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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]

Second-generation high-temperature superconductors, being brittle, need to be deposited on a substrate and converted to tapes with additional reinforcement layers. These tapes are wound on a core to make superconducting wires and cables. The mechanical loads during bending, twisting, winding, etc., can degrade the superconductor. It is necessary to determine the strain-induced during these processes. Variations in induced strain with bending of superconductors have been investigated analytically and numerically for various configurations of Rare-Earth Barium Copper Oxide (REBCO) tape. The numerical simulation has been carried out using COMSOL Multiphysics software; the analytical examination of the effect has been performed applying the Flexure formula. The geometrical configuration of the superconducting tape is varied in terms of the bending radius, the thickness of constituting layers made of Hastelloy and copper. The results showed that the variation in the thickness of the substrate has a large influence on the induced strain. Electrical performance analysis is also carried out using the power law of strain dependence and the 5% critical current degradation point is noted. Results of these numerical and analytical investigations are expected to help in finding out critical parameters to avoid degradation while being in use. [ABSTRACT FROM AUTHOR]

Both the pancake coiling and conductor on round core (CORC) cabling processes consist of winding second-generation high-temperature superconducting tape (REBCO) in a core. The contact behavior between the tape and the core during the winding process directly affects the critical current, cable flexibility, mechanical support and electrical contact resistance. Therefore, the winding process needs to be optimized to produce pancake coils and CORC cables with the desired electrical and mechanical properties. This paper comprehensively considers the role of the relaxation and Poisson effects. The theoretical model and the finite element model (FEM) for calculating the contact stress during the winding process are established, and a formula for the estimation of the contact force is proposed. The suitable winding pre-tension force, winding curvature radius of SCS2030 (2 mm width and 30 m substrate thickness) and SCS4050 (4 mm width and 50 m substrate thickness) of REBCO tapes are obtained and discussed by accounting for the relaxation effect and the critical current degradation limit. With consideration of the Poisson effect, the nonlinear contact behavior is investigated. In addition, the relaxation effect, Poisson effect and plasticity of the REBCO tape are taken into account in the FEM model. The results show that the axial strain in the REBCO layer during the winding process is related to the contact behavior. The greater the winding pre-tension force, the smaller the helical curvature radius, and the greater the contact force. The distribution of contact stress for several winding factors (winding pre-tension force, winding angle and the core radius) is divided into three contact types: the single-line contact, the double-line contact and the surface contact. The FEM model results are consistent with the theoretical evaluation and are more suitable for practical winding cases. This research is the basis for designing and optimizing pancake coils and CORC cables. [ABSTRACT FROM AUTHOR]

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]

A proper understanding of the degradation of the transport properties of cable-in-conduit conductors (CICCs) due to a changing strain distribution or to crack formation in the filaments is essential to determine the operational limits of the conductors and to optimize their design. Based on the electrical and strain properties of the superconducting strand, the performance of short samples of the CICC can be analyzed, such as the ones tested in the SULTAN facility, or the full-size CICC used in real magnets. Mathematical fitting expressions are proposed to implement strand properties into the cable model. In combination with the strain maps generated by the mechanical model MULTIFIL, these expressions are introduced in the electromagnetic code JackPot to predict the current sharing temperature of the CICC of the international thermonuclear experimental reactor (ITER) Central Solenoid. A comparison is made with SULTAN short samples tests as well. [ABSTRACT FROM AUTHOR]

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--crossingstrand, Rc, adjacent strand, Ra, or a combination of them, Reff. Although two decades ago it was agreed that for the LHC Rc should be in the range 10-30 μΩ, more recent measurements of LHC quadrupoles have revealed Rc values ranging from 95 to 230 μΩ. This paper discusses ways in which these values can be achieved. In a heavily compacted cable Reff 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, Reff 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. [ABSTRACT FROM AUTHOR]

