Diagnosis of high energy electron beams produced by laser wakefield accelerators

This thesis discusses the production and diagnosis of electron bunches from laser produced wakefields. For the right laser and plasma parameters, monoenergetic features were observed in the electron spectrum. These mononenergetic beams are found to be sensitive to plasma density and laser parameters. In particular, the beams were found to show shot-to-shot variations in energy and pointing. Simulations were performed to study the mechanism of electron injection and acceleration. Further results demonstrate how the intermittancy of the electron beam may at times be an artifact caused by pointing instabilities. This thesis also discusses the first reported experimental observation of photon acceleration from a laser-produced wakefield. The spectrum of the transmitted light from a wakefield accelerator was measured. A large density dependent blue-shifted portion of the light was observed which cannot be explained by flash ionisation. A photon kinetic model of the experiment demonstrates that this blue-shifting occurs at the back of the pulse. Here, the moving density gradient of the wake provides a time varying refractive index suitable for photon acceleration. Thus comparison between theory and experiment allows one to optically characterise the wakefield accelerating structure. Finally the thesis presents experimental measurements of the electron bunch duration. A chirped probe pulse is passed through a birefringent (ZnTe) crystal close to the wakefield generated electron bunch. Frequency components of the probe pulse that coincide temporally with the Coulomb field of the electrons at the location of the crystal experience a rotation of polarization due to the induced Kerr effect. Measuring the spectrum of the rotated component, allows calculation of the electron bunch duration. Limitations to the technique are discussed and the methods used to overcome limitations are described. The results presented in this thesis constitute the highest resolution measurement of single laser produced electron bunches thus far.