  The gluon excitations of the QCD vacuum are investigated in the presence of a
static quark-antiquark source. It is shown that the ground state potential and
the excitation spectrum of dynamical gluon degrees of freedom, as determined in
our lattice simulations, agree remarkably well with model predictions based on
the diaelectric properties of the confining vacuum described as a dual
superconductor. The strong chromoelectric field of the static Q-Qbar source
creates a bubble (bag) in the condensed phase where weakly interacting gluon
modes can be excited. Some features and predictions of the bag model are
presented and the chromoelectric vortex limit at large quark-antiquark
separation (string formation) is briefly discussed.
