Ionization dynamics of high-intensity laser-target interactions

The ionization dynamics of thin foils irradiated by an ultrashort pulse laser is investigated with a fully relativistic 2D particle-in-cell model. The PIC model is integrated with a ionization dynamics model, which includes optical field and collisional ionizations. The spatio-temporal evolution of the ion charge and electron density of a 5 micron aluminum foil are studied for peak laser intensities 10$^{18}$-10$^{20}$ W/cm$^{2}$ and laser pulse duration of 80 fs. The optical field ionization dominates in the pre-plasma region, creating a dense plasma of highly charged ions, while the collisional ionization is most effective in the bulk of the target. A series of ionization waves launched at the front surface of the foil propagate with high velocity ($\sim $0.2$c)$ through the target.