Thesis

Laser generated plasma torch injectors for beam-driven plasma wake field accelerators

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Awarding institution
  • University of Strathclyde
Date of award
  • 2021
Thesis identifier
  • T16028
Person Identifier (Local)
  • 201694752
Qualification Level
Qualification Name
Department, School or Faculty
Abstract
  • The demand for high-quality relativistic electron beams continuously grows due to socioeconomic applications, most of which have been enabled by radio-frequency acceleratorsin the first place. However, further development of this matured technology implies anunsustainably growing footprint of these machines and therefore causes immense costsand limited availability of such facilities. At the same time, conventional accelerators mayhave reached the ceiling of their capabilities, e.g. in terms of achievable electron beamquality. Novel accelerator schemes are therefore needed.Beam-driven plasma wakefield accelerators (PWFAs) offer the necessary environmentfor a conceptual change by providing much stronger and phase-constant accelerationcompared to their conventional counterparts. Employing plasma takes advantage of anindestructible medium that offers flexible design via its fundamental property – theplasma density.While intense R&D has fostered a clearer understanding of plasma wakefields in thepast decades, challenges remain to be mastered before PWFAs reach the status of amatured technology that is ready to drive next-generation applications. These typicallynecessitate electron sources producing high-quality beams in a reliable fashion, whichare both attributes that are dominated by the employed mechanism for electron injection.Reshaping the plasma density by hydrodynamic density downramp injectors has beenshown to facilitate controlled injection in laser-driven wakefield accelerators, but proveselusive in PWFA.However, injection is readily achievable with the plasma torch injector, an all-opticaldensity downramp scheme that has enabled controlled injection in PWFA experimentsfor the first time. It relies on the superposition of laser-generated plasma rather thanhydrodynamic reorganisation of gas media and offers the capability for 3D shaping ofdensity distributions. In this context, results from the first experimental implementationof this technique at SLAC FACET are studied by extensive particle-in-cell simulations(PIC) revealing the involved dynamics in addition to strategies for further improving theinjector’s resilience to operational instabilities.Combining experimental observations and simulations, a phenomenological modelis derived that quantifies the charge of injected electron populations based on thegeometry and distribution of the wakefields and plasma torch. This approach opens newperspectives on downramp physics and may facilitate the design and optimisation ofdownramp injectors while avoiding computationally costly PIC simulations.Based on the congruence of the experiment, model and numerical investigations, furtherPIC studies are carried out to examine two extreme cases of plasma torch injection. Forplasma torches wider than the wakefields, conventional downramp physics are recoveredand high-quality electron beams are obtained rivalling the state-of-the-art of conventionallinear accelerators and even offer the potential to supersede those. In contrast to thevarious gas density downramp injection schemes, the plasma torch additionally enablesinjection from density spikes much narrower than the plasma accelerator. This paves theway for new regimes of electron generation, such as counter-oscillating twin-beamlets– structures that have not been observed before in wakefield accelerators. This lays thefoundation for novel modalities, which bear particular potential for controlled generationof highly polarised x-rays.
Advisor / supervisor
  • Jaroszynski, D. A. (Dino A.)
  • Hidding, Bernhard
Resource Type
Note
  • Previously held under moratorium from 22nd July 2021 until 26th July 2023.
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