(originally equal to the field energy) will be unchanged. The net result is a change by a factor 2.5. In a type II superconductor the magnetic flux and the energy density are more concentrated close to the center of a flux tube or vortex line, and there is a repulsive (attractive) interaction between parallel (antiparallel) vortices, with a logarithmic dependence on the separation.

The simplest model for a colour flux tube is the bag model [@Johnson:1975zp; @Johnson:1975sg], which is analogous to a type I superconductor. The tension in an unconstrained flux tube is here proportional to $\sqrt{C_2}$ [@Johnson:1975sg], in contradiction to the lattice result. However, for a flux tube constrained to the width of an elementary string, the energy is indeed proportional to the lattice result $C_2$ [@Semay:2004br].

Lattice calculations by Cardoso *et al.* [@Cardoso:2009kz] show that the energy in the longitudinal colour-electric field is dominating over the transverse colour-magnetic field, with other components small, and the result shows similarities with a type II superconductor. However, recent calculations by Cea *et al.* [@Cea:2014uja] indicate, that the best analogy with a dual superconductor is close to the transition between type I and type II, but with the Ginzburg–Landau parameter within the 