  We perform numerical simulations of magneto-rotational instability in a local
patch of accretion disk in which radiation pressure exceeds gas pressure. Such
conditions may occur in the central regions of disks surrounding compact
objects in active galactic nuclei and Galactic X-ray sources. We assume
axisymmetry, and neglect vertical stratification. The growth rates of the
instability on initially uniform magnetic fields are consistent with the linear
analysis of Blaes & Socrates 2001. As is the case when radiation effects are
neglected, the non-linear development of the instability leads to transitory
turbulence when the initial magnetic field has no net vertical flux. During the
turbulent phase, angular momentum is transported outwards. The Maxwell stress
is a few times the Reynolds stress, and their sum is about four times the mean
pressure in the vertical component of the magnetic field. For magnetic pressure
exceeding gas pressure, turbulent fluctuations in the field produce density
contrasts about equal to the ratio of magnetic to gas pressure. These are many
times larger than in the corresponding gas pressure dominated situation, and
may have profound implications for the steady-state vertical structure of
radiation-dominated disks. Diffusion of radiation from compressed regions damps
turbulent motions, converting kinetic energy into photon energy.
