Energy partition, gamma-ray emission, and radiation reaction in the near-quantum electrodynamical regime of laser-plasma interaction

When extremely intense lasers (I >= 10(22) W/cm(2)) interact with plasmas, a significant fraction of the pulse energy is converted into photon emission in the multi-MeV energy range. This emission results in a radiation reaction (RR) force on electrons, which becomes important at ultrahigh intensities. Using three-dimensional particle-in-cell simulations which include a quantum electrodynamics model for the gamma-photons emission, the corresponding RR force and electron-positron pair creation, the energy partition in the laser-plasma system is investigated. At sufficiently high laser amplitudes, the fraction of laser energy coupled to electrons decreases, while the energy converted to gamma-photons increases. The interaction becomes an efficient source of gamma-rays when I > 10(24) W/cm(2), with up to 40% of the laser energy converted to high-energy photons. A systematic study of energy partition and gamma-photon emission angle shows the influence of laser intensity and polarization for two plasma conditions: high-density carbon targets and a low-density hydrogen targets. We find that in the opaque region, the laser-to-photon conversion efficiency scales as I-0(3/2) for linearly polarized and I-0(2-2.5) for circularly polarized lasers, respectively. In the relativistically transparent regime, the power-laws merge into I-0(1/2) for both polarizations and photon emission peaks in the forward direction with a relatively small divergence angle (