M. C. Jewell

Results of the TF conductor performance qualification samples for the ITER project

The performance of the toroidal field (TF) magnet conductors for the ITER machine are qualified by a short full-size sample (4 m) current sharing temperature (T-cs) test in the SULTAN facility at CRPP in Villigen, Switzerland, using the operating current of 68 kA and the design peak field of 11.8 T. Several samples, including at least one from each of the six ITER Domestic Agencies participating in TF conductor fabrication (China, European Union, Japan, Russia, South Korea and the United States), have been qualified by the ITER Organization after achieving T-cs values of 6.0-6.9 K, after 700-1000 electromagnetic cycles. These T-cs values exceed the ITER specification and enabled the industrial production of these long-lead items for the ITER tokamak to begin in each Domestic Agency. Some of these samples did not pass the qualification test. In this paper, we summarize the performance of the qualified samples, analyze the effect of strand performance on conductor performance, and discuss the details of the test results.

Evidence that filament fracture occurs in an ITER toroidal field conductor after cyclic Lorentz force loading in SULTAN

We analyzed the ITER TFEU5 cable-in-conduit conductor (CICC) after the full SULTAN conductor qualification test in order to explore whether Lorentz force induced strand movement inside the CICC produces any fracture of the brittle Nb3Sn filaments. Metallographic image analysis was used to quantify the change in void fraction of each sub-cable (petal); strands move in the direction of the Lorentz force, increasing the void space on the low force side of the CICC and producing a densification on the high force side. Adjacent strand counting shows that local increases in void space result in lower local strand–strand support. Extensive metallographic sampling unambiguously confirms that Nb3Sn filament fracture occurred in the TFEU5 CICC, but the filament fracture was highly localized to strand sections with high local curvature (likely produced during cabling, where strands are pivoted around each other). More than 95% of the straighter strand sections were free of filament cracks, while less than 60% of the bent strand sections were crack free. The high concentration of filament fractures on the tensile side of the strand–strand pivot points indicates that these pivot points are responsible for the vast majority of filament fracture. Much lower crack densities were observed in CICC sections extracted from a lower, gradient-field region of the SULTAN-tested cable. We conclude that localized filament fracture is induced by high Lorentz forces during SULTAN testing of this prototype toroidal field CICC and that the strand sections with the most damage are located at the petal corners of the high field zone.

Study of Filament Cracking Under Uniaxial Repeated Loading for ITER TF Strands

In a tokamak, such as ITER, superconducting strands suffer from bending and uniaxial strain due to Lorentz force loading/unloading and thermal cool down which may de- grade the performance over time due to specific cable-in-conduit conductor (CICC) design choice. Under repeated uniaxial loading the Cu(Sn) matrix which surrounds the brittle filaments allows the possibility of some elastic-plastic deformation that can initiate filament cracking. Here we present a metallographic study of filament cracking under increasing uniaxial loading cycles (0, 1000, 10,000 and 30,000 cycles) for one ITER Toroidal field (TF) bronze-process strand (tested at 0.4%, 0.6% and 1% strain) and one ITER TF internal tin strand (tested at 0.4%, 0.6% and 0.7% strain). Significant cracking of filaments was found at close to the respective fracture limits (strain at which strand breaks under uniaxial tensile loading) for both strands. After 0.6% strain, filament cracking in the bronze-process strand tends to increase with increasing number of loading cycles up to 10,000 and then remains almost constant after increasing the loading cycles from 10,000 to 30,000. The internal tin strand on the other hand showed an increase in filament cracking with increasing loading cycles to 10,000 up to 0.6% strain. For both types of strand and in all conditions the cracks were most likely to be found adjacent to voids.

Current sharing temperature of NbTi SULTAN samples compared to prediction using a single pinning mechanism parametrization for NbTi strand

NbTi strands to be used in four of the six ITER poloidal field (PF) coils, all the correction coils (CC) and all the superconducting feeder busbars are being produced in China. Short full-size qualification conductor (cabled and jacketed) samples have been developed at ASIPP and tested at CRPP. Single pinning mechanism parametrization for this Chinese strand (type S2) has been obtained using the Bottura scaling law. The determination of the scaling parameters using a Kramer-type regression method will be described. A comparison between the critical temperature at the operating current and field of a single strand as determined by the parametrization and the current sharing temperature (TCS) of a few conductor samples tested at the SULTAN facility will be made. The validity and limitation of the estimation will be discussed. The estimated TCS dependence on various (superconducting critical as well as geometric and volumetric) parameters will be assessed using the modelled critical surface. Errors propagated from critical current (Ic) measurements of the strands and parameter fitting, and other uncertainties, will be quantified.

