Strain control of composite superconductors to prevent degradation of superconducting magnets due to a quench: I. Ag/Bi₂Sr₂CaCu₂O_x multifilament round wires

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 Bi₂Sr₂CaCu₂O_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, σ _a, however, T _allowable decreases nonlinearly, dropping to 280 K for σ _a = 120 MPa and to 160 K for σ _a = 145 MPa. T _allowable(σ _a) is shown to be nonlinear and independent of magnetic field from 15 to 30 T. T _allowable(σ _a) 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(σ _a) can be predicted accurately by a general strain model, showing that strain control is the key to preventing degradation of superconductors during a quench.