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Wavelength-dependent variations of the electron characteristics in laser-induced plasmas: a combined hydrodynamic and adiabatic expansion modelling and time-gated, optical emission imaging study

Research output: Contribution to journalArticle

Original languageEnglish
JournalJournal of Applied Physics
DateSubmitted - 16 Aug 2018
DateAccepted/In press (current) - 5 Feb 2019

Abstract

The spatial and temporal evolution of the absolute electron densities and temperatures in plasmas formed by nanosecond pulsed laser ablation of silicon in vacuum at two wavelengths (1064 and 532 nm), at similar irradiances, have been explored by complementary simulation (using combined hydrodynamic and adiabatic models) and experiment. Modelling the laser-target and laser-plume
interactions with the POLLUX code revealsthe evolving composition and
dynamics of the laser induced plasma (LIP) during the incident laser pulse: 532 nm irradiation causes more ablation, but the LIP formed by 1064 nm excitation has a higher average charge state and expands faster. The experimental data, from analysis of Stark broadened line shapes of SiIII and SiIV cations in time-gated, position- and wavelength-resolved images of the plume emission, allow characterisation of the plume dynamics at later times. These dynamics are compared with predictions from two forms of adiabatic expansion model. Both take as input parameters the plume properties returned by the POLLUX simulations for the end of the laser pulse, but differ according to whether the initial plasma is assumed isothermal or isentropic. The study illustrates: the important -dependences of the target absorption coefficient (in establishing the ablated material density) and of electron–ion inverse bremsstrahlung absorption (in coupling laser radiation into the emergent plasma); the extents to which these interactions, the relative ablation yields and the plume expansion dynamics depend on ; and the importance of identifying appropriate initial conditions for adiabatic expansion modelling of LIP in vacuum.

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