Langmuir probe characterization of laser-generated plasmas

Non-equilibrium plasmas expanding in vacuum at supersonic velocities produced by interaction between the 9 ns Nd:YAG laser and metallic bulk targets (Ta, Ag, Fe, Ti) are investigated. The plasma plume is emitted mainly along the normal to the irradiated target surface and the plasma temperature and density are strongly dependent on the expansion time and distance from the target surface. Plasma characterization measurements are performed in situ with a millimetric Langmuir probe. The probe tip current vs. the polarization voltage is measured as a function of the expansion time. 3D plots of the probe I vs. V curve with respect to the time are reconstructed in order to describe the plasma plume dynamics. Investigations on the plume structure, electron density, electron temperature and ion saturation currents are reported. Obtained results indicate that high temperature and density gradients are present and that strong ion accelerations occur. The temperatures and densities measured by means of the Langmuir probe have been compared with those obtained by other analysis techniques and estimations of such parameters in proximity of target surface (i.e. in the early stages of plasma plume expansion) are also given.