Coupling losses induced in cable-in-conduit conductors (CICC) when subject to a time-varying magnetic field are a major issue commonly encountered in large fusion tokamaks (e.g., JT-60SA, ITER, DEMO). The knowledge of these losses is crucial to determine the stability of CICC but is yet difficult to achieve analytically (thus in a short computation time) given the specific and complex architecture of these conductors although numerical solutions such as THELMA and JACKPOT already exist. In an attempt to ease the resolution of this problem, we have previously presented a theoretical generic study of a group of elements twisted together (representing a cabling stage of a CICC) and derived the analytical expression of its coupling losses. We have now extended this study to a two cabling stage conductor by establishing an analytical model to calculate its coupling losses as function of its effective features. In a second part, we compare our results to these of THELMA and JACKPOT on geometries representing ITER CS and JT-60SA TF conductors. Finally, we have set up a specific algorithm to reconstruct strand trajectories from X-ray images and have extracted the effective geometrical parameters of a JT-60SA TF conductor. Our next objective is then to extract its effective electrical parameters from interstrand resistivity measurements to be able to compare the coupling losses predicted by our analytical model with those measured within the SULTAN facility. [ABSTRACT FROM AUTHOR]

ITER Nb3Sn strand quality verification tests require large quantities of precise measurements. Therefore, regular cross-checking between testing laboratories is critically important. In this paper, we present results from a cross-checking test of 140 samples between the National High Magnetic Field Laboratory, USA, and the University of Twente, The Netherlands. The tests comprise measurements at 4.2 K on critical current, residual resistance ratio, and hysteresis loss, while at room temperature the chromium layer thickness, Cu/non-Cu ratio, filament twist pitch, and diameter were determined. Our results show very good agreement between the two laboratories. The reasons for small random discrepancies are discussed. [ABSTRACT FROM PUBLISHER]

China fusion engineering test reactor (CFETR) is a new tokamak device, whose magnet system includes the toroidal field, central solenoid (CS), and poloidal field coils. In order to develop the manufacturing process for the full-size CS coil, the CS model coil (CSMC) project was launched first. The cable-in-conduit conductor used for CFETR CSMC refers to ITER CS conductor with the same short twist pitch cable pattern. They have the similar layout. In order to qualify the manufacturing process and performance of conductor, two short sample conductors with two different jacket materials (316LN and high-Mn steel) were manufactured. The conductors were tested at Swiss Plasma Center successfully. In future, the CSMC will be tested at the Institute of Plasma Physics, CAS (ASIPP). In this paper, the manufacturing processes for conductor are described in detail. The strand damage analysis and conductor performance were also discussed. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

Magnetic field, temperature and axial strain dependencies of critical current must be defined for all strands intended for the production of ITER TF windings. Such a characterization was performed for three Russian bronze routed strands. The design features of the strands under investigation are given. The measurements of critical current was carried out in the Pacman facility at the University of Twente over a range of magnetic field from 5 to 14 T, temperature from 4.2 to 11.5 K and applied axial strain from -0.7 to +0.7%. A study on irreversibility strain limit is also performed indicating that critical current degradation starts at an intrinsic strain level above 0.5%. Along with critical current data derived from voltage-current curves, a number of voltage-temperature characteristics were measured and used for further analysis. The data obtained allow one to make calculations of ITER TF coil performance as well as analysis of short conductor sample test results. [ABSTRACT FROM AUTHOR]

Multistrand cables may exhibit two classes of parasitic magnetization both of which can distort the bore-field of the host magnet. They are: (1) a dynamic magnetization that is produced by interstrand coupling currents generated by time-varying magnet excitation and moderated by the interstrand contact resistances (ICR), (2) a static magnetization ("hysteretic") resulting from the intrastrand persistent currents. This paper (i) compares the ICRs of two sets of cables with and without stainless steel cores and subjected to three levels of compaction during cabling, (ii) presents the results within the context the previously measured ICRs of a series of similar cables with cores of various widths, and (iii) concludes by comparing the LHC-ramp-rate induced coupling magnetization of a typical Rutherford cable with its transport-current-moderated persistent-current magnetizations at low and high fields. [ABSTRACT FROM AUTHOR]

In ITER CICCs, the strands experience varying mechanical strain levels due to thermal compression and electromagnetic loading, cumulating into severe periodic bending and contact strain. Depending on the choice of the cabling pattern, the strain may exceed the irreversibility limit leading to cracks in the Nb3Sn filaments and degraded performance of the conductors. We present a systematic microscopy study of filament fracture at gradually varied axial tensile strain levels in a bronze and internal tin processed type of Nb3Sn ITER strand. The axial stress-strain relation of the wires was measured at 4.2 K, up to a different peak load for each sample, successively increasing from 0.0% to 0.7%. Longitudinal cross sections were prepared from cut sections of the specimens. The crack pattern and distribution in the isolated filaments could thus be studied as a function of applied strain, revealing different fracture mechanisms. Electrical measurements were performed with applied strain with special attention for the tensile strain range. Finally, the microscopic information was correlated back to the observed macroscopic Ic degradation. [ABSTRACT FROM AUTHOR]

