Multi-pulse laser wakefield acceleration

Thesis

This thesis presents results towards the realisation of high-repetition rate laser wakefield acceleration (LWFA). I have framed the contributions around the development of multi- pulse laser wakefield acceleration (MP-LWFA), which is a scheme using a train of laser pulses rather than a single pulse to excite a plasma wakefield. High energy but low peak power lasers that can reach multi-kHz repetition rates are already commercially available, and could become viable as drivers in the MP-LWFA scheme in the near future.

First, I discuss the development of a waveguiding mechanism capable of overcoming the diffraction length of the tightly focused laser pulses without suffering laser damage after continuous operation. I describe the novel solution of plasma channel waveguides and focus on the measurement of their densities using optical methods. This is challenging due to the small size (∼ 100 μm) and often low densities (n e ∼ 10 17 cm −3 ) of the plasma structures, leading to a low phase contrast. I discuss several techniques for measuring such structures, and methods for analysing the obtained data. Finally, I show how these methods are applied using data acquired from a recent experiment that was performed to produce plasma channels waveguides.

Second, I discuss the issue of ion motion and wakefield decay. MP-LWFA requires that the wakefields after each pulse in the train interact coherently to excite a wakefield through resonance. However, the interaction between the wakefield and the plasma ions lead to several instabilities which moves energy into other modes, eventually breaking up the wakefield. I present new experimental results measuring the wakefield decay time to be on the order of 100 plasma periods in hydrogen, which is likely to be sufficient for the MP-LWFA scheme.

Authors: Jonnerby, J

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: wakefield lifetime; laser wakefield acceleration
• Subjects: Laser-plasma interactions