Hydrodynamic optical-field-ionized plasma waveguides for laser plasma accelerators

This thesis documents the experimental development of low-density hydrodynamic optical-field-ionized (HOFI) plasma channels. Results from two experiments are presented; the first, a proof of concept experiment, performed in Oxford, demonstrating for the first time that the formation of low-density (∼1017 cm-3) hydrodynamic plasma channels was possible using optical-field-ionization. The second experiment, performed at Astra-Gemini TA2, aimed to demonstrate the generation of low-density axicon-formed HOFI plasma waveguides and subsequently to demonstrate high-intensity guiding in those waveguides at the highest available repetition rate. In the Oxford experiments, short (2-4mm) HOFI plasma channels were generated in hydrogen with a spherical lens and probed with longitudinal interferometry. The channels had on-axis densities between 1.6 à 1017 cm-3 and 1 à 1018 cm-3 and matched spot sizes of order 30μm to 60μm. A computational investigation into the guiding properties of the measured plasma channels indicated that they would be suitable as plasma waveguides. In the experiments at Astra-Gemini TA2, HOFI plasma channels up to 16mm in length were generated in hydrogen using an axicon. These were probed with both longitudinal and transverse interferometry and had on-axis densities as low as 2 à 1017 cm-3 . These waveguides were subsequently used to successfully guide high-intensity (>1017 W cm-2) pulses over the full 16mm or 14.5 Rayleigh ranges, with a maximum throughput of 60 % observed for the higher intensities. The variation in guiding with pressure and with guided beam intensity was investigated experimentally. It was found that the high-intensity guided pulse modified the HOFI plasma channel, with the result of increasing the relative throughput of the guided beam.