Reversible and irreversible mechanical effects in real cable-in-conduit conductors

Abstract

The strong strain sensitivity of the critical properties of Nb 3 Sn is well established. However, the roles played by both the reversible strain sensitivity and the susceptibility to brittle fracture of Nb 3 Sn filaments is still leading to unexpected results and resulting design modifications of conductors. Practical conductors require acceptance of less than perfect superconducting behaviour because such conductors actually operate continuously in a slightly resistive mode. Performance testing of the ITER conductors has provided a unique database of both strand and corresponding conductor performance. The test database includes strand characterization under uniaxial and bending strain, superconducting measurements on full-size conductor samples and microscopic investigations into filament fracture. A simple mechanical and electrical model of the strands and a multistage cable (capable of operation at 12 T and 45 kA) that focuses on predicting the imperfect superconducting behaviour is used to provide insight into the processes governing the observed conductor behaviour.The conclusions show that a short twist pitch at the first cabling stage provides the best operating conditions for the strands, and further that some strand mechanical properties, essentially a high stiffness to bending and a low stiffness to pinching, increase the ability of the strands to tolerate less well optimized cables.

Progressing in cable-in-conduit for fusion magnets: from ITER to low cost, high performance DEMO

“…Both the compressive and tensile tails of the strain distribution depress T cs : the compressive tail via reversible reduction, while the tensile tail via irreversible reduction of I c . Which tail has the major effect in TF conductors is still debated, see for example the exhaustive review in [13]. Filament breakage is often considered responsible of reduction of T cs during cycling, but in [14] and [15] it was shown that the broadening of the strain distribution alone (without including breakage) can result in T cs reduction during cycling and low n values which were in agreement with the measurements.…”

“…The margin of improvement is basically two to three times larger than the reduction observed after 1000 electromagnetic cycles in TF conductors. [12] and [13]) are also reported. The dashed lines are for hypothetical conductors made with the same strands but working at effective strain of -0.37%, -0.2% and 0% .…”

“…The optimization of the void fraction and of the twist pitches have a beneficial, but modest, impact on T cs and were incorporated in the new ITER design [19]. In fact (see fig 2 in [13]) in CS conductors with standard twist pitch (prone to extensive filament breakage), the T cs reduction after 1000 cycles is <0.5 K and the initial values of T cs is comparable to the one measured in the CS conductor with short twist pitches (no filament breakage), whose T cs is constant with cycling. In TF conductors the degradation during cycling is also small (<0.5 K in fig.…”

The Mechanical Behavior of the Cable-in-Conduit Conductor in the ITER Project

“…Here, D stands for the diameter of the strand. If the maximum strain is larger than 1%, the strand would be considered as cracking [10,[24][25][26]. Based on these equations, the coefficient a ij can be calculated, as well as the buckling deflection, the relationship between the radius of curvature, and the thermal compression strain ε Thermal or slid strainε Slid .…”

Mechanical Characterization and Modeling of a Powder-In-Tube MgB2 Strand

“…This method is applied to a commercial MgB2 strand. The constitutive equations are built for future use in multi-scale finite element simulations to predict the mechanical behavior [15] or the electrical behavior [16], [17].…”