Parametric study of density down-ramp injection in laser wakefield acceleration

Abstract: In laser wakefield acceleration (LWFA) a high intensity laser pulse is used to excite a plasma density wave with an associated electric field. This electric field can be used to accelerate electrons. However, to be accelerated the electrons first of all need to enter the plasma wave. This process is called injection. In this thesis a scheme for injecting electrons into a laser wakefield accelerator is studied. Focus lies on particle-in-cell simulations performed on a computer cluster. A parametric scan is performed where a density down-ramp’s slope and length is varied. A linear relation between the density down-ramp length and injected charge is shown. Furthermore a small density difference is shown to yield higher electron energies. A logarithmic relation between the density down-ramp slope and injected charge is shown. The slope can be optimised to control the spatial distribution of injected electrons within the plasma wave. A high peak current is shown to preserve a mono-energetic distribution over the acceleration length. A number of simulations is performed to explain experimental results where a large variation of injected charge is shown. A second injection mechanism is identified as the source of a large variation in injected charge. An imaging diagnostic system with a resolution of 2 μm looking at the Thomson scattered light from the laser pulse is designed and implemented. The Thomson scattered light is proportional to the background density of the electrons, and could therefore be used to detect density gradients.