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Colliding plasmas in air

Davitt, Stephen orcid logoORCID: 0000-0003-3950-8525 (2019) Colliding plasmas in air. PhD thesis, Dublin City University.

When two, or more, laser produced plasma plumes are forced to collide there are two extremes in the resulting outcome, (i) the plasmas may interpenetrate, or, (ii) a layer of stagnated plasma is formed at the collision front. The degree of interpenetration (or stagnation) is determined by the so-called ‘collisionality parameter’ which is the ratio of the separation of the individual ‘seed’ plasmas to the ion-ion mean- free path. Previous work on colliding plasmas in vacuo show that when ‘stagnation layers’ are formed, they exhibit an enhancement in plasma emission duration and a reasonably uniform spatial distribution of electron density, electron temperature and atomic/ionic density. These characteristics have made stagnation layers of interest to the laser plasma community, however to date research has been almost exclusively in vacuo. This work concerns experimental efforts to produce similar conditions for plasmas formed on solid targets in air at atmospheric pressure. This was realised by creating plasmas on the walls of V-shaped aluminium targets irradiated by a single, defocused laser pulse. Channels of varying vertex angles (namely, 30o, 60o, 90o) were employed to alter the relative collisional velocity of the expanding plumes and thus the collisionality parameter. The formation and evolution of the plasmas were tracked using time resolved imaging and spectroscopy. It was observed that the confined plasma plumes exhibited three distinct phases. For early time delays (<100 ns) they were similar to plasmas formed on flat targets with a rapidly expanding plasma front that separates from the primary plasma located close to the target vertex. At later times the primary plasma splits into two components, a stationary or stagnated plasma remaining close to the target vertex and a lobe-like plume. The stationary or stagnated plasmas exhibited some of the characteristics of stagnation layers in that they were formed at the collision plane, showed little sign of expansion, and exhibited reasonable spatial uniformity and slower rates of decay for various key plasma parameters. Furthermore line emission from plasmas formed within the V-channel targets had enhancements of intensity upwards of 20% over that of a single plume formed from a flat target. Additionally, a compact, single shot, Fourier Transform Spectrometer was developed for recording weak spectral lines from laser produced plasmas and other pulsed sources. Details of the spectrometer design and its performance are also given in the thesis.
Item Type:Thesis (PhD)
Date of Award:March 2019
Supervisor(s):Costello, John T. and Kelly, Mossy
Uncontrolled Keywords:Compact Fourier Transform Spectrometer; LIBS; Colliding Plasmas
Subjects:Physical Sciences > Laser plasmas
Physical Sciences > Lasers
Physical Sciences > Physics
Physical Sciences > Spectrum analysis
Physical Sciences > Plasmas
DCU Faculties and Centres:DCU Faculties and Schools > Faculty of Science and Health > School of Physical Sciences
Research Institutes and Centres > National Centre for Plasma Science and Technology (NCPST)
Use License:This item is licensed under a Creative Commons Attribution-NonCommercial-No Derivative Works 3.0 License. View License
Funders:Science Foundation Ireland, FP7 EMJD 'EXTATIC'
ID Code:22751
Deposited On:03 Apr 2019 11:44 by John Costello . Last Modified 13 Dec 2019 13:58

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