Strain control of composite superconductors to prevent degradation of superconducting magnets due to a quench: I. Ag/Bi₂Sr₂CaCu₂O_xmultifilament round wires

Abstract

The critical current of many practical superconductors is sensitive to strain, and this sensitivity is exacerbated during a quench that induces a peak local strain which can be fatal to superconducting magnets. Here, a new method is introduced to quantify the influence of the conductor stress and strain state during normal operation on the margin to degradation during a quench, as measured by the maximum allowable hot spot temperature T allowable , for composite wires within superconducting magnets. The first conductor examined is Ag-sheathed Bi 2 Sr 2 CaCu 2 O x round wire carrying high engineering critical current density, J E , of 550 A/mm 2 at 4.2 K and 15 T. The critical axial tensile stress of this conductor is determined to be 150 MPa and, in the absence of Lorentz forces, T allowable is greater than 450 K. With increasing axial tensile stress, ! " , however, T allowable decreases nonlinearly, dropping to 280 K for ! " = 120 MPa and to 160 K for ! " = 145 MPa. T allowable (! ") is shown to be nonlinear and independent of magnetic field from 15 T to 30 T. T allowable (! ") dictates the balance between magnetic field generation, which increases with the magnet operating current and stress, and the safety margin, which decreases with decreasing T allowable , and therefore has important engineering value. It is also shown that T allowable (! ") can be predicted accurately by a general strain model, showing that strain control is the key to preventing degradation of superconductors during a quench.

Superconducting Accelerator Magnets Based on High-Temperature Superconducting Bi-2212 Round Wires

“…The Ag-0.2 wt% Mg outer sheath of commercial Ag/Bi-2212 strands is an oxide-dispersion strengthened alloy for providing strength along the axial direction. For a commercial Ag-0.2 wt% Mg/Ag/Bi-2212 (area ratio AgMg: Ag: Bi-2212 = 0.25: 0.5: 0.25) round wire, the working maximum of tensile axial stress is around 150-160 MPa at 4.2 K, determined using an ITER barrel coil method [64] whereas the irreversible tensile axial strain, beyond which J c degrades irreversibly, is between 0.4% and 0.6%, determined using a U-shaped bending spring [65], a 77 K I c -tensile strain coupled with in situ X-ray strain measurement [66], a Walter-spring measurement [67,68]. The irreversible I c degradation is caused by filament cracks.…”

Designs and Prospects of Bi-2212 Canted-Cosine-Theta Magnets to Increase the Magnetic Field of Accelerator Dipoles Beyond 15 T

“…Its critical current is sensitive to strain. Its critical axial stress along the wire is ∼150 MPa [2], whereas the critical current of a fiberglass insulated, epoxy impregnated Bi-2212 Rutherford cable degrades irreversibly with increasing transverse pressure above 60 MPa applied to the wide surface of the cable, and above 100 MPa applied to the narrow surface of the cable [3]. The Canted-Cosine-Theta (CCT) magnet technology provides the possibility of intercepting the Lorentz forces of each turn of the winding and transferring them to the support structure, preventing stress accumulation between turns [4].…”

Strain control of composite superconductors to prevent degradation of superconducting magnets due to a quench: II. High-strength, laminated Ag-sheathed Bi-2223 tapes

“…We developed a spiral coil quench technique capable of evaluating all these strains within a test [10]. We have applied it to Bi-2212 round wire, which is made into magnets using a wind-and-react technique that features zero winding strain, to probe the influence of the operational stress and strain state on its margin to degradation during a quench.…”