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

Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2022
Thesis identifier
  • T16200
Person Identifier (Local)
  • 201773352
Qualification Level
Qualification Name
Department, School or Faculty
  • This thesis reports on experimental investigations of laser-solid interactions for intensities at the frontier of what is possible using current laser technology. Peak laser intensities of up to 5 × 1021 Wcm−2 were achieved through the focusing of picosecond laser pulses to near-wavelength sized focal spots with a novel, elliptical focusing plasma mirror. The influence of these high intensities and small focal spot sizes on proton acceleration in the relativistic transparency regime and on the temperature scaling and dynamics of fast electrons is explored. These two aspects of laser-solid interactions are of critical importance to the realisation of many envisioned applications, in addition to providing insight into the fundamental underpinning physics. The work reported here is structured into two main sections. 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/1 focusing plasma optics, on proton acceleration from ultra-thin foil targets, for which the highest proton energies to date are achieved. In this regime, acceleration occurs via a transparency-enhanced, TNSA-RPA hybrid mechanism. When comparing the spectral properties of protons accelerated using F/1 focusing to a F/3 focusing geometry, significant reductions in both maximum proton energy and laser-to-proton energy conversion efficiency were observed, despite the higher nominal laser intensity. Furthermore, the measured holeboring velocity was also found to be reduced for F/1 focusing, when compared with the F/3 case. These findings 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 enhancement in 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 focusing geometry. This increased intensity subsequently results in enhanced proton energies, with both the peak intensity and proton energy maximised for an optimal focal 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 target remains opaque for the duration of the interaction, as self-focusing cannot take place. This result will help guide the design of future experiments, by showing that optimal proton energies in the transparency regime are obtained for more conventional focusing conditions, significantly reducing the technical challenges and 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 and proton acceleration via the TNSA mechanism. Using measurements of copper Kα photons from 25 µm thick copper targets and protons accelerated via the TNSA mechanism from 6 µm thick aluminium targets, the fast electron temperature scaling with intensity was determined. This was found to scale more slowly with increasing intensity than would be expected from existing models, resulting in reduced electron temperatures. Analytical modelling shows that this slower scaling is likely due to the inhibition of electron heating as a result of the relativistic skin-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 slowing of the temperature scaling. Modifications to the plasma density within the skindepth, based on relativistic effects or radiation pressure induced compression are discussed, 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 scaling with intensity is shown to adversely affect the scaling of maximum proton energies generated via the TNSA mechanism. This result highlights that, when moving to higher intensities, the gains in electron temperature may not be as significant as previously predicted, which has a significant impact on the generation of high energy particles and ionising radiation.
Advisor / supervisor
  • McKenna, Paul
Resource Type
  • Previously held under moratorium from 12th April 2022 until 12th April 2023.
Date Created
  • 2021