ABOUT THE CRITICAL CURRENT
The current-carrying capacity of a superconductor is limited by the critical current density, jc. The critical current of a type I superconductor is set by the critical field; the current induces a magnetic field that adds to the applied field, and the total cannot exceed the critical field. In fact, this limit is not very restrictive unless the applied field is already a considerable fraction of Hc. As we show below, type I conductors can carry very high current densities without inducing fields that approach Hc. Their use in magnets and motors is restricted by the low value of Hc rather than by the field-free critical current.
The critical current of a type II superconductor is set by a completely different mechanism that may limit it to a very small value. When the conductor is in the mixed state, the current imposes a force (the Lorentz force) on the magnetic vortices that thread through it. If the vortices move in response to this force their motion produces an electrical resistance. For this reason, a pure, perfect type II conductor becomes resistive as soon as it enters the mixed state, at Hc1, and is even less useful than a type I superconductor. However, the vortices are linear features that interact with microstructural defects such as precipitates and grain boundaries. Just as microstructural defects pin dislocations and increase strength, they also pin superconducting vortices so that there is a finite critical current in the mixed state. The technologically useful type II superconductors have microstructures that are engineered to provide the strong vortex pinning needed to support high critical currents at high fields.
The current-carrying capacity of a superconductor is limited by the critical current density, jc. The critical current of a type I superconductor is set by the critical field; the current induces a magnetic field that adds to the applied field, and the total cannot exceed the critical field. In fact, this limit is not very restrictive unless the applied field is already a considerable fraction of Hc. As we show below, type I conductors can carry very high current densities without inducing fields that approach Hc. Their use in magnets and motors is restricted by the low value of Hc rather than by the field-free critical current.
The critical current of a type II superconductor is set by a completely different mechanism that may limit it to a very small value. When the conductor is in the mixed state, the current imposes a force (the Lorentz force) on the magnetic vortices that thread through it. If the vortices move in response to this force their motion produces an electrical resistance. For this reason, a pure, perfect type II conductor becomes resistive as soon as it enters the mixed state, at Hc1, and is even less useful than a type I superconductor. However, the vortices are linear features that interact with microstructural defects such as precipitates and grain boundaries. Just as microstructural defects pin dislocations and increase strength, they also pin superconducting vortices so that there is a finite critical current in the mixed state. The technologically useful type II superconductors have microstructures that are engineered to provide the strong vortex pinning needed to support high critical currents at high fields.
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