Proton Shock Acceleration in Laser-Plasma Interactions

The formation of strong, high Mach number (2–3), electrostatic shocks by laser pulses incident on overdense plasma slabs is observed in one- and two-dimensional particle-in-cell simulations, for a wide range of intensities, pulse durations, target thicknesses, and densities. The shocks propagate undisturbed across the plasma, accelerating the ions (protons). For a dimensionless field strength parameter $a_0=16$ ($I{\lambda }^2\approx 3\times {10}^{20}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}\mu {\mathrm{m}}^2$, where $I$ is the intensity and $\lambda$ the wavelength), and target thicknesses of a few microns, the shock is responsible for the highest energy protons. A plateau in the ion spectrum provides a direct signature for shock acceleration.