  A scalar adjoint field is introduced as a spatial average over (anti)calorons
in a thermalized SU(2) Yang-Mills theory. This field is associated with the
thermal ground state in the deconfining phase and acts as a background for
gauge fields of trivial topology. Without invoking detailed microscopic
information we study the properties of the corresponding potential, and we
discuss its thermodynamical implications. We also investigate the gluon
condensate at finite temperature relating it to the adjoint scalar field.
