Gamma-ray emission in ultra-intense laser interaction with solid targets

Electrons moving in ultra-intense laser fields emit hard radiation due to radiation reaction and non-linear Compton scattering. Multi-MeV $\gamma$-rays were measured by scattering of electrons generated from laser wakefield with a focused laser of intensity $a_0 \sim 1$. However, non-linear Compton scattering and radiation reaction is also an efficient mechanism for generating copious amount of $\gamma$-rays in laser interaction with solids at intensities approaching $\sim 10^{22}$ W/cm$^{2}$. Emission of $\gamma$-rays due to radiation reaction and bremsstrahlung are investigated here in the high intensity regime of laser-solid target interaction by using a combination of Particle-in-Cell and Monte Carlo radiation transport simulations. The relative contribution of these processes is analyzed as a function of the target parameters. We concentrate on the influence of the target thickness, material, preplasma conditions or a surface structure on the generation of high energy photons and study separately their energy and angular distributions. It is demonstrated that the presence of preplasma or a special surface structure may significantly enhance emission of hard $\gamma$ photons and their cut-off energy and change their angular distribution.