Why do makers of superconducting magnet wire try to pin fluxoids in place?

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Figure 1: Specific heat in the normal (Cen) and superconducting (Ces) states of a classic superconductor as a function of absolute temperature. The two functions are identical at the transition temperature (Tc) and above Tc. — superconductivity Figure 1: Specific heat in the normal (C_en) and superconducting (C_es) states of a classic superconductor as a function of absolute temperature. The two functions are identical at the transition temperature (T_c) and above T_c.

In type II superconductors, magnetic fields penetrate the material in the form of quantized fluxoids. When a significant electric current flows through the superconductor, these fluxoids are subjected to forces that cause them to move.

This motion is problematic because it leads to energy dissipation, which generates heat. If the temperature rises sufficiently, the material is driven out of its superconducting state and back into a normal, resistive state. To prevent this and maintain a high critical current density, manufacturers intentionally introduce inhomogeneities into the composition of the wire. These structural variations act as pinning centers that hold the fluxoids in place, effectively preventing the energy-dissipating motion that would otherwise limit the wire's performance in high-field magnets.1