UCLA

Plasma-based acceleration (PBA) refers to a class of advanced accelerator concepts in which the electromagnetic fields sustained in a plasma wave are used to accelerate charged particle beams. The strength of these fields can be orders of magnitude higher than those achievable in conventional radio-frequency accelerators. Thus, PBA offers the prospect of dramatically reducing the accelerator size and cost for a given beam energy. This makes PBA attractive for a variety of accelerator-based applications including high-energy collider physics, microscopy using free electron lasers, radiotherapy for cancer treatment, and ion implantation in semiconductors. Laser wakefield acceleration (LWFA), in which an intense laser pulse drives the plasma wave, is one of two concepts of interest for PBA. In this dissertation, new results on LWFA and new capabilities for its simulation are presented through a combination of theory and particle-in-cell computer simulations.In LWFA, the acceleration length and the total energy gain for an accelerating beam are critical system parameters. Lu et al. provided estimates for how these parameters scale in the nonlinear, self-guided regime as a function of basic quantities such as the laser wavelength, laser power, and plasma density. It was assumed that the laser is self-guided over a length greater than the length over which the beam dephases with respect to the wake, but it was recognized that this assumption may break down for sufficiently low plasma density. New results on this physics are presented in this dissertation. Using the quasistatic particle-in-cell (PIC) code QPAD, a parameter scan of simulations is carried out with a fixed laser vector potential, a laser profile which scales with the plasma skin depth c/ωp, and a varying ratio of the laser frequency ω0 to the plasma frequency ωp. It is found that the Lu et al. scaling laws are accurate for densities above a threshold value, below which the self-guiding length is less than the dephasing length and scales as Lguide ≈ 25zR where zR is the Rayleigh range. This results in similar scalings for the acceleration length Lacc ≈ 25zR and energy gain ∆E ≈ 25zR(ωp/c)mec 2 for sufficiently low density. The results indicate that near-term 10 PW lasers may be able to generate 30-GeV electron beams in a single 2-m plasma stage. They also inform longer-term design considerations for LWFA in the context of a linear collider.Spatiotemporally structured laser pulses, which feature coupled spacetime-dependent properties, have been proposed to enable or enhance a variety of laser-plasma interactions, including LWFA. Many of these methods require an accurate understanding of the linear propagation of the pulse. Previous work on linear pulse propagation has mostly considered the transverse or longitudinal evolution of the laser pulse separately, but these must be considered simultaneously to describe spatiotemporally structured pulses. To address this need, theory is developed to describe the linear evolution of the full spatiotemporal structure of a laser pulse in a linear dielectric. The approach uses the dispersion relation to derive integral expressions for the evolution of the pulse. Taylor expansion of the dispersion relation leads to systematic estimates for length scales over which spatiotemporal coupling emerges for a laser pulse with an envelope which is initially separable transversely and longitudinally. These are derived in vacuum and in cold, uniform plasma. The results are also described from the standpoint of single photons propagating within the pulse. Simulations confirm accuracy of the integral formulation and its predictions for the length scales over which spatiotemporal coupling emerges.Exploration of applications of spatiotemporally structured laser pulses requires an understanding of the kinds of spatiotemporal structure that can exist in nature. It is also desirable to be able to describe a given kind of spatiotemporal structure with a mathematical model that captures its essential features. In this dissertation, a general formalism describing spatiotemporally structured laser pulses is introduced which meets both of these demands. This concept of arbitrarily structured laser (ASTRL) pulses employs a superposition of prescribed pulses to create a desired field structure. The ASTRL formalism is capable of describing a variety of kinds of spatiotemporal structure, including pulses with an evolving focal point, spot size, orbital angular momentum, or polarization. Several examples are presented to illustrate the versatility of ASTRL pulses to address a broad range of laser-based applications, including LWFA, inertial confinement fusion, nanophotonics, and attosecond physics.Laser wakefield acceleration (LWFA) may enable the next generation of TeV-scale lepton colliders. Current designs are based on the use of multiple LWFA stages to overcome limitations on the energy gain achievable in a single stage. The use of stages, however, introduces challenges such as alignment, adiabatic matching between stages, and a lower average accelerating gradient. Flying focus pulses, a specific type of spatiotemporally structured pulse, have been proposed to circumvent dephasing. This would reduce both the number of stages and the total accelerator length required for a target energy by enabling operation at higher plasma densities. Adding to these concepts, LWFA driven by a discrete flying focus based on the ASTRL formalism is proposed. A sequence of laser pulses with staggered focal points and delays drives a plasma wave in which an electron beam experiences a near-constant accelerating gradient over distances beyond those attainable with a conventional pulse. Simulations with non-optimized parameters demonstrate that a discrete flying focus with a total energy of 150 J can transfer 40 GeV per electron to a 50-pC beam in a single 30-cm stage, corresponding to 50 dephasing lengths. Extension of the concept to positron acceleration through the use of a discrete set of higher-order Laguerre-Gaussian pulses is also presented.Fully relativistic particle-in-cell (PIC) simulations are crucial for understanding a variety of processes in plasma physics, including LWFA. Modern supercomputers based on graphics processing units (GPUs) offer the potential to perform PIC simulations of unprecedented scale, but require robust and feature-rich codes that can fully leverage their computational resources. To address this demand, GPU acceleration was implemented in the PIC code OSIRIS. An overview of the algorithm, which features a CUDA extension to the underlying Fortran architecture, is given. Detailed performance benchmarks for thermal plasmas are presented, which demonstrate excellent weak scaling on NERSC’s Perlmutter supercomputer and high levels of absolute performance. The robustness of the code to model a variety of physical systems is demonstrated via simulations of Weibel filamentation and laser-wakefield acceleration run with dynamic load balancing. Finally, measurements and analysis of energy consumption indicate that the GPU algorithm is up to 14 times faster and 7 times more energy efficient than the optimized CPU algorithm on a node-to-node basis. The described development addresses the PIC simulation community’s computational demands both by contributing a robust and performant GPU-accelerated PIC code and by providing insight into efficient use of GPU hardware.