  Synchrotron emission from a supernova necessitates a magnetic field, but it
is unknown how strong the relevant magnetic fields are, and what mechanism
generates them. In this study, we perform high-resolution numerical gas
dynamics calculations to determine the growth of turbulence due to
Rayleigh-Taylor instability, and the resulting kinetic energy in turbulent
fluctuations, to infer the strength of magnetic fields amplified by this
turbulence. We find that Rayleigh-Taylor instability can produce turbulent
fluctuations strong enough to amplify magnetic fields to a few percent of
equipartition with the thermal energy. This turbulence stays concentrated near
the reverse shock, but averaging this magnetic energy throughout the shocked
region (weighting by emissivity) sets the magnetic fields at a minimum of 0.3
percent of equipartition. This suggests a minimum effective magnetic field
strength ($\epsilon_B > 0.003$) which should be present in all interacting
supernovae.
