Effect of quasi-hydrostatical radial pressure onI_cof Nb₃Sn wires

Abstract

High-performance Nb3Sn conductors are intended to be used in large-scale magnets like the International Thermonuclear Experimental Reactor (ITER) and in the upgrade of the Large Hadron Collider (LHC). Due to the occurrence of high electromagnetic forces, a detailed knowledge of the response of the critical current to the three-dimensional mechanical loads acting on the wires inside the cables is required. A detailed analysis of transverse stress effects on the critical current for powder-in-tube and bronze route Nb3Sn wires is presented. In an earlier publication, we have described the effect of transverse stress exerted on a Nb3Sn wire by means of two parallel plates. In the present paper, we analyse the effect of transverse stress exerted simultaneously by four walls on a wire being confined in a U-shaped groove. In order to get a more realistic picture of the situation of wires embedded in a Rutherford cable, the compression by four walls was also performed after impregnating the wire with epoxy in the same U-shaped groove. The result is very different from the case of pressing by means of two walls: the effect of pressure on Ic is now strongly reduced, which is attributed to the almost hydrostatic pressure in the U-shaped groove. This is further confirmed by the comparison between the effects of axial and transverse loads on the upper critical field and the pinning force. The present data are also compared against the effects of mechanical load on the critical current of cables in large-scale magnets.

Superconducting Materials and Conductors: Fabrication and Limiting Parameters

“…In addition, the epoxy that impregnates the wire tends to redistribute the local transverse force and produce a stress state that approaches hydrostatic conditions. This has been proven recently on modern, high-J c Nb 3 Sn wires [78], confirming that the large observed degradation in single-wire measurements quoted earlier is a measurement artifact. A defining test of the US LHC Accelerator R&D Program (LARP) TQS03 magnet at CERN [79], in which the magnet was preloaded up to 260 MPa in the windings while experiencing very limited permanent degradation, re-established Nb 3 Sn firmly as the conductor of choice for future upgrades of the LHC.…”

Critical Current Measurements of High-<formula formulatype="inline"><tex Notation="TeX">$J_{c}$</tex> </formula> <formula formulatype="inline"><tex Notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$</tex></formula> Rutherford Cables Under Transverse Compression

“…This behavior suggests that the critical current reduction is dominated by the reversible strain on the superconductor (and not by cracks). Similar results can be found analyzing the critical current measurements under transversal pressure of an impregnated single 1.25 mm PIT strand [17]. These measurements were carried out at the University of Geneva, where recently the same type of measurement was repeated on the wire used in our cable sample [18].…”

Implications of the strain irreversibility cliff on the fabrication of particle-accelerator magnets made of restacked-rod-process Nb3Sn wires

“…By incorporating the brittle Nb 3 Sn superconductor in a particle-accelerator machine, where mechanical forces on the conductor are going to be intense, especially at high magnet fields, strain and stress limits for conductor handling and operation naturally come into play in the design, fabrication, transportation, and cool-down and powering of accelerator magnets 47–53 . Both axial and transverse strain components should be taken into account 54–56 . In this paper, we will discuss the axial strain limits and irreversible effects.…”