  SU(2) Yang-Mills theory at finite extension or, equivalently, at finite
temperature is probed by a homogeneous chromomagnetic field. We use a recent
modified axial gauge formulation which has the novel feature of respecting the
center symmetry in perturbation theory. The characteristic properties of the
Z_2-symmetric phase, an extension-dependent mass term and antiperiodic boundary
conditions, provide stabilization against magnetic field formation for
sufficiently small extension or high temperature. In an extension of this
investigation to the deconfined phase with broken center symmetry, the combined
constraints of thermodynamic and magnetic stability are shown to yield many of
the high temperature properties of lattice SU(2) gauge theory.
