Novel injection and targetry in laser wakefield acceleration

Abstract

This thesis presents experimental research into laser wakefield accelerators. It is divided into two main themes; the first was the examination of a novel injection mechanism for wakefield acceleration using a clustered medium and the second was the use of density-length scans to study the optimum conditions for different injection mechanisms.

First, we address an experiment that examined cluster injection, where methane was used for the clustering medium. A clustered target, with cluster radii of 5-10nm, were observed to generate electron beams with 60+/-2.5 pC, which was an increase in the charge of the electron beam by a factor of 24 when compared to molecular methane in a gas cell, when driven with a 10TW laser pulse. The clustered target generates 20% more charge, than the highest charge beams from self-injection in the gas cell. This demonstrates that the injection mechanism was distinct from ionisation injection.

Next, a clustered target was examined at a higher power laser facility, where a 100TW laser pulse was used to drive a wakefield accelerator. In the same gas jet, helium (SI), helium + 1% nitrogen (II) and methane (CI) were used. The total charge of the electron beam generated for all three targets was 100pC. For cluster injection at higher densities, the electron beam was observed to develop transverse oscillations which were attributed to the onset of the hosing instability. This oscillation also coincides with a large increase in x-ray flux. The highest flux betatron beam from CI was 38% higher than II and 8 times greater than SI.

The latter two chapters of this thesis discuss a multi-parameter scan, where density and length were varied sequentially. For self-injection, the highest beam energy observed was 210+/-19MeV and the highest beam charge measured was 50+/-2.3 pC and the lowest beam divergence measured was 1.5+/-0.1 mrad$.

The final chapter examined a density-length parameter scan using ionisation injection. The maximum energy observed was 159+/-22MeV, the highest charge measured was 35.7+/-3.4pC and the lowest divergence measured was 1.6+/-0.1 mrad. The lowest eccentricity for II was measured to be 23% lower for II than SI. We conclude that for the highest charge beam, SI in a gas cell or CI is the ideal target. Electron beams from SI reach the highest energies and the lowest divergence. Electron beams from II have the lowest eccentricity, pointing deviation and transverse momentum.