Ion acceleration by collisionless shocks and solitons in laser plasma interactions

The excitation of non-linear electrostatic waves, such as shocks or solitons, by ultraintense laser interaction with over dense plasmas and related acceleration of ions by reflection from the moving wave front have been investigated numerically by 1D particle-in-cell simulations. Linearly polarized pulses with dimensionless amplitude a₀ ~ n_e/n_c drive solitary waves or multi-peak structures depending on the pulse duration. Such nonlinear waves drive secondary ion acceleration in the plasma bulk, the acceleration dynamics being more complex than “specular” reflection of ions from the wave front. Possibly novel features observed in the dynamics of solitary waves include a strong collective oscillation of the electrostatic field and the pulsed nature of ion acceleration. In a cold ion background, wave loading effects prevent “true” shock wave formation and efficient mono-energetic acceleration. Mono-energetic peaks appear only as a result of such pulse acceleration in which the corresponding number of ions is relatively low. Acceleration of large fraction of ions lead to quenching and slowing down of the wave, resulting in broadening of the energy spectrum. The background ion distribution., i.e; ions with some initial energy spread, plays an important role in the ion acceleration dynamics. For instance, appearance of “true” shock waves with steady ion reflection from the wave front is observed only for warm ions. For long pulses, we envision a possible novel mechanism of “ion surfing” acceleration in a nonlinear ion wave driven by pulsed radiation pressure at the laser-plasma interface. Conditions on laser and plasma parameters for the generation and stability of both shock- and soliton-like waves are discussed. Circularly polarized pulses drive “hole boring” or “pure piston” acceleration at the plasma surface, accelerated ions propagate thro- gh the plasma in a purely ballistic way causing almost no perturbation in the plasma and do not generate nonlinear waves in initially cold plasmas.