Ionization dynamics of high-intensity laser-target interactions

The ionization dynamics of a thin aluminum foil irradiated by an ultrashort high-intensity laser is investigated with a two-dimensional relativistic electromagnetic particle-in-cell model, which includes optical field and collisional ionizations. The spatio-temporal characteristics of the ion charge and electron density have been studied for peak laser intensities between 10(22) and 10(24) W m(-2) and a laser pulse duration of 80 fs. A series of ionization waves, launched near the front target surface, propagate through the target with a velocity of about two tenths the speed of light. In the pre-plasma region the aluminum is almost fully ionized due to optical field ionization, while in the bulk of the target the collisional ionization is more efficient. The ion charge in the bulk is a result of a complex sequence of events, the major role in which is played by the deposition of laser energy in the system and its distribution among the various degrees of freedom.