When extremely intense lasers (I ≥ 1022 W/cm2) 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 γ–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 γ-photons increases. The interaction becomes an efficient source of γ-rays when I > 1024 W/cm2, with up to 40% of the laser energy converted to high-energy photons. A systematic study of energy partition and γ-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 I03/2 for linearly polarized and I022.5 for circularly polarized lasers, respectively. In the relativistically transparent regime, the power-laws merge into I01/2 for both polarizations and photon emission peaks in the forward direction with a relatively small divergence angle (<20°), resulting in a collimated γ-ray beam.

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