Fluxon

In physics, a fluxon is a quantum of electromagnetic flux. The term may have any of several related meanings.

Superconductivity
In the context of superconductivity, in type II superconductors fluxons (also known as Abrikosov vortices) can form when the applied field lies between $$B_{c_1}$$ and $$B_{c_2}$$. The fluxon is a small whisker of normal phase surrounded by superconducting phase, and Supercurrents circulate around the normal core. The magnetic field through such a whisker and its neighborhood, which has size of the order of London penetration depth $$\lambda_L$$ (~100 nm), is quantized because of the phase properties of the magnetic vector potential in quantum electrodynamics, see magnetic flux quantum for details.

In the context of long Superconductor-Insulator-Superconductor Josephson tunnel junctions, a fluxon (aka Josephson vortex) is made of circulating supercurrents and has no normal core in the tunneling barrier. Supercurrents circulate just around the mathematical center of a fluxon, which is situated with the (insulating) Josephson barrier. Again, the magnetic flux created by circulating supercurrents is equal to a magnetic flux quantum $$\Phi_0$$ (or less, if the superconducting electrodes of the Josephson junction are thinner than $$\lambda_L$$).

Magnetohydrodynamics modeling
In the context of numerical MHD modeling, a fluxon is a discretized magnetic field line, representing a finite amount of magnetic flux in a localized bundle in the model. Fluxon models are explicitly designed to preserve the topology of the magnetic field, overcoming numerical resistivity effects in Eulerian models.