Models and physics of plasma wakefield accelerators in beam-ionized gases

Plasma-based accelerators have attracted great attention recently for their high accelerating gradients. One of the leading plasma accelerator schemes is the plasma wakefield accelerator (PWFA), in which an intense relativistic electron or positron beam propagates in pre-ionized plasma and creates plasma waves. Most of the previous theories and experiments are concerned wakes created in pre-ionized plasmas. The work presented in this dissertation investigates the propagation of a high-energy electron beam through a gas that is self-consistently ionized by the beam's space charge. As shorter and shorter high-density beams are used in current experiments to get higher accelerating fields, the self-fields of these beams can ionize neutral gases or further ionize a plasma. This beam ionization can provide a new way of creating plasma sources for plasma wakefield accelerators or after the wakes in pre-formed PWFAs. The wake generation in self-ionized plasma is explored through 2-D and 3-D particle-in-cell (PIC) simulations. New physics phenomena such as ionization hosing in self-ionized plasma are found and studied analytically and with 3-D PIC simulations.; The implementation of 2-D and 3-D impact and field ionization models in the particle-in-cell code OSIRIS is introduced. The new OSIRIS model is then used to find the optimal gas density for maximizing the plasma wakefield in a self-ionized plasma. Simulation results and physical explanations are given.; Different gas types, beam parameters and gas densities are studied to support the design and analysis of current E-164 and E-164X PWFA experiments at the Stanford Linear Accelerator Center (SLAC). Simulations are performed for actual experimental conditions and the effects of experimental realities such as tilted and asymmetric beams and inhomogeneous plasmas are investigated. The simulations support the interpretation of the results in E-164 and E-164X. Excellent agreement between the energy gain observed in experiments and the simulations is obtained (only) when ionization and realistic gas profiles are included.; A new instability mechanism is observed and studied analytically and with simulations. The instability arises from the coupling between a beam and the offset plasma channel it creates when it is perturbed slightly from a straight path. The coupling can either add to the traditional electron hose instability in a pre-formed or replace it with a much slower growth depending on the radius of the ionization channel compared to the distance that electrons are blown outward by the beam space charge. (Abstract shortened by UMI.)...