Intra-pulse dynamics of laser-driven ion acceleration in ultra-thin foils

  • Hersimerjit Padda

Student thesis: Doctoral Thesis


This thesis reports on experimental and numerical investigations of ion acceler-ation driven by the interaction of short, intense laser pulses with ultra-thin, solid targets in which relativistic transparency is induced. In particular, it explores the multiple laser-ion acceleration mechanisms that take place over the duration of the laser pulse. Investigating these acceleration mechanisms is important for understanding the underlying physical dynamics and optimising laser-driven ion acceleration.;The investigations featured in this thesis result from intense laser-solid in-teractions conducted at the Rutherford Appleton Laboratory, using the Vulcan Petawatt laser system. The first investigation explores the spatial-intensity profile of the proton beam accelerated from thin (tens of nanometre) aluminium targets.The beam of accelerated protons displayed a variety of features, including a low-energy annular profile, a high energy component with a small divergence and Rayleigh-Taylor-like instabilities. A particularly interesting observation is the low-energy annular profile, which is shown to be sensitive to target thickness and proton energy.;Numerical investigations using particle-in-cell (PIC) simulations exhibit the same trends and demonstrate that the radiation pressure from the laser pulse drives an expansion of the target ions within the spatial extent of the laser focal spot. This induces a radial deection of relatively low energy sheath-accelerated protons to form an annular distribution.;Through variation of the target foil thickness, the opening angle of the ring is shown to be correlated to the point in time during the laser pulse interaction at which the target becomes transparent to the laser (in a process termed relativistic induced transparency). The ring is largest when transparency occurs close to the peak of the laser intensity.;The second investigation focuses on the rising edge profile of the laser pulse and the correlation between its temporal width and the resultant maximum proton energy. An important parameter to consider when irradiating nanometre-thick foils is the laser contrast. However, the effect of the temporal width of the laser pulse at 1% of the peak, where the intensity is ~1018 Wcm-2, has not been previously explored. Using CH targets with a fixed thickness, a range of proton energies, from 20-70 MeV, are measured experimentally.;The temporal width ofthe laser pulse is measured using a second order autocorrelator and is used to model the rising edge of the laser pulse on target. The temporal width at 50%, 10% and 1%, of the peak of the pulse, is measured. The measured proton energies are found to strongly correlate with the temporal width at the 1% level, and as the duration at this pulse width increased the maximum proton energy decreased.;Using particle-in-cell simulations, a detailed numerical investigation is carried out to understand the effect the rising edge of the laser pulse has on proton energies. By increasing the temporal width at 1%, the expansion of the target increased, resulting in a less efficient acceleration of protons. Furthermore, by inducing a small expansion in the target before the peak of the pulse arrives, the hole boring mechanism of RPA can be optimised along the laser axis.;However, in the case where the temporal width at 1% is relatively larger, the hole boring mechanism no longer dominates the interaction, as the target undergoes relativis-tic induced transparency on the rising edge of the pulse, limiting the effect of hole boring. Improving the laser contrast on the picosecond time-scale could result in higher and stable proton energy.
Date of Award13 Jun 2017
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsEPSRC (Engineering and Physical Sciences Research Council)
SupervisorPaul McKenna (Supervisor) & Dino Jaroszynski (Supervisor)

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