Proton and Positron Acceleration from Ultra-Intense Lasers

• Author / Creator

  Kerr, Shaun M

• Ultra-intense lasers (I > 10^18 W/cm^2) irradiating solid targets can produce bright, energetic (MeV) beams of protons, with potential applications from fusion energy to tumour treatment. Spatially and spectrally controlling these particle beams remains a challenge, however, and experimental and computational work performed for this thesis seeks to address these issues. In one typical acceleration process relativistic electrons driven by the laser form an escaping electron cloud, which accelerates protons away from the target surface, called target normal sheath acceleration (TNSA). For thick (mm), high-Z targets, positrons are generated in the target from high-energy x-rays and accelerated by the same TNSA electron cloud. Due to their low mass and rapid acceleration, positrons can act as a probe of transient field features. Experimental results from the OMEGA EP laser show multi-peak modulations developing in the positron spectra when the laser energy exceeds one kilojoule. Detailed 2D particle-in-cell simulations using the LSP code were carried out, and suggest that for high laser energies, multiple acceleration phases are present in the TNSA process and leave signatures in the positron spectrum. They also give insight into the influence of the proton contaminant layer and target geometry on the positron acceleration process. Experimental work was also done to demonstrate an alternative acceleration mechanism - collisionless shockwave acceleration (CSA) - which could offer better control of the proton beam spectrum. CSA occurs when plasma pressure drives an electrostatic shock that reflects ions ahead of it, generating quasi-monoenergetic ion beams. The CSA experiment was performed using the ultra-intense, wavelength = 1.054 micrometer Titan laser at the Jupiter Laser Facility. Near-critical density targets were used to maximize laser coupling, while tailored density profiles on both the front and rear sides of the target were necessary to create a shock and inhibit the strength of TNSA fields that would obscure the CSA spectrum. Density shaping was achieved experimentally by using a nanosecond beam to expand Mylar foils, and a carefully timed, ultra-intense picosecond pulse to drive shocks in the decompressing foils at the optimal density profile. Narrow energy spread proton and ion beams were observed using an Imaging Proton Spectrometer (IPS), with characteristics consistent with generation from CSA. This is believed to be the first observation of highly energetic (>> 1 MeV) proton beams from CSA with a wavelength = 1.054 micrometer laser.

• Subjects / Keywords

  • Laser-generated positrons
  • Ion acceleration
  • Ultra-intense laser-plasma interaction

• Graduation date

  Spring 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3PN8XW6C

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.