  The singlet coupling to the topological charge density in the instanton
vacuum, causes the instantons and antiinstantons to be screened over distances
of the order of 1/2 fm. Dilute instanton systems behave as a free gas, while
dense instanton systems behave as a plasma. The free gas behaviour is favored
by a density of 1 fm$^{-4}$. Owing to the Higgs mechanism, the $\eta$' mass is
heavy (1100 MeV). The vacuum topological susceptibility is small (0.07
fm$^{-4}$) and consistent with the QCD Ward identity. In the chiral limit, the
singlet screening vanishes, leading to a four dimensional plasma state with a
temperature given by the nonet decay constant $f$ (90 MeV). The phase is Debye
screened. In the presence of matter, the screening is quenched, and a
Kosterlitz-Thouless transition may occur signaling the restoration of the
$U_A(1)$ symmetry. We suggest to use the spatial asymptotics of the static
topological charge correlator together with the topological susceptibility to
probe the interplay between the $U_A(1)$ restoration and the chiral restoration
in finite temperature QCD. The relevance of these results to the bulk
thermodynamics in the instanton vacuum is discussed.

High Energy Physics - Phenomenology