Spectroscopic diagnostics of plume rebound and shockwave dynamics of confined aluminum laser plasma plumes

Generation and expansion dynamics of aluminum laser plasma plumes generated between parallel plates of varying separation (ΔZ = 2.0, 3.2, 4.0, and 5.6 mm), which confined plume expansion normal to the ablation surface, were diagnosed. Space and time resolved visible emission spectroscopy in the spectral range λ = 355-470 nm and time gated visible imaging were employed to record emission spectra and plume dynamics. Space and time resolved profiles of N_e (the electron density), T_e (the electron temperature), and T_ionz (the ionization temperature) were compared for different positions in the plasma plume. Significant modifications of the profiles of the above parameters were observed for plasma-surface collisions at the inner surface of the front plate, which formed a barrier to the free expansion of the plasma plume generated by the laser light on the surface of the back plate. Shockwave generation at the collision interface resulted in delayed compression of the low-density plasma plume near the inner ablation surface, at late stages in the plasma history. Upon exiting the cavity formed by the two plates, through an aperture in the front plate, the plasma plume underwent a second phase of free expansion.