Laser Wakefield Acceleration of Multi-GeV Electron Bunches with Petawatt-Scale Laser Pulses Guided in a Laser-Heated Capillary Discharge

Laser wakefield accelerators (LWFAs), which accelerate electrons in the fields of a plasma wave driven by the ponderomotive force of an intense laser pulse, have attracted intense research interest in recent years due to the high gradients they can support. These devices hold promise as a new class of compact accelerators for applications including free-electron lasers, particle colliders, and Thomson-scattered gamma ray sources. To date, the highest energy gains for a given laser power in LWFAs have been achieved by guiding the driving laser pulse in a pre-formed plasma channel. This dissertation covers the development and demonstration of a novel guiding structure: the laser-heated gas-filled capillary discharge waveguide. These structures are capable of guiding intense laser pulses over many diffraction lengths at low plasma densities required to mitigate bunch dephasing and accelerate electrons to high energies. In the work presented here were used to accelerate electrons to 7.8 GeV in 20 cm using 850 TW of power from the BELLA laser at Lawrence Berkeley National Laboratory (LBNL). Low power guiding experiments demonstrating the feasibility of the technique are described. Plasma heating was demonstrated through visible light plasma spectroscopic measurements, which through coincidence with improved guiding of a sub-ns probe beam indicated formation of a waveguide by plasma expansion driven by inverse-bremsstrahlung heating. The density and matched spot size of the plasma channel formed by laser-heating were diagnosed using measurements of the group velocity and spot-size oscillation of a guided probe beam. Two spectral interferometers were constructed for the group velocity measurements: a fiber-based Mach-Zehnder interferometer installed on a target prototyping vacuum chamber, and a two-color common path interferometer on the main BELLA beam line. The design of these setups as well as the algorithms used for analyzing the interferograms are described. These diagnostic measurements revealed strategies for optimizing waveguide performance through tuning of plasma and laser parameters, and were found to be in excellent agreement with magnetohydrodynamic simulations using the MARPLE code. Guiding of petawatt pulses and acceleration of electrons to multi-GeV energies in laser-heated capillary discharges is demonstrated, showing good agreement between low power guiding measurements, particle-in-cell simulations with the INF\&RNO code, and the results of high power laser experiments. Additionally, electron beams were produced using ionization injection in a localized region of high-Z gas in the capillary entrance with channel and laser parameters tuned to suppress self-trapping. Finally, a numerical model of third harmonic generation for femtosecond laser pulses is presented, which is used to simulate a possible design for an ultraviolet beamline for a future demonstration of laser-triggered bunch injection. The work described in this dissertation constitutes a foundation for future LWFA experiments aimed at the production of low energy spread electron beams and staged acceleration at the multi-GeV level through demonstration of controlled bunch injection with multi-GeV energy gain and development of a nonlinear optical modeling tool useful for the design of laser sources required for triggered bunch injection schemes.

Advisor: Leung, Ka-Ngo
Gonsalves, Anthony