  The emission of multi-MeV ($\gamma$-ray) photons from the interaction of a
high-powered laser pulse with a dense plasma target is studied using
particle-in-cell simulations. A new set of diagnostic techniques is presented
and applied to analyze the intense field and ultra-relativistic electrons. Such
methods elucidate the dominant processes responsible for efficient
laser-to-$\gamma$ energy conversion, which include nonlinear Compton scattering
and magneto-bremsstrahlung radiation, and provide a clear picture of the
interaction on a microscopic level. We identify regions in the plasma target of
high photon energy-density and obtain an energy conversion efficiency as high
as 30%. The essential characteristics of the interaction are validated with
full-3D simulations.
