Mev to Gev Direct Forward and Backward Acceleration of Electrons from Undersense Plasma using a Radially Polarized Ultra-Intense Laser Focus*

In this work we perform a computational investigation of the direct acceleration of electrons produced during ionization of undersense neon gas using tightly focused and radially polarized Petawatt-class short pulse lasers with wavelength range from 0.8 to 2µm by numerically solving the relativistically invariant Lorentz equations with non-paraxial fields, incorporating semi-classical tunneling ionization and Monte-Carlo type sampling of the focal volume. The accelerated electrons energy gain increases at longer laser wavelengths and GeV energies are reached for electrons ionized from the neon inner shell. Backward acceleration of electrons is observed for a range of initial positions and phases of ionization of neon charge states. This apparent counterintuitive phenomenon is directly linked to the radial polarization state of the incident laser beam that results in a strong longitudinal electric field (E_z) when tightly focused. Electrons ionized near the focal center at the phase when E_z is pointed toward the forward propagation direction experiences an initial push in the backward direction. A parametric study of the phenomenon by varying laser parameters will be presented.