Low density plasma waveguides for multi-GeV laser Wakefield accelerators

Particle accelerators are vital tools for discovery in the fields of medicine, engineering, life sciences and physics. These machines, however, are becoming increasingly large, technically challenging to construct, and uneconomical. Laser wakefield accelerators (LWFAs) provide immense accelerating fields, and offer the potential to reduce the size of particle accelerators by a factor of 1000, though several challenges are yet to be overcome. One such challenge is the development of a low density plasma waveguide capable of guiding high-intensity laser pulses of distances up to a metre to form the basis of a multi-GeV LWFA stage. It has been shown that Hydrodynamic Optical-Field-Ionsied (HOFI) plasma channels can support low-density LWFA stages operating at a high repetition rate (f_rep ≳ 1 kHz). This thesis focuses on the development of low density, HOFI plasma channels through experiments and simulation. Optical guiding is demonstrated of laser pulses with a peak input intensity of 6 × 10¹⁷ W cm⁻² through 100 mm long plasma channels with on-axis densities measured to be as low as ne0 = (1.0 ± 0.3) × 10¹⁷ cm⁻³. It is found that the attenuation lengths of HOFI plasma channels were limited by the channel depth and radial extent to L_att ≲ 100 mm. To account for this, a new method for extending such plasma channels to the metre-scale is proposed. Through transverse interferometry measurements taken as the guided pulse arrives, it is shown that a neutral collar of gas which surrounds the initial HOFI channel is ionised by the guided pulse. The attenuation length increased instantaneously from L_att = (15 ± 8) mm to L_att = (0.42 ± 0.01) m. Particle-in-cell simulations demonstrate that the leading edge of a conditioning pulse injected into this structure ionises the neutral gas in its transverse wings, forming a deep plasma channel which guides the bulk of the pulse with low propagation losses. Design of an experiment employing a metre-scale CHOFI plasma channel and of electron acceleration to the multi-GeV level is presented, including a novel gas cell design. Simulations demonstrating propagation and electron acceleration in CHOFI plasma channels for the first time are discussed. Two different injection mechanisms capable of injecting electrons without significantly increasing experimental complexity are considered — ionisation injection, and density transition injection. It is shown that electron bunches with peak energies ΔW = 3.57 GeV with a relative energy spread of just 2.5% can be generated. It is clear that CHOFI plasma channels are likely to be very well suited to high repetition rate, multi-GeV LWFA stages.