The multifaceted role of relativistic transparency in laser-plasma interactions

The nature of how light interacts with plasma is fundamentally altered when the bulk of the electrons become relativistic, manifested as an enhanced transparency of the plasma. Three dimensional particle-in-cell simulations demonstrate that this enhanced transparency of a relativistically hot plasma is sensitive to how the energy is partitioned between different degrees of freedom. For an anisotropic electron distribution, propagation characteristics, like the critical density, will depend on the polarization of the electromagnetic wave. Despite the onset of the Weibel instability in such plasmas, the anisotropy can persist long enough to affect laser propagation. This plasma can then function as a polarizer or a wave plate to dramatically alter the pulse polarization. We further demonstrate using numerical simulations that a high intensity laser pulse propagating through a classically overcritical, relativistically transparent plasma can generate a strong azimuthal magnetic field, leading to copious quantities of synchrotron radiation. An optimal channel setup is proposed and tested to produce a collimated beam of multi-MeV photons.