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

Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2021
Thesis identifier
  • T16028
Person Identifier (Local)
  • 201694752
Qualification Level
Qualification Name
Department, School or Faculty
  • The demand for high-quality relativistic electron beams continuously grows due to socioeconomic applications, most of which have been enabled by radio-frequency accelerators in the first place. However, further development of this matured technology implies an unsustainably growing footprint of these machines and therefore causes immense costs and limited availability of such facilities. At the same time, conventional accelerators may have reached the ceiling of their capabilities, e.g. in terms of achievable electron beam quality. Novel accelerator schemes are therefore needed. Beam-driven plasma wakefield accelerators (PWFAs) offer the necessary environment for a conceptual change by providing much stronger and phase-constant acceleration compared to their conventional counterparts. Employing plasma takes advantage of an indestructible medium that offers flexible design via its fundamental property – the plasma density. While intense R&D has fostered a clearer understanding of plasma wakefields in the past decades, challenges remain to be mastered before PWFAs reach the status of a matured technology that is ready to drive next-generation applications. These typically necessitate electron sources producing high-quality beams in a reliable fashion, which are both attributes that are dominated by the employed mechanism for electron injection. Reshaping the plasma density by hydrodynamic density downramp injectors has been shown to facilitate controlled injection in laser-driven wakefield accelerators, but proves elusive in PWFA. However, injection is readily achievable with the plasma torch injector, an all-optical density downramp scheme that has enabled controlled injection in PWFA experiments for the first time. It relies on the superposition of laser-generated plasma rather than hydrodynamic reorganisation of gas media and offers the capability for 3D shaping of density distributions. In this context, results from the first experimental implementation of 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 the injector’s resilience to operational instabilities. Combining experimental observations and simulations, a phenomenological model is derived that quantifies the charge of injected electron populations based on the geometry and distribution of the wakefields and plasma torch. This approach opens new perspectives on downramp physics and may facilitate the design and optimisation of downramp injectors while avoiding computationally costly PIC simulations. Based on the congruence of the experiment, model and numerical investigations, further PIC studies are carried out to examine two extreme cases of plasma torch injection. For plasma torches wider than the wakefields, conventional downramp physics are recovered and high-quality electron beams are obtained rivalling the state-of-the-art of conventional linear accelerators and even offer the potential to supersede those. In contrast to the various gas density downramp injection schemes, the plasma torch additionally enables injection from density spikes much narrower than the plasma accelerator. This paves the way for new regimes of electron generation, such as counter-oscillating twin-beamlets – structures that have not been observed before in wakefield accelerators. This lays the foundation for novel modalities, which bear particular potential for controlled generation of highly polarised x-rays.
Advisor / supervisor
  • Jaroszynski, D. A. (Dino A.)
  • Hidding, Bernhard
Resource Type
  • Previously held under moratorium from 22nd July 2021 until 26th July 2023.