Laser ablation of Cu and plume expansion into 1 atm ambient gas

A one-dimensional gas-dynamic model is presented for the laser ablation of Cu and the expansion of the Cu vapor in a background gas (He) at 1 atm. The ionization of Cu and He, the inverse bremsstrahlung absorption processes and photoionization process, and the back flux onto the target are considered simultaneously. The binary diffusion, the viscosity, and the thermal conduction including the electron thermal conduction are considered as well. Numerical results show that the consideration of ionization and laser absorption in the plume greatly influences the gas dynamics. The ionization of Cu enables the recondensation at the target surface to happen even during the laser pulse. The ionization degree of Cu and He may change greatly with the location in the plume. For laser irradiances ranging from 2 to 9x10(12) W/m(2), the simulations show that the second-order ionization of Cu competes with the first-order ionization. In the region close to the target surface, the first-order ionization of Cu dominates. In the core of the plasma, the second-order ionization of Cu may dominate over the first-order ionization at laser irradiances higher than 7x10(12) W/m(2). In the mixing layer, the first-order ionization of Cu is always more important than the second-order ionization although the latter increases monotonously with laser irradiance. The ionization of He is only important in the mixing layer. The plume expansion velocity is much larger than that without ionization and laser absorption by the plume. The relative importance of different laser absorption mechanisms may change with time. Close to the surface photoionization and electron-neutral inverse bremsstrahlung are always important. Once the ionization in the plume starts, at later time, electron-ion inverse bremsstrahlung can become more important than photoionization in the plume core until the shock wave front. Unlike in the vacuum case, electron-neutral inverse bremsstrahlung is very strong due to the relatively high number density of neutral atoms in the plume in the presence of a dense ambient gas. A similar laser irradiance threshold is found for the ablation rate and the plasma formation in the plume, which agrees well with the case of nanosecond laser ablation of metals in vacuum. (C) 2005 American Institute of Physics.