Identification of discontinuities in plasma plume evolution

TL;DR

This paper investigates the formation and evolution of discontinuities such as shock waves and ionization fronts in plasma plumes during laser ablation, using shadowgraphy to analyze different stages and mechanisms.

Contribution

It provides a detailed experimental analysis of plasma plume discontinuities and introduces a method to distinguish ionization fronts from contact surfaces based on refractive index contributions.

Findings

Identified irradiance threshold for different ablation mechanisms

Mapped stages of plasma plume evolution

Separated ionization front from contact surface using refractive index analysis

Abstract

The ejection of material during laser ablation gives rise to the development of discontinuities in the ambient gas. Several of these discontinuities are observed and characterized, including externally and internally propagating shock waves, contact surface, and the ionization front. Qualitative experimental observations and analysis of these discontinuities is presented. Results from shadowgraphy enabled determination of an irradiance threshold between two different ablation mechanisms, and determination of several stages of plasma plume evolution. Consideration of the refractive index as a dynamic sum of the contributions from gas and electrons led to separate identification of ionization front from the contact surface. Furthermore, ionization front was observed to lead the shock wave at the earlier stage of the ablation.