Determination of recombination length of a non-equilibrium plasma produced by laser ablation

An experimental study of the laser ablation produced plasma evolution is necessary for its deeper understanding, since plasma expansion has both spatially and temporally varying characteristics. We irradiated a Cu target with a KrF laser beam. A small Faraday cup array and an axial Faraday cup were used as diagnostic systems, in order to study the spatial variation in the total charge carried by plasma ions. Charge loss during the plasma expansion was observed, which was attributed to the charged species recombination. This occurred upstream to the critical distance where the plasma density is high enough. Downstream the critical distance the plasma particles collisions were negligible and the ion charge remained frozen. In these experiments it was observed that the critical distance for charge recombination was a function of laser fluence.