QCD Effective Lagrangian and Condensation of Chromomagnetic Flux Tubes

We compute the effective action for covariantly constant gauge fields that are solutions of the sourceless Yang-Mills equation and have the form of magnetic flux tubes. They represent a superposition of infinite many alternating monopole/ani-monopole pairs situated at infinity, with each pair having a structure similar to the Nielsen-Olesen magnetic flux tube. The chromomagnetic flux tubes condensation is stable and indicates that the Yang-Mills vacuum state is highly degenerate.