Study of Laser Produced Plasma Plume and its Dynamics in Nickel thin Film

Abstract

Plasma plume formation from a nickel thin film in rear ablation geometry, its propagation and in teraction with ambient gas using lasers of different pulse widths and wavelengths has been studied in a range of background pressures. The plasma plume dynamics is studied using fast imaging, Langmuir probe and spectroscopic diagnostics. Stark broadening of neutral nickel lines is studied in correlation with H↵ line broadening and the Stark broadening parameters of nickel lines are estimated. The Stark parameters estimated for nickel lines are used for the estimation of plasma density at different background pressures. In the rear ablation geometry, the evolution of plasma plume, directionality, splitting and expansion are found to depend on parameters like laser pulse width, wavelength,fluence, film thickness and background pressure with which the plume inter acts. The experimental results show that the background pressure confines the plasma plume and thus helps increase in the electron density. In the case of fs ablation, the plume shows a linear expansion at lower background pressures, while at higher pressures the expansion appears to fol low shock wave like nature. For ps ablation the expansion appears to follow a blast wave model at lower pressures, which, however, fits with drag model at higher pressures. In case of ns ab lation, shock wave like nature is observed for all the pressures. The dependence of wavelength and laser fluence on plume formation is also investigated. It is observed that the dynamics of constituent species in plasma plume say, neutrals and ions exhibit distinct behavior which can be seen from the spectroscopic and fast imaging. The TOF spectra for the neutral and ionic species of the plasma plume show that it has multiple distributions probably, exhibiting a signature of their origin. The numerical fit of Shifted Maxwell Boltzman distribution provides some information into the generation mechanism or the energy gain of different constituents of the plasma. The dif ferent diagnostics used are well cross correlated and provide valuable information about the plume constituents and its dynamics. The experimental results also give some information about the com bined effect of laser pulse width and thickness of the thin film used for generation of plasma plume in rear ablation geometry. The experimental results throw some light on the multi layer formation and its effects on neutral and ionic species resulting in the acceleration of ionic species which in turn generates fast neutrals by means of recombination. The acceleration mechanism appears to be of double-layer nature, however, the field estimated from the observed enhancement of velocity of ionic components shows significantly large values suggesting further extensive study in the rear ablation geometry opted in the present study. In this work we observed a significant asymmetry in spectral shape for one neutral Ni line (712.2 nm) even at an extended delay time as well as at moderate distance indicating the probability of presence of micro-electric field for an extended duration, which may again result in continuous acceleration of the ionic species.