Brilliant gamma-ray emission from near-critical plasma interaction with ultraintense laser pulses*

$\gamma$-ray is the electromagnetic radiation having the highest photon energy and smallest wavelength, which has a broad range of applications in material science, nuclear physics, astrophysics and so on. In this talk, I shall report recent progresses [1-5] on theoretical and numerical studies of laser-driven brilliant gamma-ray radiation in near critical plasmas at Peking University (PKU), where an intense circularly polarized (CP) lasers. A novel resonant acceleration scheme can be achieved [1, 4] for generating dense relativistic electron bunches and emitting brilliant $\gamma$-ray pulses, where the laser frequency matches with that of electron betatron oscillation under quasistatic electromagnetic fields and radiation reaction in plasma. 3D PIC simulations show that brilliant $\gamma$-ray radiation with energy of 3J and brightness of $10^{24}$photons/s/mm$^{2}$/mrad$^2$/0.1\%BW (at 3MeV) can be produced by using CP lasers at intensity $10^{22}$W/cm$^{2}$. It is found [3, 4, 5] that the total number of radiated photons scales as $a^{2}/S^{1/2}$ and the conversion efficiency scales as $a^{3}/S$, where $S=(n_e/n_c)a$ and a is the laser normalized amplitude. Further studies show [4,5] that if the laser intensity is increased to $10^{23}$W/cm$^{2}$, the quantum electrodynamic (QED) effects are in favor of trapping and achieving resonance acceleration of electrons, resulting in production of brilliant $\gamma$-ray pulses with unprecedented power of 6.7PW and brightness of $10^{25}$photons/s/mm$^{2}$/mrad$^{2}$/0.1\%BW (at 15MeV). To the best of our knowledge, this is the $\gamma$-ray source with the highest peak brightness in tens-MeV regime ever reported in the literature. [1] B. Liu et al., PRL 110, 045002 (2013). [2] B. Liu et al., PoP 22, 080704 (2015). [3] H. X. Chang, B. Qiao et al., PRE 92, 053107 (2015); [4] H. X. Chang, B. Qiao et al., under Review, PRL (2016); [5] T. W. Huang, et al., PRE 93, 063203 (2016).