Electron Beam Dynamics in a Proton-driven Plasma Wakefield

  • Barney Williamson

Student thesis: Phd


The scale of frontier particle physics experiments is expected to go on increasing, precipitated by a physics case that demands ever higher energy collisions of more particles, more frequently. All working colliders in particle physics today operate on the basis of radiofrequency (RF) acceleration in metallic cavities. However, fundamental limits on the achievable accelerating gradient per cavity mean that to obtain higher energy gain requires longer accelerators, and larger and more costly beam facilities. Electrostatic density waves driven in plasma can accommodate accelerating electric fields that are orders of magnitude greater than those found in metallic cavities, offering a route to lower-cost beam facilities. Experimentally, plasma wakefield accelerators have been demonstrated as a compact source of high energy electrons with features that could be well suited to specific applications. One such scheme is the proton-driven plasma wakefield accelerator, where a highly relativistic proton beam sustains an accelerating field over many metres of plasma, owing to the substantial energy it carries. Because of this property, an accelerated witness beam may reach the energy frontier in a single plasma stage, decreasing the complexity of a plasma-based collider. The reproducibility and quality of the accelerated witness beam are key for the successful operation of any collider, and for the most part these considerations still remain to be addressed for plasma-based accelerators. \pagestyle{plain} This thesis makes use of extensive particle-in-cell simulations to investigate those aspects of the proton-driven scheme on which achievable beam quality depends and gives guidance for future proton-driven plasma wakefield acceleration experiments, which could prove significant for the development of plasma-based electron-positron colliders. The first simulation study presented in this thesis demonstrates that an LWFA can provide short duration, narrow, and low emittance electron beams at injection, which also have a sufficient energy and charge, to facilitate beam quality preservation during proton-driven plasma wakefield acceleration. Significantly, the required duration and transverse beam size are not readily achievable with conventional RF photoinjectors. The second study characterises the betatron emission from an electron beam accelerated in a proton-driven plasma wakefield under a range of experimental conditions. It demonstrates that a significant quantity of detectable photons are emitted by the accelerated electron beam into high angles away from the proton driver, therefore enabling a non-intercepting emittance measurement. Such a measurement will allow the characterisation of electron beam quality inside the plasma wakefield itself, a unique tool to diagnose the evolution of electron beam emittance within a single accelerating stage. Finally, long-range simulations show the impact of radiation reaction on proton-driven plasma wakefield acceleration, demonstrating the point at which radiative damping of the electron beam emittance becomes significant. Radiative damping of electron beams ultimately defines the long term dynamics of plasma wakefield-accelerated beams, and may be exploited to reach novel luminosity regimes at the interaction point of a plasma-based collider. The emittance preservation of a radiating positron beam, accelerated in a nonlinear electron-driven plasma wakefield, is also shown, which could simplify the design of a plasma-based electron-positron collider.
Date of Award31 Dec 2020
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorDarren Graham (Supervisor) & Guoxing Xia (Supervisor)

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