the indented sample was obtained from the test results. The impact of local plastic deformation on the thermo-magnetic instability of two types of Nb3Sn strand was confirmed by cross-sectional metallographic observation and quantitative microstructure analysis. It was shown that flux jumps were suppressed at indentation depths below 0.3 mm. Further indentation increase leads to severe flux jumps. [ABSTRACT FROM AUTHOR]

For the International Thermonuclear Experimental Reactor magnets, a significant fraction of the Nb3Sn conductors will be operating at a low field (B < 4 T), such as the outer turns of the central solenoid modules and toroidal field winding packs, while so far, the Ic(B,T, Ɛ) parameterization has been derived from measurements carried out at higher fields (B ≥ 7 T). It may not be accurate enough to adopt this parameterization for the extrapolation of the conductors’ operating performance in terms of critical current (Ic) and current sharing (Tcs). The critical current characteristic can be effectively extended to the lower field region by means of a relation between the magnetization and critical current. The parameterization of the derived critical current scaling law Jc(B) is processed for the field range of the magnetization measurement (0 < B ≤ 9 T) and the critical current measurements (7 ≤ B < 12 T) in order to cover the full range up to 12 T. [ABSTRACT FROM AUTHOR]

The Segregated Copper Ratio Experiments on Transient Stability is considering the role of segregated copper in Nb3Sn cable in conduit conductors.

The procurement of Nb3Sn cable in conduit conductors for the Segregated Copper Ratio Experiment on Transient Stability is discussed.

The results of interstrand contact resistance and AC loss on 48-strand Nb3Sn conductors with Cr/Cr oxide coatings are reported.

An ITER TF model coil conductor (DP4) was examined to determine electromagnetic and mechanical AC loss under transverse cyclic loading.

An evaluation of a special cryogenic press constructed to study the electrical and mechanical characteristics of full-size ITER conductor samples under transverse, mechanical loading conditions is presented. A superconducting dipole coil is used to provide the required magnetic field for magnetization measurements. The results, obtained after several tens of full loading cycles, indicate a significant reduction of the cable coupling loss time constants after a number of cycles.

Research was conducted to examine the mechanical and electrical characteristics of full-size ITER multistrand Nb3Sn cable-in-conduit conductor samples under transverse, mechanical loading. The transverse magnetic force that takes place when the conductors are used in a coil was simulated. A summary of findings derived with up to several tens of full loading cycles are presented.

Research was conducted to examine the mechanical and electrical properties of full-size ITER-CS cable samples under transverse, mechanical loading. Fatique tests were conducted including loading the conductor from zero up to 260 kN for a large number of cycles to investigate the evolution of n-tau adn R(sub c) under realistic ITER lifetime conditions. Results of the mechanical calculations and the electromagnetic losses and interstrand contact resistances are presented and discussed.

A study was conducted on the effects of Lorentz force on the alternating current losses in cable-in-conduit superconductors for ITER coils with operating currents over 40 kA and fields of up to 13 T. The transverse Lorentz force was found to decrease effective contact resistance among cable strands and boost coupling loss. This was observed in a sub-size jacketed cable with 81 Cr-coated Nb3Sn strands.

The cable in conduit conductors for the various ITER coils are required to function under pulse conditions and fields up to 13 T. A parametric study, restricted to a limited variation of the reference cable lay out, is carried out to clarify the quantitative impact of various cable parameters on the coupling loss and to find realistic values for the coupling loss time constants to be used in ac loss computations. The investigations cover ac coupling loss measurements on jacketed sub- and full size conductors with variations in type of cabling, twist pitch, void fraction, and thickness of the chromium coating. The results are summarised and conclusions regarding the ITER cable design are presented. Scaling laws for coupling loss versus void fraction and number of cabling stages are presented.

AC coupling loss measurements on jacketed Nb3Sn cable-in-conduit conductors with varying cabling stage, twist pitch, void fraction and chromium coating thickness have been carried out. The tests were conducted to derive the value of the coupling loss time constant for future use in ac loss calculations. The results revealed that the coupling loss time constant rise as the void fraction decreases. For braided cables, it increases by a factor of two over twisted cables.

The Comprehensive Research Facility for Fusion Technology (CRAFT) project has been launched in 2019, for developing the essential engineering technologies for Chinese Fusion Engineering Testing Reactor (CFETR). Within this project, a full-size toroidal field (TF) coil will be built as the prototype coil for CFETR. Based on design of CFETR magnet system, the TF coil will operate at 95.6 kA in a peak field of 14.5 T. The high-Jc Nb3Sn strand is taken into consideration due to the critical current density of ITER-grade Nb3Sn is too low at 14.5 T. Considering that it will be the first time to apply the high-Jc Nb3Sn strand in the large-scale cable-in-conduit conductor (CICC) for fusion magnet, a conductor sample made of high-Jc Nb3Sn strand with short twist pitch (STP) cable pattern was manufactured in ASIPP and tested in SULTAN facility, to investigate the feasibility. The test campaign focuses on the impact of cyclic electromagnetic (EM) loading and warm-up cool-down (WUCD) to the performance of the conductor, the strain distribution of the conductor before and after EM cycles was measured by inductive method to make a deeper insight of the conductor performance evolution. AC losses tests have also been carried out, providing relevant information for further coil design. [ABSTRACT FROM AUTHOR]

In this study, we analyze the properties of boron isotope (11B)‐rich powders from three different sources, that is, American, Cambridge, and Pavezyum, to fabricate the bulk Mg11B2 superconductors and evaluate their superconducting properties. While 11B‐rich powder is an essential precursor to fabricate Mg11B2 superconductors for fusion magnet applications, the properties of the 11B powder turned out to be critical to determine the quality of the final superconducting product. Therefore, appropriate control of processing conditions is needed to comply with the requirements of the nuclear fusion application. Analysis of the B isotope ratio by accelerator mass spectroscopy and neutron transmission revealed that all three types of powder are enriched with 11B to better than 99 at % quality. In addition, Pavezyum's 11B shows the lowest crystallinity and smallest crystalline domain size as evidenced by the high‐resolution X‐ray diffractometer and scanning electron microscopy. The chemical states of the boron isotope investigated with near edge X‐ray absorption fine structure spectroscopy and X‐ray photoemission spectroscopy also reveals that Pavezyum boron has amorphous structure. Mg11B2 bulks and multi‐filamentary (12‐filament) wires have been manufactured, sintered at different temperatures and characterized via the transport critical current density. The wire with Pavezyum 11B shows three times higher current carrying capacity at a particular magnetic field compared to the wire using Cambridge 11B and hence, Pavezyum 11B boron has the potential for manufacturing fusion grade Mg11B2 based magnets. The results of this study demonstrated that Boron powders with higher purity, smaller grain size and lower crystallinity are critical for improving the superconducting and electronic properties of Mg11B2 samples fabricated from the powder. Thus, the low‐neutron‐activation Mg11B2 is possibly an affordable and technically viable candidate to replace NbTi superconductors in the low field poloidal field and correction coils for the next‐generation fusion reactors. [ABSTRACT FROM AUTHOR]