Numerous manufacturers and different strand processing techniques are involved with the production of the Nb3 Sn strand material required for ITER. The superconducting transport properties of brittle Nb3 Sn layers strongly depend on their strain state. Hence, the thermal compression and the substantial transverse load in combination with the key choice for the cabling pattern of the CICCs, will determine their performance. Knowledge of the influence of axial strain, periodic bending, and contact stress on the critical current (Ic) of the used Nb3 Sn strands is inevitable to gain sufficient confidence in an economic design and a stable operation of ITER. We have measured the Ic and n-value of Nb3 Sn strands from various manufacturers in the TARSIS facility, when subjected to spatial periodic bending and contact stress. The Ic and n-values have been determined for applied axial compressive and tensile strain varying from -0.8% up to +0.5%, between T = 4.2 K and 10 K and B = 6 T to 14 T. The strain sensitivity varies appreciably for different strand types. We present a selection of the results obtained so far. [ABSTRACT FROM AUTHOR]

The voltage signals measured in short sample qualification tests for the ITER conductors often show a linear relationship with the current starting from zero. This behavior is most likely caused by non-uniform current distribution in the joints. To counteract this, two samples have been equipped with extra sets of six voltage tap pairs, connected with regular spacing, as a star configuration around the conductor. The linear relationship still showed up in the average of the extra voltage signals, and were sometimes even seen to be inverse. In some occasions, it was negative for all individual signals. A fair judgment of the approval criterion requires a proper understanding of this behavior. This paper describes the analyses with the aid of the numerical model 'JackPot'. It demonstrates that the described observation is the result of the chosen cabling pattern in combination with the position of the voltage taps. The model also shows that this problem cannot be solved by changing the positions of the voltage taps, but that completely filling the joints with solder can greatly improve the situation. [ABSTRACT FROM AUTHOR]

Recent International Thermonuclear Experimental Reactor (ITER) Model Coils and tests on Nb3Sn Cable in Conduit Conductors (CICC) showed a significant and unexpected increase in the broadness of the transition to the normal state, resulting in degradation of superconducting properties. To investigate these phenomena, two CICC samples were built with identical 144 strand cables but different conduit materials. One sample had titanium conduit with low coefficient of thermal expansion, the other had stainless steel conduit. The purpose of this experiment was to study changes in strand properties in the cable (critical current, current sharing temperature, n-value), the effects of cycling and high electromagnetic load, and the effect of the conduit on the CICC performance. [ABSTRACT FROM AUTHOR]

Determines the interstrand contact resistance (Rc) and alternating current loss in a Nb3Sn cable-in-conduit conductor with a chromium oxide coating. Key parameters in Rc; Factor influencing level of surface oxidation; Build-up of chromium coating in two layers; Comparison of time constant and atomic concentration of oxygen.

Determines the electromagnetic and mechanical alternating current loss of the Toroidal Field (TF) model coil conductor (DP4) of the International Thermonuclear Experimental Reactor under transverse cyclic loading. Representation of the coupling loss over an extended frequency range; Magnetization measurements; Contact resistance.

Part I. Discusses the measurement of the mechanical and electrical properties of full-size International Thermonuclear Experimental Reactor (ITER) conductor samples under transverse, mechanical loading. Use of superconducting dipole coils which provide the magnetic fields required to perform magnetization measurements using pick-up coils.

Discusses the construction of a special cryogenic press to study the mechanical and electrical properties of full-size International Thermonuclear Experimental Reactor multistrand niobium-tin cable-in-conduit conductor samples under transverse, mechanical loading. Variable cyclic force transmitted by the cryogenic press; Contact resistance measurements.

Discusses the measurement of the mechanical and electrical properties of full-size International Thermonuclear Experimental Reactor-Central Solenoid cable samples under transverse, mechanical loading using a cryogenic press. Mechanical properties; Electromagnetic losses and interstrand contact resistances; Description of the cryogenic press.

Presents a parametric study on the quantitative impact of cable-in-conduit conductors on coupling loss for various ITER coils. Determination of coupling loss; Specifications on strands and cables; Experimental results; Internal inspection of jacketed subcables; Variation in chromium coating thickness of cabling; Conclusions.