Dynamics of laser-induced plume expansion into an ambient gas during film deposition

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Abstract
The interaction between laser-induced plasma plume and gas background during film deposition is studied theoretically. The gas dynamic model has been developed to describe the temporal and spatial evolution of the plume expanding into a gas under a pressure of typically a few tens of pascals. The model is based upon the generation of a spherical plasma cloud whose expansion is described in a two-temperature approximation by use of the Euler equations. The dynamics of laser ablation of YBa2Cu3O7-x superconductor in an oxygen environment have been analysed using this model. The calculations show that the plume does not stop upon reaching the maximum propagation distance but moves repeatedly back and forth up to 200 mu s after ablation. Two types of shock waves determine the interaction dynamics: series of primary (external) waves which propagate through the ambient gas, and a secondary shock wave which is formed in the plume region and executes a periodic motion. The conversion of cloud ionization energy into gas dynamic flow during expansion approximately doubles the kinetic energy of the plasma, but has no significant effect on the plume propagation distance. The results are in quantitative agreement with the previously observed oscillation behaviour of the plume.