Experimental studies of the dynamics of neutral electron-positron beams

Abstract

This thesis documents the experimental investigation of laser-driven positron beams, and the interaction between relativistic quasi-neutral electron-positron beams and an ambient electron-ion plasma. In both investigations, the Astra-Gemini laser system was used to produce laser-wakefield accelerated electron beams, that subsequently seeded a quantum electromagnetic cascade in a high-Z material to produce the necessary positron and quasi-neutral electron-positron beams. The development of hybrid accelerators that fuse the best features of plasma-based and radio-frequency acceleration techniques, has gained a lot of interest recently, with the potential of reducing the cost, size and exclusivity of conventional particle accelerators. In this thesis, high-quality laser-driven positron beams are characterised spectrally and spatially, with the results showing that such beams are of a sufficient quality to be used in the injection phase of a larger radio-frequency accelerator. In the second investigation, the development of a current-driven instability within an electron-positron beam is observed for the first time. This was accomplished through the technique of proton radiography, which probed the remnant magnetic fields left in a background electron-ion plasma post-propagation of a quasi-neutral electron-positron beam. These strong, long-lasting remnant fields (> 1 T), along with the other experimental parameters, return an equipartition parameter of εB ≈ 10-3, which correlates well with values for models of astrophysical phenomena involving pair-plasma jets and outflows interacting with the intergalactic medium. Experimental results are supported with simulations and analytical estimates. The work garnered from these experiments lays the foundation for further investigations to be built upon. Since the time these experiments were performed, significant advances have been made in the field of laser-wakefield acceleration, with higher energy electron beams carrying more charge now possible. Repeating the experiments with such an electron source could yield higher energy electron-positron beams with higher densities. This offers the potential to study greater development of current-driven instabilities and the generation of stronger magnetic fields. There is also the potential to improve the positron beam characteristics, with the possibility of using magnetic chicanes and quadrupole systems to select narrow-energy bands, as well as improve the beam divergence.

Date of Award	Jul 2020
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Gianluca Sarri (Supervisor) & Marco Borghesi (Supervisor)