Metallographic autopsies of full-scale ITER prototype cable-in-conduit conductors after full testing in SULTAN: 1. The mechanical role of copper strands in a CICC

Cables made with Nb3Sn-based superconductor strands will provide the 13 T maximum peak magnetic field of the ITER central solenoid (CS) coils and they must survive up to 60 000 electromagnetic cycles. Accordingly, prototype designs of CS cable-in-conduit-conductors (CICC) were electromagnetically tested over multiple magnetic field cycles and warm-up-cool-down scenarios in the SULTAN facility at CRPP. We report here a post-mortem metallographic analysis of two CS CICC prototypes which exhibited some rate of irreversible performance degradation during cycling. The standard ITER CS CICC cable design uses a combination of superconducting and Cu strands, and because the Lorentz force on the strand is proportional to the transport current in the strand, removing the copper strands (while increasing the Cu:SC ratio of the superconducting strands) was proposed as one way of reducing the strand load. In this study we compare the two alternative CICCs, with and without Cu strands, keeping in mind that the degradation after the SULTAN test was lower for the CICC without Cu strands. The post-mortem metallographic evaluation revealed that the overall strand transverse movement was 20% lower in the CICC without Cu strands and that the tensile filament fractures found were less, both indications of an overall reduction in high tensile strain regions. It was interesting to see that the Cu strands in the mixed cable design (with higher degradation) helped reduce the contact stresses on the high pressure side of the CICC, but in either case, the strain reduction mechanisms were not enough to suppress cyclic degradation. Advantages and disadvantages of each conductor design are discussed here aimed to understand the sources of the degradation.

Procedures for evaluating filament cracking during fatigue testing of Nb3Sn strand

In Tokamak fusion reactors, such as ITER, superconducting strands are subjected to repeated Lorentz force loading and unloading which may degrade performance over time. The Cu matrix which surrounds the brittle Nb3Sn filaments allows the possibility of some elastic-plastic deformation that can initiate filament cracking. We seek to understand if there are strand design variables that might ameliorate such degradation but before being able to do such experiments, we need to establish procedures that can unambiguously detect the cracking caused by loading, rather than by subsequent metallographic examination. Here we make a first report of our procedures after fatigue testing at 77K. Filament crack densities were quantified from large montages covering ≈20 mm length of strand. Three types of cracks were present. The most common were cracks transverse to the filament axis adjacent to voids. Cracks away from voids were of low density until close to the fracture strain. A third kind of crack which generally in...

Error Estimation in the

The main parameter measured in the tests of the TF conductors for the ITER machine is the current sharing temperature (TCS). In the voltmetric assessment of TCS, the voltages are measured on the conductor jacket, due to the technical difficulties to introduce voltage taps inside the stainless steel conduit. The average voltages measured on the jacket do not exactly correspond to those that arise along the single strands, as shown by the presence of early voltages arising during the current ramp up, when the cable is still in the superconducting phase. It is therefore not trivial to evaluate the difference between the cable and jacket voltages, that gives rise to an experimental error in the measurement of TCS. This paper reports the results of a vast simulation campaign performed with a detailed electromagnetic model of the Cable in Conduit Conductor, in which the origin and the extent of these differences have been investigated, giving an estimate for the experimental error. The impact on measurements of other sources of error such as the signal/noise ratio and the error in the temperature measurements is also reported.

T_{cs}

Brittle fracture of Nb3Sn filaments is one mechanism by which the current-carrying capacity of composite Nb3Sn wires is degraded. However, there are relatively little data in the literature on the intrinsic material fracture properties of Nb3Sn filaments, because the complex composite structure (matrix, secondary phases, and defects such as voids) acts as an integrated mechanical unit. In this study, we extracted individual Nb3Sn filaments from a fusion-style Nb3Sn composite wire and conducted tensile testing to determine the fracture strength distribution of the isolated filaments. The distribution is modeled using a Weibull function. The relative fracture propensity of fully reacted filaments versus those with unreacted Nb cores is compared. The presence of a Nb core reduces, on average, the fracture strength of a filament by 38% and the strain to failure by 29%. Understanding the fracture probability of Nb3Sn as a function of both stress and volume will allow strand and conductor modeling efforts to more accurately represent the relative contributions of fracture and other effects (such as plasticity) to irreversible current density degradation, and may assist wire manufacturers in assessing the mechanical impact of wire design changes.