- We investigate the effect of external field boundary conditions of a background chromomagnetic field on the instability (discovered by Nielsen and Olesen) for the vacuum of a non-Abelian field theory. We find that the vacuum is neither stabilized by a one-dimensional nor a two-dimensional cutoff of this magnetic field. However, the vacuum in the presence of flux tubes whose length is restricted to be under a critical value, L0, is stable. Therefore, there is a tendency for flux tubes of lengths greater than L0 to spontaneously fragment into segments each of which is smaller than L0. This corresponds to a dual picture which allows stable electric flux tubes.