The singlet coupling to the topological charge density in the instanton vacuum, causes the instantons and anti-instantons 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 η′ mass is heavy (1100 MeV). The vacuum topological susceptibility is small (0.07 fm −4) and consistent with the QCD Ward identify. In the chiral limit, the singlet screening vanishes, leading to a four-dimensional plasma state with a temperature given by the nonet decay constant ƒ (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.