Thesis

Investigation of laser-solid interaction physics with tightly focused, ultra-intense laser pulses

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Awarding institution
  • University of Strathclyde
Date of award
  • 2022
Thesis identifier
  • T16200
Person Identifier (Local)
  • 201773352
Qualification Level
Qualification Name
Department, School or Faculty
Abstract
  • This thesis reports on experimental investigations of laser-solid interactions forintensities at the frontier of what is possible using current laser technology. Peaklaser intensities of up to 5 × 1021 Wcm−2 were achieved through the focusing ofpicosecond laser pulses to near-wavelength sized focal spots with a novel, elliptical focusing plasma mirror. The influence of these high intensities and smallfocal spot sizes on proton acceleration in the relativistic transparency regime andon the temperature scaling and dynamics of fast electrons is explored. These twoaspects of laser-solid interactions are of critical importance to the realisation ofmany envisioned applications, in addition to providing insight into the fundamental underpinning physics. The work reported here is structured into two mainsections.The first study reports on an investigation of the influence of ultra-high intensity and near-wavelength sized focal spot, achieved through the use of F/1focusing plasma optics, on proton acceleration from ultra-thin foil targets, forwhich the highest proton energies to date are achieved. In this regime, acceleration occurs via a transparency-enhanced, TNSA-RPA hybrid mechanism. Whencomparing the spectral properties of protons accelerated using F/1 focusing toa F/3 focusing geometry, significant reductions in both maximum proton energyand laser-to-proton energy conversion efficiency were observed, despite the highernominal laser intensity. Furthermore, the measured holeboring velocity was alsofound to be reduced for F/1 focusing, when compared with the F/3 case. Thesefindings are explained in terms of transparency-induced self-focusing, which occurs very strongly in the F/3 case, but to a negligible extent for F/1 focusing,and is shown by 2D particle-in-cell simulations. This results in an enhancementin the peak intensity achieved by the F/3 following the onset of transparency,boosting the intensity beyond the nominal peak intensity of the F/1 focusinggeometry. This increased intensity subsequently results in enhanced proton energies, with both the peak intensity and proton energy maximised for an optimalfocal spot size (ϕL = 5 µm) and target thickness (ℓ = 100 nm). Limited enhancement occurs for F/1 focusing to close to the laser wavelength or when the targetremains opaque for the duration of the interaction, as self-focusing cannot takeplace. This result will help guide the design of future experiments, by showingthat optimal proton energies in the transparency regime are obtained for moreconventional focusing conditions, significantly reducing the technical challengesand financial expense involved.The second study presents findings related to the scaling of fast electron temperature within thin foil targets, and the effect of this on electron refluxing andproton acceleration via the TNSA mechanism. Using measurements of copper Kαphotons from 25 µm thick copper targets and protons accelerated via the TNSAmechanism from 6 µm thick aluminium targets, the fast electron temperaturescaling with intensity was determined. This was found to scale more slowly withincreasing intensity than would be expected from existing models, resulting inreduced electron temperatures. Analytical modelling shows that this slower scaling is likely due to the inhibition of electron heating as a result of the relativisticskin-depth, which becomes on the order of ∼ 10 nm for intensities > 1021 Wcm−2.The decreasing skin-depth alone is however not suffice to fully explain the slowingof the temperature scaling. Modifications to the plasma density within the skindepth, based on relativistic effects or radiation pressure induced compression arediscussed, supported by analytical modelling and 2D particle-in-cell simulations,are shown to produce better agreement with the results measured experimentally.The electron temperatures measured are also shown to result in significantly increased electron refluxing within the target, whilst the effect of the slower scalingwith intensity is shown to adversely affect the scaling of maximum proton energiesgenerated via the TNSA mechanism. This result highlights that, when movingto higher intensities, the gains in electron temperature may not be as significantas previously predicted, which has a significant impact on the generation of highenergy particles and ionising radiation.
Advisor / supervisor
  • McKenna, Paul
Resource Type
Note
  • Previously held under moratorium from 12th April 2022 until 12th April 2023.
DOI
Date Created
  • 2021

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