Investigation of the expansion characteristics and dynamics of colliding laser-produced plasmas

Al-Juboori, Haider Mahdi; von Plessen, Gero (Thesis advisor); Kull, Hans-Jörg (Thesis advisor)

Aachen : RWTH Aachen University (2020, 2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020. - Dissertation, University College Dublin, 2020

Abstract

Colliding laser-produced plasmas (CLPP) are formed when two high power laser beams with a short pulse are focused onto a surface. There are many promising applications for CLPP including; analytical spectroscopy, electron/ion beam generation, VUV/X-ray sources for lithography, microscopy, radiography and thin-film deposition. The characteristics of CLPP are strongly dependent on the choice of experimental conditions. The main investigations in this thesis are based on time-resolved measurements of CLPP recorded in the visible part of the spectrum. The key parameters that affect CLPP characteristics are the plasma material, the shape and geometry of the target, the laser wavelength, pulse duration, energy and spot size and the ambient background pressure. The delay time of any observation as well as the structural setup of the optics will also affect measurements made. The aim of the work presented here is to further advance and study colliding plasma techniques, as well as other methods to realize and control species density, with a view to gaining a deeper understanding of the complex mechanisms involved. This knowledge may inform the development of CLPP physics to optimize plasmas aimed at specific applications. In this work, the experimental arrangement was designed to systematically observe CLPP. In this setup a single Nd:YAG laser was used with a wedge prism to create two beams which were subsequently focused onto the target to create two seed plasmas using a single lens. Two different focusing lenses with effective focal lengths of approximately 100mm and 125mm were used and this resulted in a seed separation of around 1.66mm and 2.16mm respectively. Spatially and temporally resolved observations techniques were employed to observe the interaction zone between the two plasmas. This allowed the geometrical characteristics of the stagnation layer and its expansion dynamics to be investigated. Plasmas were created from homogenous target materials, specifically Aluminum and Silicon, as well as an interaction zone created from heterogeneous target material, Silicon and Aluminum. For all three types of CLPP three different wedge targets were used with angles ranging from flat (180o) to (80o) between the surfaces where the two seeds were created. The pulse length of the laser, used in the fundamental, was of the order of 10ns and laser energies ranging from 254 to 670 mJ were used. Time-resolved emission imaging was used to track the response and behavior of the stagnation layer features giving a good indication of the degree of plume interpenetration for all cases studied. The work focused on observations using filters centered at 400nm and 450nm.The study provided a considerable amount of detailed data related to the geometrical analysis and expansion velocity of the interaction zone. The use of the same experimental arrangement for the whole set of experiments has allowed a consistent, comprehensive data set to be collected. This approach means that the data are directly comparable and inter-relatable, and this coupled with fast-frame imaging using spectroscopic methods has revealed the essential elements of the stagnation layer dynamics. Consequently, this work extends the understanding of the behavior of CLPP.