On the onset of relativistic transparency in intense laser-solid interactions

Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2021
Thesis identifier
  • T15849
Person Identifier (Local)
  • 201559863
Qualification Level
Qualification Name
Department, School or Faculty
  • This thesis reports on experimental and numerical investigations of the onset of relativistic transparency in initially solid density plasmas, during interactions with an ultra-intense short laser pulse and the effect this has on the resulting plasma dynamics. The research presented was conducted with the aim of increasing understanding of the complex dynamics of these interactions to accelerate the development of particle and radiation sources towards potential applications. These range from being a driver for inertially confined fusion to proton therapy for cancer treatment. To this end, control of the interaction conditions and optimisation of the particle and radiation sources generated are essential. The thesis reports results which support and deepen understanding of plasma dynamics in the relativistic transparency regime. In the first of these experiments, the onset time of transparency in initially opaque plasmas was investigated as a function of target thickness, pulse energy and pulse duration for pulses with a maximum intensity on the order of 3 × 1020 Wcm−2 , on-target energies of ∼3 J and minimum pulse lengths of ∼50 fs. The fractional transmitted energy was found to match well with predictions using an established transparency model. Simultaneous frequency-resolved optical gating (FROG) measurements of the transmitted pulse profile compare favourably to particle-in-cell (PIC) code simulations. These simulations indicate that the coherent transition radiation generated by accelerated electrons interferes with the transmitted pulse in those targets which undergo transparency, resulting in fringes in the spectrum of the light measured at the target rear. This novel application of spectral interferometry is shown to be capable of high resolution measurements of the transmitted pulse spectrum and offers the potential ability to control and optimise the onset time of transparency in expanding thin foil targets. These results accelerate the development of ion acceleration in the transparency regime as the onset time greatly affects the ion beam properties, including maximum energy. In the second of these experiments, it was found that as target thickness is decreased from the micron scale to tens of nanometres that the absorption of laser energy to electrons transitions from surface to volume dominated processes. Experimental measurements of the energy absorption and the population of fast electrons escaping the target show a maximum of ∼ 80% total absorption at an optimum target thickness of ∼380 nm, with a slow decrease with increasing target thickness, for incident pulses with intensities on the order 9 × 1019 Wcm−2 and pulse lengths of ∼700 fs. For thinner targets, particle-in-cell code simulations indicate that transparency occurs increasingly early, which results in a decrease in absorption. However, the experimental results suggest that with decreasing thickness more electrons escape the target, with a higher average energy, with the particle-in-cell simulations predicting similar results. This increase in “escaped” electron fraction is attributed to a direct laser acceleration of the bulk target electrons. These findings indicate a trade-off between total energy coupling to the electrons and the spectral properties of the electron beam produced. In the third of these experiments, the role of transparency in laser driven ion acceleration is investigated. This mode consists of hybrid phases of RPA, TNSA and enhancement due to the onset of transparency, with recent experimental results suggesting this mechanism can accelerate protons to maximum energies on the order of 100 MeV, for pulses with intensities on the order 3 × 1020 Wcm−2 . By comparing results from aluminium and plastic targets, it is demonstrated that the chosen target material has an effect on the maximum energy and flux of the accelerated proton beam. Plastic targets are shown to produce higher energy protons, on laser-axis, compared to those from metallic targets due to the presence of hydrogen throughout the target volume rather than just in hydrocarbon surface contaminants. Particle-in-cell code simulations highlight the differing behaviour of the two target types investigated post-transparency where it is found that the peak accelerating field structures act to drive the proton population more efficiently in plastic targets. These results provide a key motivation for the choice of target material for a number of applications, where high energy, high flux proton beams are desired. The results of these investigations enable a greater understanding of the complex dynamics of initially solid density foil targets dynamically expanding to the point at which they become relativistically transparent to the intense laser light, accelerating the development of these novel ion sources towards potential applications.
Advisor / supervisor
  • McKenna, Paul
Resource Type
  • This thesis was previously held under moratorium from 26th May 2021 until 26th May 2023.
Date Created
  • 2020
Former identifier
  • 9912982093202996